Of the four fundamental forces of nature, gravity is the one we experience most directly ‪—‬ it's what keeps our feet on the ground and the sun in the sky. Yet we still can't pin down its exact strength. Since the 1980s, scientists have made more than a dozen measurements to calculate the precise value of gravity, and many of those numbers contradict one another.

So why is it so hard to figure out how strong gravity is?

One problem is that gravity is weak. Gravity feels strong because we constantly feel the pull of Earth. But the force of gravity between any two objects in everyday life — or any two objects that can fit in an experimental lab — is extraordinarily weak.

"It's weak, and you have to measure this against the background of the Earth's gravitational field," Stephan Schlamminger, a physicist at the National Institute of Standards and Technology, told Live Science. "If we measure gravity, we have to use everyday objects, because these are the only ones where we know the mass. What you have to do in the lab is basically use two very controlled masses, bring them close together, and measure the force between them."

In an April 2026 study, Schlamminger and colleagues replicated a precision experiment to determine the strength of gravity and calculated a value different from the previous result. They used 13 tons (12 metric tons) of mercury to run their experiment, but even then, "the change in the gravitational field was only a millionth of the change that we have here from local gravity," he said.

The team’s measured value was 6.67387x10^-11 m³kg^-1s^-2, which was 0.0235% lower than the previous result — a small difference in everyday terms, but significant in the field of metrology.

Christian Rothleitner, a physicist at the German National Metrology Institute, co-authored a comprehensive review of all gravity measurements with Schlamminger in 2017 but was not involved in the new study.

"This small force has to be determined to six or more decimal places," Rothleitner told Live Science in an email. "This is equivalent to trying to measure the weight of 7 human cells."

Physics, engineering and psychology

One explanation for the discrepancy in values could be that all of the measurements are so imprecise that the true value lies somewhere within them. But each experiment reports a small margin of error, and those ranges don't overlap.

Schlamminger thinks there are three possible reasons for this.

"I have it as a handy-dandy acronym: It's PEP: P stands for physics, E stands for engineering, and the second P stands for psychology," Schlamminger said. "It's sorted by excitement."

The least probable explanation, he said, is the physics one: Maybe there's some element of physics that scientists don't yet understand. Just as general relativity extended scientists' understanding of gravity, there may be another realm of physics yet to be discovered.

"I think it's a remote possibility, but we should not exclude it," Schlamminger said.

Then, there's the engineering explanation: Every experiment uses a slightly different setup, resulting in different values. Some use a torsion balance, a device that senses tiny forces by measuring the twisting of a small fiber. Others use pendulums or free-falling objects. Each approach has its own potential sources of error, and those mistakes are difficult to untangle from the gravitational signal.

"I personally do not believe that the reason lies in the physics, but in the measurement technology," Rothleitner said.

Human error is another part of the engineering explanation. "Such an experiment requires expert knowledge in many areas of physics and measurement technology," Rothleitner said. "You cannot be an expert in all those fields. This kind of measurement is on the cutting edge of measurement science."

The most likely possibility, Schlamminger said, relates to psychology.

"There is a driver for these people who measure these numbers to give really, really small uncertainties" — that is, margins of error — "because it makes them famous," Schlamminger said. "Because the pressure is there, the uncertainties may be a little bit too small, and that's why they don't agree with each other."

In the end, though, a precise measurement of gravity may not matter. We know the product of G times Earth's mass, and that's enough for practical applications like launching rockets into space. That may be all we need for now.

"The value of Newton's gravitational constant is rather of academic interest," Rothleitner said. "If it were different, nations would have spent much more effort in determining it better."

Schlamminger still finds it exciting, though. "We live in a society where we think everything is discovered," he said. "But if you look, there's still terra incognita. There are still problems, and the problems may be small, but they're still problems we can solve and contribute to and find mesmerizing and intriguing. And this is one of those problems."

The sun has been Earth's constant companion ever since our planet emerged. But if the sun were to suddenly disappear, what would happen to our home planet?

To understand the fate of a sunless Earth, it's important to know how both arose. The sun formed about 4.6 billion years ago, when a massive spinning cloud of gas and dust collapsed in on itself and condensed, creating the biggest object in what would become our solar system and eventually reaching a temperature of 27 million degrees Fahrenheit (15 million degrees Celsius) at its core.

Much of the remaining material nearby then clumped up to form Earth and the other rocky planets, including Mercury, Venus and Mars, as well as moons and asteroids. Since its formation, Earth has been heavily reliant on its star. The sun's gravitational pull keeps our planet in orbit in the "Goldilocks zone," the just-right distance from its star where it's not too hot or too cold for water to exist as a liquid on a planet's surface. The sun also drives photosynthesis and water cycles, and it provides sunlight and heat, which influence our climate. Plus, the sun's ultraviolet light helps our bodies make vitamin D, which is needed for healthy bones and teeth.

If the sun suddenly vanished, Earth and the vast majority of life would be in dire straits. It would start "a ticking time bomb on the survival of every living thing on earth that relies on photosynthesis, which is the vast majority of surface life and all of humanity," Timothy Cronin, an associate professor of atmospheric science at MIT, told Live Science over email.

For at least 8 minutes, 20 seconds, no one would know the sun went missing ‪—‬ that's how long it takes light from the sun to reach Earth. During that time, "we'd almost certainly have no idea that anything had happened," Cronin said.

Then, the real trouble would begin.

After the sun's eight-minute swan song, there would be "a sudden blackout," Cronin said. Without sunlight, artificial lighting from electricity, oil or gas would be the main ways we could still generate light, along with fire, bioluminescence and fluorescence. We'd lose track of day and night. The moon, which reflects the sun's light, would go completely dark, although distant stars in the sky would still be visible. And without the sun's mass and gravity keeping the planets and other celestial bodies in orbit, "all the planets would fly off in the direction of their current travel," Cronin said.

But humanity would have more immediate problems than flying off into interstellar space. No sunlight would mean crucial processes, such as growing food, would become much more complicated.

Photosynthetic organisms would be done for, Michael Summers, a professor of planetary sciences and astronomy at George Mason University in Virginia, told Live Science. Most plants that weren't grown under artificial lighting would quickly suffer. And while some "might stay dormant for weeks to months, like they do in the wintertime, eventually all photosynthetic organisms would die."

Fungi, meanwhile, feed on living or dead matter, and "there would be a great deal of dead material available," Summers said. So fungi likely wouldn't die from a lack of food, but from the cold.

Cold planet

It wouldn't take long for frigid temperatures to change the Earth as we know it.

At first, Earth would cool by an average of roughly 36 F (20 C) every 24-hour period, Summers said. "That plunges almost the whole world into subfreezing temperatures within just two to three days," although as it got colder, the temperature change per day would decrease, he said. Small ponds might freeze over within a week, whereas lakes might take weeks or months. The oceans could persist "for many years, maybe decades," and in certain places, like "the deepest parts of the oceans where you have volcanoes, they might stay liquid for potentially as long as the volcanoes last," Summers said. "And that could be billions of years."

To understand how cold Earth would ultimately get, let's consider Pluto. Right now, Pluto is "about 40 times as far from the sun as Earth is, and the temperature there now is about minus 400 degrees Fahrenheit [minus 240 C]," Summers said. "Once you eject the Earth out of our solar system, it's going to get much further away than Pluto very quickly."

But Earth's temperature wouldn't reach absolute zero, thanks to the Big Bang that happened around 13.8 billion years ago. Even "the lowest temperatures in the universe are limited by heat that's left over from the Big Bang," Summers said. "Take any object very far away from a star and let it cool for a million years," and it will still remain a few degrees above absolute zero. The temperature of the leftover radiation known as the cosmic microwave background is about minus 454 F (minus 270 C), whereas absolute zero is slightly chillier at about minus 459 F (minus 273 C).

At an ultracold temperature, human civilization and most of life would almost certainly collapse. "It's conceivable that people could survive underground in caves, sustained by geothermal or nuclear energy, with plants grown under artificial lighting," Cronin said, "but this would be an extinction event to make all others look puny."

What would survive?

One thing that might survive? Near-microscopic animals called tardigrades, also known as water bears. "Ugly little critters," Summers said, but "hard to kill." They can be zapped with radiation or immersed in certain types of alcohol and still survive; perhaps hitting them with a hammer would kill them, he suggested. "Otherwise, they're pretty much one of the hardiest forms of life on Earth."

Likewise, bacteria that don't require photosynthesis, such as types that live around deep ocean vents, would likely survive. That's because certain microbes, including some bacteria and archaea, use chemosynthesis, as opposed to photosynthesis, to "live off of chemical bonds in rocks and minerals," Summers added.

Fortunately for humanity, there is no reason to believe the sun will vanish in the blink of an eye. Over time, however, the sun will die. It will continue to create heat and light for another 5 billion years or so, but once its fuel runs out, it will expand into a red giant, swallowing Mercury and Venus and perhaps Earth. Regardless, humans likely won't last that long; the sun's gradual increase in brightness is expected to vaporize Earth's oceans in a little over a billion years from now.

While those impacts may be a long way away, Summers said it's important to consider the potential outcomes. When "we understand more about stars and how they can change over time, on short timescales and on long timescales, we understand the universe better."

Sun quiz: How well do you know our home star?

Astronomers have produced one of the most accurate, comprehensive cosmic maps ever made, revealing a brilliant "sea of light" that permeated the early universe.

Unlike other universal maps, this 3D representation is composed of light emitted by a single element: hydrogen, the simplest and most abundant element in the universe, which emits large quantities of a specific wavelength of light when it becomes excited by energy from nearby stars.

By measuring this light across a vast patch of sky, astronomers got a glimpse of what the universe looked like 9 billion to 11 billion years ago, during an epoch of vigorous star formation.

The new research, described in a paper published March 3 in The Astrophysical Journal, is part of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), a sky survey that aims to illuminate how dark energy and gravity shape the universe. The researchers can now compare their simulations with this new data, gathered with the Hobby-Eberly Telescope at the McDonald Observatory in Texas, to assess how cosmological models differ from observations.

An exciting way to see the baby universe

When hydrogen atoms are bombarded by stellar radiation, they become excited and emit Lyman-alpha light, a specific wavelength in the ultraviolet part of the electromagnetic spectrum.

Massive, bright galaxies are easier to detect, but fainter galactic structures and the immense interstellar gas clouds that form stars and galaxies have remained largely undetected — until now.

To reveal the sea of light that permeated the fledgling cosmos, the researchers used a technique called line-intensity mapping, which focuses on the telltale wavelengths, or signature spectral emissions, given off by different elements. Astronomers can therefore use line mapping to chart the concentration and distribution of those specific elements throughout the cosmos, forming a map of the luminous galaxies and glowing gas clouds lit up by excited hydrogen atoms.

Cosmology is about zooming out

When studying individual galaxies, stars or other discrete celestial objects, astronomers analyze their characteristics by zooming in. Cosmology, however, requires zooming out. Accordingly, the HETDEX survey doesn't observe individual galaxies but rather the combined light from every object in a designated region of the sky. As a result, astronomers can gather integrated data from a multitude of galaxies and intergalactic gas clouds simultaneously.

"Imagine you're in a plane looking down," study co-author Julian Muñoz, a theoretical cosmologist at The University of Texas at Austin, said in a statement. "The 'traditional' way to do galaxy surveys is like mapping the brightest cities only: you learn where the big population centers are, but you miss everyone that lives in the suburbs and small towns. Intensity mapping is like viewing the same scene through a smudged plane window: you get a blurrier picture, but you capture all the light and not just the brightest spots."

In the quest to understand dark energy and chart more than 1 million bright galaxies, HETDEX "has gathered more than 600 million spectra over an area equivalent to more than 2,000 full moons, creating an unprecedented dataset," the researchers said in a different statement.

A golden age of cosmic mapping

The mapping method made possible by HETDEX offers another way to examine cosmology’s driving forces and how mass is distributed throughout the universe.

"These new 3D maps allow us to study how galaxies cluster together," study co-author Karl Gebhardt, a professor of astrophysics at The University of Texas at Austin, told Live Science via email. "The culprit that causes galaxies to come together is gravity. So by studying the clustering properties, we are understanding the properties of gravity and how much mass exists," Gebhardt explained.

Seeing galactic structures as a collective is invaluable for measuring large-scale density fluctuations across the cosmos to explore the influence of dark energy, the mysterious entity that appears to be accelerating the universe's expansion.

Unsurprisingly, detecting the signals from ancient galaxies is hard, "but excluding the faint signal from everything else — faint galaxies in the foreground, noise from the detector, artifacts produced by the analysis techniques, scattered light sources like the moon, weak absorption/emission lines from the Earth's atmosphere, is even harder," study co-author Robin Ciardullo, a professor of astronomy and astrophysics at Penn State and the observing manager of HETDEX, told Live Science via email.

The next step is to improve noise-reduction techniques and separate the desired signals from the numerous astronomical and Earthly contaminants. The researchers can then use fainter sources and lower-mass objects as tracers of cosmic evolution to more robustly constrain gravity models.

"The Hobby-Eberly is a pioneering telescope," Muñoz said. "And with new, complementary instruments coming online, we’re entering a golden age for mapping the cosmos."

Researchers have developed a method for steering microscopic swimming robots using light patterns and the principles of Einstein's theory of relativity. The technology is a potential first step toward deploying tiny robots in applications ranging from medicine to manufacturing.

One of the major challenges of developing microrobots for practical applications is creating ones capable of navigation without the inclusion of bulky sensors and other electronics, which would make the machines too large to operate at the desired scale (like inside a human body). In an attempt to overcome this issue, physicists at the University of Pennsylvania created "artificial space-time" to direct machines to travel in the same way that spacecraft or light does while crossing the universe.

In the study, researchers submerged 100-micron (roughly the width of a human hair) electrokinetic (EK) swimming robots in an ionized solution and tasked them with navigating a simple maze. The bots were covered with tiny solar cells with electrodes on both ends; when the solar cells were exposed to light, they powered the electrodes, which created an electric field that propelled the robots through the solution.

The challenge was to guide the microscopic machines with enough precision for them to reach a specific point in space, without being stymied by the maze's walls. That's where relativity came in. According to Einstein's theory of general relativity, gravity bends space-time around objects with mass. Light and objects follow "straight" geodesics ‪—‬ the shortest paths ‪—‬ that look bent around masses. A great example of this is gravitational lensing: Although light travels in a straight line across the cosmos, it can appear bent and magnified when passing through the gravitational well of a massive object, such as a large galaxy cluster.

"We showed that the way EK robots behave in patterned light fields is identical to the paths light follows in general relativity," lead study author Marc Miskin, an assistant professor of electrical and systems engineering at the University of Pennsylvania, told Live Science in an email. "Amazingly, you can use the robots as a gravity analog since the correspondence is exact. Alternatively, you can turn general relativity ideas around to use them to guide robots: in the same way gravity pulls objects together, you can guide robots to a specific spot."

Artificial space-time

To mimic the effect, the team modeled the maze as curved virtual space using relativity equations. Paths to the target inside the maze became simple straight lines in the model. Then, they converted the model back to a 2D light map. Dark spots naturally attracted the bots, while brighter spots repelled them. The end point of the maze was the darkest spot (a kind of faux black hole), with obstacles being more brightly lit.

Regardless of where they were initially placed, the EK bots naturally followed these geodesics, dodging walls automatically, as if sliding downhill in warped space. The team published their findings in November 2025 in the journal npj Robotics.

For Miskin, the study is a bridge between the worlds of physics and technology, "rather than a competition between them," he said. "On the one hand, relativity and light are very well understood; connecting reactive control to them invites new ways of thinking and established tools for robotics. On the other hand, general relativity and optics are also very abstract (think bending spacetime), while robotics is mechanistic and concrete (it's very easy to understand why the robot does what it does). In addition to showing how new types of robots behave according to known theories of optics, the experiments give researchers "a bit more" insight into general relativity, particularly in exploring the impact of "flat space-times" in 2D spaces, Miskin added.

While the maze study is a very early step, Miskin said practical applications may emerge over the next 10 years.

"Some use cases we're interested in exploring include checking up on teeth following a root canal, a kind of dental biopsy to make sure everything was cleared, eliminating tumors after making local measurements to confirm cells are cancerous, or even, outside of bio, assembly of microchips with tiny robotic helpers," Miskin said. "The microworld is a fascinating place; I wouldn't be surprised if these ideas are just the tip of the iceberg."

Elevators have a strange way of messing with your sense of gravity. The moment an elevator lurches upward, you feel it in your feet. For a second, the floor presses harder than usual. When the elevator slows, that pressure eases, leaving you briefly lighter.

If you stand on a scale inside an elevator going up, the number jumps. When it slows to a stop, the number dips. On the way down, the opposite happens.

So what's really going on? Do you weigh more when the elevator goes up or when it goes down?

The short answer is that you can feel heaviest at two points: when the elevator starts moving up (accelerating upward) and when it's slowing down at the very end of a downward trip (decelerating downward). But the explanation depends on what "weight" actually means and what your body can feel.

"The word 'weight' in physics has different meanings," Miguel Morales, a physics professor at the University of Washington in Seattle, told Live Science. In physics, weight can refer to at least three related ideas: your mass (how much matter you're made of), the gravitational force pulling on you, or how hard the scale beneath you is pushing up, Morales explained.

"When you're just standing still, those can all be the same thing," Morales said. "But as soon as the elevator starts to speed up or slow down, you get three different answers. It's just physics."

Your mass never changes, no matter what the elevator does. Gravity near Earth's surface also stays essentially the same between the bottom and top of a building. What does change is the third definition: how hard the scale pushes upward. That upward push is what a scale actually measures.

Looking at gravity

This distinction reveals something counterintuitive: "You can't feel gravity. You never could," Jason Barnes, a physics professor at the University of Idaho, told Live Science.

Barnes pointed to astronauts aboard the International Space Station. "The actual gravity of the Earth up there is almost the exact same as here," he said. "But they don't feel it."

That's not because gravity disappears in orbit. At the station's altitude (about 250 miles, or 400 kilometers, above our planet), Earth's gravitational pull is still about 90% as strong as it is at the surface. The difference is that astronauts and the space station are in continuous free fall toward Earth.

The station is moving sideways at more than 17,000 mph (27,300 km/h). As it falls, Earth curves away beneath it. Instead of hitting the ground, it keeps missing it. The result is a constant fall around the planet.

Because the astronauts and the space station are falling together at the same rate, the floor never needs to push up on them. And that upward push is what we actually feel as weight (also called the normal force).

On Earth, the ground constantly prevents you from falling by pushing upward against you. In orbit, there's no such push. The astronauts are still under the influence of gravity, but nothing is stopping them from falling. Without the floor pressing upward, they feel weightless.

Why do elevators make you feel heavier or lighter?

An elevator briefly changes how hard the floor pushes back on you. When the elevator starts rising, it must accelerate you upward, too. "To start going up, that's when you feel heavier," Barnes said. "The elevator pushes back harder than normal in order to accelerate you upward."

In a typical building elevator, that extra acceleration might be about 1 meter per second squared. That is roughly one-tenth of Earth's gravity. For someone who normally weighs 150 pounds (68 kilograms), that would briefly add about 10% to the scale reading. Instead of 150 pounds, the scale might show around 165 pounds (75 kg).

Morales described the same effect from the scale's perspective. "The force of gravity hasn't changed at all," he said. "But now, in order for you to be speeding up, something's got to be pushing you harder than gravity. And so your weight on the scale will go up."

Once the elevator reaches a steady speed, the acceleration stops. Gravity and the upward push balance again, and the scale returns to its normal reading, even though you're still moving.

At the top, when the elevator slows to a stop, the opposite happens. Even though you're still moving upward, the elevator must accelerate downward slightly to slow you down.

The force of gravity hasn't changed. But because the elevator is now accelerating downward, the floor doesn't need to push up as hard to control your motion. With less upward push (normal force), the scale reading drops.

"You kind of feel yourself get a little light," Morales said.

The same pattern repeats on the way down. When the elevator accelerates downward, you feel lighter because the floor pushes up less than usual. But as it approaches the bottom and slows to a stop, the acceleration flips upward again, making you feel heavy again.

This everyday experience turns out to be connected to one of the most important ideas in modern physics.

"It is an effect that Einstein first noted when he was developing general relativity," Barnes said. That insight, known as the equivalence principle, helped Einstein understand gravity not as a force but as a consequence of acceleration and the curvature of space-time itself.

Astronomers have discovered an exciting new "sungrazing" comet that will have a perilously close encounter with our home star in less than two months. Some experts predict the hefty ice ball could become bright enough to be visible to the naked eye, even in daylight — but only if the comet survives its deadly solar slingshot.

The newfound comet, dubbed C/2026 A1 (MAPS), was discovered Jan. 13 by a team of French astronomers at the AMACS1 Observatory in Chile's Atacama Desert. It is likely around 1.5 miles (2.4 kilometers) wide and, when it was first spotted, was just over twice as far from the sun as Earth is, according to Sky & Telescope magazine.

C/2026 A1 belongs to the Kreutz family of "sungrazing" comets — a group of at least 3,500 comets with orbits that take them within 850,000 miles (1.4 million km) of our home star. The Kreutz sungrazers are suspected to be fragments of a single massive comet that was ripped apart by the sun around 1,700 years ago, according to Live Science's sister site Space.com.

The icy ball of rock and gas will reach its closest point to the sun, called perihelion, on April 4, when it will come within around 500,000 miles (800,000 km) of our home star — or roughly 70 times closer to the sun than Mercury is. At such proximity, the comet will whip around the sun at more than 2 million mph (3.2 million km/h), causing it to experience intense gravitational pressure, high temperatures, and a hefty dose of solar radiation.

This immense strain may end up ripping the comet apart, which happens to most other sungrazing comets. But if it survives its perilous perihelion, C/2026 A1 will be so changed by the event that it will shine like an incredibly bright star — potentially even during the daytime.

How bright will it get?

C/2026 A1 comes from a particularly noteworthy subgroup of Kreutz sungrazers that are thought to be fragments of the "Great Comet of 1106," which was itself a remnant of the family's massive progenitor. Previous alumni of this subgroup include Comet Ikeya-Seki, which shined brighter than the full moon in 1965, and Comet Lovejoy, which became a "headless wonder" after being largely ripped apart in 2011.

As these comets made their own solar flybys, they became uncommonly bright, largely due to the significant amounts of gas that were released as they soaked up solar radiation. This has also caused several sungrazers to grow spectacular "broom-like" tails as they neared the sun, which could also happen to C/2026 A1, according to Sky & Telescope.

It is too early to accurately predict how bright C/2026 A1 will become. However, some researchers speculate that it could get several times brighter than the full moon, which would make it visible to the naked eye in the daytime sky, according to an article in The Conversation. But this will happen only if it survives perihelion; if not, it won't get anywhere near its max brightness.

Most sungrazing comets are small and are discovered mere days before they reach perihelion. Usually, they also get ripped apart by the encounter. For example, during the total solar eclipse of April 2024, researchers discovered a tiny sungrazer just hours before it disappeared forever.

How to see C/2026 A1 (MAPS)

If C/2026 A1 survives its deadly dance with the sun, it will likely reach its max brightness a few days later as it nears its closest point to Earth later in the month.

Observers in the Southern Hemisphere will get the best views of the comet, according to Sky & Telescope. However, people in the global north will still be able to see the object low above the southwestern horizon just before sunset.

But even if the comet falls apart, it will still be visible from late March with a decent telescope or pair of stargazing binoculars.

Later in April, another potentially spectacular comet, C/2025 R3 (PanSTARRS), could also become visible to the naked eye as it nears its own perihelion on April 20. This ice ball was previously (and perhaps prematurely) dubbed the "Great Comet of 2026."

Time will tell if C/2026 A1 can wrestle that title for itself.

The structure of the local universe is surprisingly flat, according to new research, and this cosmic quirk may save our Milky Way from colliding with countless other massive, nearby galaxies — except one.

For decades, astronomers have made the puzzling observation that our nearest galactic neighbor, Andromeda, is speeding toward a possible collision with our galaxy, while other nearby galaxies are moving away from us. Now, a new study may finally reveal why: A vast, flat sheet of dark matter is drawing those galaxies into deep space.

Dark matter anchors and attracts visible matter, and the gravitational pull from the far-out dark matter sheet, which lies slightly beyond the bounds of Andromeda and the Milky Way, overwhelms the attraction between our galaxy and other neighboring galaxies, researchers reported in a paper published Jan. 27 in the journal Nature Astronomy.

"The observed motions of nearby galaxies and the joint masses of the Milky Way and the Andromeda Galaxy can only be properly explained with this 'flat' mass distribution," the researchers said in a statement.

Future simulations could further explain how gravity sculpts our surroundings and why the local universe looks the way it does.

Going with the flow

The motion of galaxies throughout the expanding fabric of space-time is known as the Hubble flow. It's mathematically described by Hubble's law, named after astronomer Edwin Hubble, who discovered the expansion of the universe in the 1920s. His eponymous law constrains an observational phenomenon: Galaxies are moving away from Earth at speeds that are proportional to their distance. The farther a galaxy is from our vantage point, the faster it seems to be receding.

So why is Andromeda, located 2.5 million light-years away, hurtling toward us at 68 miles per second (110 kilometers per second), while most other large, nearby galaxies are following the flow? Curiously, these receding galaxies appear to resist the immense gravitational attraction of our Local Group, which includes the Milky Way, Andromeda, the Triangulum Galaxy and dozens of gravitationally bound, smaller galaxies.

This universal enigma has endured for more than half a century. In 1959, astronomers Franz Kahn and Lodewijk Woltjer found evidence of dark matter situated around Andromeda and the Milky Way. They calculated that to reverse the initial expansion imparted by the Big Bang, these two galaxies would require a combined mass much greater than all their stars put together.

It turns out that a significant portion of the mass of the Milky Way and Andromeda is contained in dark matter halos that surround each galaxy and facilitate the galaxies' rapid approach toward each other.

However, this attraction does not seem to affect nearby galaxies outside the Local Group, where "material is actually moving away from the Milky Way faster than the Hubble flow," study co-author Simon White, director emeritus of the Max Planck Institute for Astrophysics in Germany, said in a statement.

"Thus, galaxies closer than [roughly 8 million light-years] are moving away from us slower than predicted by Hubble's Law, whereas galaxies farther than [that] are actually receding faster than predicted," White told Live Science via email.

Building a universe from scratch

To find out why, the researchers built their own universe. They ran a multitude of simulations to explore the interactions among dark matter, our Local Group, and the receding galaxies just outside it, to a distance of around 32 million light-years.

The simulations modeled the evolution of the local universe from the beginning of space-time, starting with the mass distributions observed in the cosmic microwave background, the oldest light in the cosmos, emitted when the universe was just 380,000 years old. The researchers then had the model reproduce certain salient characteristics observed in nearby galaxies, including the mass, position and velocity of Andromeda and the Milky Way, as well as the positions and velocities of 31 galaxies located just outside the Local Group.

This revealed that the mass just slightly beyond the Local Group, including both dark matter and visible matter, is distributed in a vast, flat sheet that stretches for tens of millions of light-years and continues beyond the boundaries of the simulation.

Because nearby galaxies are embedded in this flattened sheet of dark matter, any gravitational pull from our Local Group is counteracted by the gravitational pull from the more distant mass in the sheet, drawing them away from us.

"If the mass were distributed approximately spherically around the Local Group, rather than being flat, then the external galaxies would be moving away from us slower than predicted by Hubble's law for the cosmic expansion, because they would be slowed down by the gravitational pull of the Milky Way and Andromeda," White told Live Science. "Instead, the flattened distribution of the surrounding matter pulls these galaxies outwards in a way which almost exactly compensates for the inward pull of the [Milky Way] and [Andromeda]."

Equally important, the regions above and below the sheet are devoid of galaxies. Such sparse regions occur throughout the cosmos, and the deep Local Voids around our Local Group formed in areas where the initial density of the universe was a bit lower than average.

"As a result these regions expanded faster than average, and their matter was 'pushed' outwards," White said via email. "By the present day these low-density regions fill most of space and gravitational effects have concentrated most of their material into the 'walls' that separate them."

Reconciling experiments, observations and models

The location of the voids is essential. These sparse regions are where any existing structures would fall toward the Local Group; any galaxies there would indeed be moving toward us. So we don't see any other objects careening toward the Milky Way, as Andromeda is doing, because there simply aren't any galaxies there to do so.

Overall, when accounting for the vast sheet of mass, the simulations accurately modeled the distribution of nearby galaxies and the voids, thereby reconciling experimental results with astronomical observations of galactic motions as well as with the leading model of cosmology, known as lambda cold dark matter.

"We are exploring all possible local configurations of the early universe that ultimately could lead to the Local Group," lead study author Ewoud Wempe, a cosmologist at the University of Groningen in the Netherlands, said in a different statement. "It is great that we now have a model that is consistent with the current cosmological model on the one hand, and with the dynamics of our local environment on the other."

Interestingly, the researchers report that high-latitude galaxies farther out in the cosmos have been observed to be falling toward the flat sheet of matter at several hundred kilometers per hour. Finding additional structures infalling from the directions of the voids could lend further support to the results of this study.

Astronomers have solved the mystery of how some stars stay youthfully bright and blue, despite being almost as old as the universe itself: They cannibalize their stellar siblings.

Known as blue straggler stars, these age-defying celestial objects have mystified astronomers for more than 70 years. "Blue stragglers are anomalously massive core hydrogen-burning stars that, according to the theory of single star evolution, should not exist," researchers wrote in a paper published Jan. 3 in the journal Nature Communications.

To investigate these puzzling stars, researchers used the James Webb Space Telescope (JWST) to analyze 48 galactic globular clusters and more than 3,400 blue straggler stars in the Milky Way. Their observations revealed that these "forever young" stars stay youthful by siphoning the gas from their partners, like stellar vampires. This fuel injection allows the vampire stars to shine more brightly, and to appear more blue and youthful, long after they should have started fading away.

Searching for age-defying stars

Scientists previously posited that blue stragglers can form in two ways: through violent collisions between two stars, or through more subtle interactions in binary systems as pairs of stars orbit each other and trade gas.

The team found that the latter scenario is more likely.

Galactic globular clusters provide the perfect place to study stellar interactions between gas-siphoning binary systems. These spherical clusters contain thousands or millions of stars, held together by their collective gravity. With so many stars inhabiting a region only tens or hundreds of light-years across, clusters are among the most dense stellar environments in the cosmos. Therefore, they host many stellar collisions and plenty of binary systems.

Clusters are also incredibly ancient. "Their age is of the order of 12 [billion years], hence comparable to the age of the Universe," which is 13.8 billion years old, Francesco Ferraro, lead author of the study and an astronomy professor at the University of Bologna in Italy, told Live Science via email. "In fact they are the oldest population in our Galaxy." This means the single stars in each cluster hosting the blue stragglers formed at the epoch of galaxy formation.

Older stars also emit different wavelengths of radiation. So the researchers utilized JWST's ultraviolet filters to distinguish blue stragglers from their elderly cluster-mates — because hotter -- younger stars emit more radiation at shorter wavelengths than older, redder populations that emit poorly in this part of the electromagnetic spectrum.

A surprising stellar scenario

Perhaps counterintuitively, the researchers found that blue stragglers are rarer in dense stellar environments, even though these regions are more likely to facilitate interactions between stars. Instead, stragglers are significantly more common in calm, low-density regions where stars are spaced farther apart and "fragile binary systems are more likely to survive."

The researchers used an established, quantitative measure that relates the number of blue stragglers to the host cluster's characteristics, like luminosity. This measure revealed that blue straggler populations vary greatly, from three to 58 blue stragglers per unit of luminosity — equivalent to the brightness of 10,000 suns. Accordingly, luminosity is related to a cluster’s overall mass and, therefore, its density.

Using that same measure, the researchers calculated that the number of regular stars in a cluster remains relatively constant. This suggests that stragglers and binary systems are especially sensitive to the density of their environments.

"Crowded star clusters are not a friendly place for stellar partnerships," study co-author Enrico Vesperini, an astronomer at Indiana University, said in a statement. "Where space is tight, binaries can be more easily destroyed, and the stars lose their chance to stay young."

Therefore, dense environments, such as those nearer the centers of clusters, may not be the stellar speed-dating venues they were assumed to be. The gravitational influences from large stellar populations create a cosmic-bumper-car-like effect that disrupts binary systems early in their evolution, before they can turn into blue straggler stars. As a result, the formation and survival efficiency of stragglers is 20 times higher in calmer, low-density environments, the researchers found.

A new way to understand stellar evolution

In addition to solving an astronomical mystery, this study offers a "new way to understand how stars evolve over billions of years," study co-author Barbara Lanzoni, an astronomer at the University of Bologna, said in the statement.

But after billions of years, blue stragglers may not get to live out their quiet lives in peace. Because they are significantly more massive than their sibling stars, they are more likely to sink to the core of their clusters through a process called dynamical friction. Although this is unfortunate for these calm-loving stars, astronomers can then use them as a "dynamical clock" to extrapolate a cluster's age based on the distribution of its blue stragglers.

Finally, these sprightly, fresh-faced stars highlight a dynamic stellar balance. Had they been born more massive, they would have died long ago as supernovas or white dwarfs. Their modest size, below 0.8 solar masses, have allowed them to survive long enough to renew their lifespans — at the cost of consuming their siblings.

If you've seen illustrations or models of the solar system, maybe you noticed that all the planets orbit the Sun in more or less the same plane, traveling in the same direction.

But what is above and below that plane? And why are the planets' orbits aligned like this, in a flat pancake, rather than each one traveling in a completely different plane?

I'm a planetary scientist who works with robotic spacecraft, such as rovers and orbiters. When my colleagues and I send them out to explore our solar system, it's important for us to understand the 3D map of our space neighborhood.

Which way is 'down'?

Earth's gravity has a lot to do with what people think is up and what is down. Things fall down toward the ground, but that direction depends on where you are.

Imagine you're standing somewhere in North America and point downward. If you extend a line from your fingertip all the way through the Earth, that line would point in the direction of "up" to someone on a boat in the southern Indian Ocean.

In the bigger picture, "down" could be defined as being below the plane of the solar system, which is known as the ecliptic. By convention, we say that above the plane is where the planets are seen to orbit counterclockwise around the Sun, and from below they are seen to orbit clockwise.

Even more flavors of 'down'

Is there anything special about the direction of down relative to the ecliptic? To answer that, we need to zoom out even farther. Our solar system is centered on the Sun, which is just one of about 100 billion stars in our galaxy, the Milky Way.

Each of these stars, and their associated planets, are all orbiting around the center of the Milky Way, just like the planets orbit their stars, but on a much longer time scale. And just as the planets in our solar system are not in random orbits, stars in the Milky Way orbit the center of the galaxy close to a plane, which is called the galactic plane.

This plane is not oriented the same way as our solar system's ecliptic. In fact, the angle between the two planes is about 60 degrees.

Going another step back, the Milky Way is part of a cluster of galaxies known the the Local Group, and — you can see where this is going — these galaxies mostly fall within another plane, called the supergalactic plane. The supergalactic plane is almost perpendicular to the galactic plane, with an angle between the two planes of about 84.5 degrees.

How these bodies end up traveling paths that are close to the same plane has to do with how they formed in the first place.

Collapse of the solar nebula

The material that would ultimately compose the Sun and the planets of the solar system started out as a diffuse and very extensive cloud of gas and dust called the solar nebula. Every particle within the solar nebula had a tiny amount of mass. Because any mass exerts gravitational force, these particles were attracted to each other, though only very weakly.

The particles in the solar nebula started out moving very slowly. But over a long time, the mutual attraction these particles felt thanks to gravity caused the cloud to start to draw inward on itself, shrinking.

There would have also been some very slight overall rotation to the solar nebula, maybe thanks to the gravitational tug of a passing star. As the cloud collapsed, this rotation would have increased in speed, just like a spinning figure skater spins faster and faster as they draw their arms in toward their body.

As the cloud continued shrinking, the individual particles grew closer to each other and had more and more interactions affecting their motion, both because of gravity and collisions between them. These interactions caused individual particles in orbits that were tilted far from the direction of the overall rotation of the cloud to reorient their orbits.

For example, if a particle coming down through the orbital plane slammed into a particle coming up through that plane, the interaction would tend to cancel out that vertical motion and reorient their orbits into the plane.

Eventually, what was once an amorphous cloud of particles collapsed into a disc shape. Then particles in similar orbits started clumping together, eventually forming the Sun and all the planets that orbit it today.

On much bigger scales, similar sorts of interactions are probably what ended up confining most of the stars that make up the Milky Way into the galactic plane, and most of the galaxies that make up the Local Group into the supergalactic plane.

The orientations of the ecliptic, galactic and supergalactic planes all go back to the initial random rotation direction of the clouds they formed from.

So what's below the Earth?

So there's not really anything special about the direction we define as "down" relative to the Earth, other than the fact that there's not much orbiting the Sun in that direction.

If you go far enough in that direction, you'll eventually find other stars with their own planetary systems orbiting in completely different orientations. And if you go even farther, you might encounter other galaxies with their own planes of rotation.

This question highlights one of my favorite aspects of astronomy: It puts everything in perspective. If you asked a hundred people on your street, "Which way is down?" every one of them would point in the same direction. But imagine you asked that question of people all over the Earth, or of intelligent life forms in other planetary systems or even other galaxies. They'd all point in different directions.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Scientists have found that time moves slightly faster on the Red Planet than it does on Earth. Clocks on Mars tick, on average, 0.477 milliseconds (477 microseconds) faster over 24 hours when measured from Earth compared with time recorded on our planet, a new study finds. Knowing this difference may help in establishing an "internet" across the solar system.

Over the next few decades, humanity's presence in the solar system is set to boom, with missions like those in NASA's Artemis program expected to pave the way for permanent settlements on the moon and beyond. Developing a standard clock for each cosmic locale would help astronauts navigate these worlds while coordinating communications with Earth.

But there's a catch: Time doesn't run at the same pace everywhere. Albert Einstein's theory of general relativity shows that time in a given area depends on how strong the gravity is there. Clocks in areas of high gravity tick more slowly than those where gravity is weaker, which is why people residing atop mountains age a fraction of a millisecond faster than sea-level dwellers. (Time appears to move faster at high altitudes, where Earth's gravitational tug is reduced.)

Additionally, time on a planet depends on its velocity around its parent star; the faster the orbital rate, the faster the passage of time.

Time keeps on slippin'

Together, velocity and gravity cause time on different solar system bodies to tick at different rates when measured from Earth. A 2024 study calculated that clocks on the moon would run an average of 56 microseconds (millionths of a second) faster than Earth-based ones. Having established this, the researchers — Neil Ashby and Bijunath Patla, both physicists at the National Institute of Standards and Technology in Boulder, Colorado — turned their attention to Mars.

First, they chose a reference level on Mars — an equivalent to Earth's sea level called the areoid. Then, they used physics-based formulas to calculate how, at the areoid, Mars' and Earth's gravities and velocities would influence Martian time. Although Mars' slower orbital speed relative to Earth slows down Mars-based clocks, the planet's weaker surface gravity — five times lesser at the areoid than Earth's sea-level gravity — speeds them up much more.

But this analysis neglected the orbits' shapes. Mars' orbit is more egg-shaped than Earth's, having been contorted by the gravitational tugs of Earth and its moon. (Mars' moons, Deimos and Phobos, have a negligible impact, Patla told Live Science in an email, because of their puny size. They're just a few miles wide, compared with 2,159 miles, or 3,475 kilometers, for Earth's moon.) So, Ashby and Patla factored Mars' orbital shape, the sun's gravity and Earth moon's gravity into their equations.

Setting clocks on Mars

The analysis showed that Martian clocks tick faster, when measured from Earth, than Earth-based ones by an average of 477 microseconds per Earth day. Strikingly, though, this value varies daily by 226 microseconds (about half the offset's value itself) over a Martian year. The variation stems from the egg-like shape of Mars’ orbit and changes in the gravitational tugs of its celestial neighbors as they approach and twirl away from Mars.

Additionally, the researchers found that the clocks change by an extra 40 microseconds over seven of Mars' synodic periods, with a synodic period being how long the planet takes to reappear in the same position of the sky.

"The fluctuation and the Earth-Mars planetary dance (synodic period) variation was a surprise," Patla said, because their magnitudes were larger than he expected.

The findings, published Dec. 1 in The Astronomical Journal, will help scientists synchronize time across the solar system, allowing them to establish rapid communications channels in an interplanetary internet in the distant future, although the large fluctuations will complicate this effort, Patla said. He added that the study "provides a baseline for future tests of general relativity and fundamental physics, which explore the nature of spacetime."

But the calculations were still inaccurate by about 100 nanoseconds (0.1 microseconds) per day over long timescales, because tiny shifts in the planets' movements weren't factored in. Although this imprecision is minuscule, it would mean resetting Martian clocks every 100 days.

The study also didn't account for factors like how the planets' orbits precess, or gradually wobble, and the effects of Earth's and Mars' gravitational quadrupole moments, which is a measure of how their mass is arranged within their structures. Taken together, these limitations may make it more challenging to obtain more precise time calculations, the researchers said.

Astronomers have observed a pair of stars locked in a death spiral, and their dance of doom is revealing more about how gravity works.

The system, called ZTF J2130, sits about 4,000 light-years away. Although astronomers have known about this system for a while, this is the first time they have observed it with such high clarity.

The two stars that make up the system will merge soon. Their spiraling motion agrees with theoretical predictions, which means even more refined future observations will allow researchers to use this system to test our understanding of gravity, the team wrote in a study submitted for publication in the journal Astronomy & Astrophysics in October.

This is a very old system. One of the stars is a white dwarf, the white-hot leftover core of a sunlike star. The other is what's known as a subdwarf star, which is a small star near the end of its life cycle. The two stars are so close together that they complete an orbit in just under 40 minutes. In fact, they've already started to kiss. Their mutual gravity is so strong that they've stretched and distorted, with the subdwarf's material flowing onto the white dwarf companion.

Because the stars are pretty hefty and moving very quickly, they emit gravitational waves, which are ripples in the fabric of space-time first predicted by Albert Einstein and confirmed to exist in 2015. This emission of gravitational waves saps energy from the system, inching the two stars ever closer every year.

Using a combination of data from the Oskar Luhning telescope at the Hamburg Observatory in Germany and the CAHA Observatory in Spain in Germany and Spain, the astronomers undertook a painstaking campaign to measure the orbital period as precisely as possible. They found that the orbit is slowly decaying; with every passing second, the orbital period shrinks by about two-trillionths of a second.

This is in line with calculations based on our current theoretical understanding of gravity. But scientists have been eager to move past Einstein's theory of general relativity for more than a century, so any opportunity to test it immediately draws interest.

The astronomers discovered that an upcoming gravitational-wave observatory, known as the Laser Interferometer Space Antenna (LISA), should be able to directly measure the gravitational waves emanating from this system. The European Space Agency plans to launch LISA in the 2030s, and this stellar pair will still be around next decade.

When the stars finally merge, they will release a supernova-level explosion that might be bright enough to be seen with the naked eye. In the meantime, before we get to enjoy that fireworks show, we'll just have to put gravity to the test.

Astronomers have discovered a surprisingly small "dark object" lurking within a distant ring of warped light. The record-breaking find could help shed light on the mysterious identity of dark matter, which would have major implications for the field of cosmology.

The hidden object, likely a clump of invisible dark matter, was spotted within B1938+666 — an "Einstein ring" located around 10 billion light-years from Earth. This luminous halo (which appears dark in the black-and-white images) is made up of light from a distant galaxy that has been bent around a closer foreground galaxy (the dark dot at the center of the ring). This is an effect of gravitational lensing, a phenomenon that was first proposed by Albert Einstein's theory of general relativity in 1915.

Gravitational lensing not only warps light but also significantly magnifies it. When the lensing object is perfectly aligned between the distant object and the observer, the light bends into a circle around the foreground object, known as an Einstein ring. However, an imperfect alignment can also create other strange shapes, such as crosses, zig-zags and question marks, and duplicate individual points of light within the same image.

B1938+666 was discovered in the 1990s. But in a pair of new studies, published Oct. 9 in the journals Nature Astronomy and Monthly Notices of the Royal Astronomical Society, researchers took a closer look at the gravitationally lensed object and found a subtle wobble within a prominent arc of radio waves in the outer ring (colored red and yellow in the image). They quickly realized this was a gravitational disturbance caused by a hidden object.

"From the first high-resolution image, we immediately observed a narrowing in the gravitational arc, which is the tell-tale sign that we were onto something," John McKean, an astronomer at the University of Groningen in the Netherlands and the University of Pretoria in South Africa, and co-author on both new studies, said in a statement. "Only another small clump of mass between us and the distant radio galaxy could cause this."

The object is around 1 million times more massive than the sun, which sounds like a lot. However, this actually makes it around 100 times smaller than the previous record-holder for the least-massive object ever detected via gravitational lensing.

The study teams uncovered this object by combining data from radio observatories located across the globe, including the Green Bank Telescope in West Virginia, the Very Long Baseline Array in New Mexico and the European Very Long Baseline Interferometry Network. This enabled the researchers to achieve the equivalent observing power of an Earth-size telescope, which helped them to detect such a subtle fluctuation in the data. But there was so much information that the researchers had to come up with a new way of sorting it.

"The data are so large and complex that we had to develop new numerical approaches to model them," Simona Vegetti, an astronomer at the Max Planck Institute for Astrophysics in Germany and co-author on both new studies, said in the statement. "This was not straightforward as it had never been done before."

While they cannot be certain, the researchers are confident that the new object is a small clump of dark matter — the invisible matter that makes up 27% of the known universe and does not interact with light. This is unsurprising, given that gravitational lensing is one of the only ways we can detect and measure dark matter, making Einstein rings and other warped objects one of our greatest weapons in unmasking its true identity.

Finding isolated dark matter clumps like this is especially useful for testing the "cold dark matter theory," which posits that dark matter can only clump together if it moves at relatively slow speeds, meaning it would give off relatively low amounts of energy, Live Science's sister site Space.com reported.

And the researchers predict that these clumps are far more common than we currently realize. "We expect every galaxy, including our own Milky Way, to be filled with dark matter clumps, but finding them and convincing the community that they exist requires a great deal of number-crunching," Vegetti said.

To date, only three other similarly small, potential dark matter clumps have been identified, the researchers wrote. However, the new methodology will make it easier to spot more clumps around existing Einstein rings, and the number of known rings is also climbing fast, thanks to the James Webb Space Telescope, which has proved to be exceptionally good at finding them.

"Having found one, the question now is whether we can find more," Devon Powell, an astronomer at the Max Planck Institute for Astrophysics and co-author on both new studies, said in the statement.

Satellites detected a strange gravity signal off the coast of Africa nearly 20 years ago, suggesting something unusual had happened deep within the planet to distort its gravitational field, according to a recent study.

The large gravitational anomaly lasted for about two years over the eastern Atlantic Ocean. It peaked in January 2007, the same month Steve Jobs announced the first iPhone (though, of course, there was no connection between the two events).

Researchers recently discovered the signal while analyzing data collected by the Gravity Recovery and Climate Experiment (GRACE) satellites between 2003 and 2015. The gravitational anomaly happened around the same time as a geomagnetic "jerk" — an abrupt change in the variation of Earth's magnetic field.

The strange anomaly, and the jerk, were caused by a previously unknown geological process, the researchers suspect. Their findings, published Aug. 28 in the journal Geophysical Research Letters, indicated that a shift in minerals may have caused a rapid redistribution of mass in the deep mantle, near the core, altering Earth's magnetic field.

Study co-author Mioara Mandea — a geophysicist at the National Centre for Space Studies (CNES) in France and a principal investigator for the European Research Council's Gravimetry, Magnetism, Rotation and Core Flow project — told Live Science that she questioned the validity of the signal at first.

"As is often the case in scientific research, my initial response was one of questioning: is the signal genuine, how can it be validated, and how should it be interpreted?" Mandea said in an email. "While the result and its publication were certainly a source of satisfaction, the dominant thought was to consider the next steps and possible implications."

The GRACE satellites were a pair of identical spacecraft operated as part of a joint mission between NASA and the German Aerospace Center (DLR). Scientists used these satellites, which were active from 2002 until they ran out of fuel in 2017, to measure variations in Earth's gravity. The satellites moved in tandem (one behind the other) around Earth, and researchers measured the distance between the two objects to look for any changes that occurred as a result of variation in Earth's gravitational force, according to NASA.

Such gravity variations are often caused by variations in the concentration of mass — more mass means more gravity. For example, water currents shift mass around in the ocean, which can lead to localized variations in Earth's gravitational field. In the new study, the researchers scoured GRACE data for anomalous gravity signals that potentially originated deep within Earth, rather than from water shifting on or near the surface.

The signal was a north-south-oriented gravity anomaly, stretching about 4,350 miles (7,000 kilometers) — close to the length of the entire African continent — from 2006 to 2008, according to the study.

Researchers are still learning about Earth’s deep mantle and the boundary between the rocky layer and our planet’s liquid outer core, but the lower section of the mantle is largely made up of magnesium silicate (MgSiO₃). The study authors suggested that the mass redistributions they attributed to the signal occurred as a result of a perovskite to post-perovskite phase transformation in this lower mantle section, whereby the structure of magnesium silicate changed under pressure, shifting mass deep within the Earth.

Mandea noted that the main message of the study was that Earth is complex and that different datasets and methods are required to understand its internal processes.

"Earth is a complex system that must be studied using diverse datasets and complementary methods of analysis," Mandea said. "This synergy gives us the opportunity to uncover and better understand hidden processes in the Earth's deep interior."

A new paper describes another possible "quasi-moon" of Earth, and the interloping asteroid may have been following our planet around for decades, undetected.

Quasi-moons, the Planetary Society states, are “like a gravitational sleight of hand.” They are asteroids, which — from our point of view on Earth — appear to be orbiting our planet like our permanent moon does. However, they actually orbit the sun, only temporarily moving through the solar system alongside our planet.

If the status of the newly detected asteroid, called 2025 PN7, is confirmed, it would not be the only object seemingly behaving as a moon of Earth; there are seven other known quasi-moons in Earth-like orbits, and they are "full of surprises," said study co-author Carlos de la Fuente Marcos of the Complutense University of Madrid.

Of these quasi-moons, 2025 PN7 is the "smallest and the least stable known quasi-satellite of Earth," de la Fuente Marcos told Live Science in an email.

The newly discovered asteroid is only 62 feet (19 meters) wide — slightly smaller than the meteor that exploded over Chelyabinsk, Russia, in 2013. The asteroid is officially classified as having a brightness of magnitude 26, meaning it is visible only through good telescopes. (The lower the magnitude, the brighter the object. For comparison, most naked-eye stars are magnitude 6 or lower, and the bright star Sirius is roughly magnitude -1.5.)

The potential quasi-moon finding was published Sept. 2 in the journal Research Notes of the American Astronomical Society (AAS). The journal, which is not peer-reviewed, aims to allow authors to "promptly" share items of interest with the astronomical community, with papers "moderated by an editor for appropriateness and format before publication" in lieu of peer review. That approach allowed study authors Carlos de la Fuente Marcos and Raúl de la Fuente Marcos, also of the Complutense University of Madrid, to publish their findings rapidly.

Related: First-ever image of China's mysterious 'quasi moon' probe revealed weeks after it secretly launched into space

An official Aug. 29 circular about 2025 PN7 from the International Astronomical Union shows data about the object dating back only to July 30, in observations by the Haleakalā Observatory's Pan-STARRS1 telescope in Maui, Hawaii.

The quasi-moon designation was first proposed for 2025 PN7 by French journalist and amateur astronomer Adrien Coffinet, who posted Aug. 30 on the Minor Planet Mailing List about how his calculations appeared to show that was the case.

"2025 PN7 seems to be a quasi-satellite of the Earth for the next 60 years," Coffinet wrote. Another in the group said it appears, from the object’s orbit, to have been flying nearby us already for about seven decades.

So, why didn't astronomers notice 2025 PN7 before now? "It is small, faint, and its visibility windows from Earth are rather unfavorable, so it is not surprising that it went unnoticed for that long," de la Fuente Marcos said.

More quasi-moons may be lurking out there. The Vera C. Rubin Observatory, which recently became operational and can scan for objects like this, "may uncover many more like 2025 PN7," de la Fuente Marcos added.

Chinese astronomers may have discovered a never-before-seen triple black hole system.

The team identified this triplet, which is locked in a complex "waltz," after spotting a hidden supermassive black hole lurking in the background of a peculiar gravitational wave event first detected six years ago.

In 2019, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a series of faint ripples in the fabric of space-time, known as gravitational waves. They appeared to be given off by the distant merger of two black holes located somewhere between 544 and 912 light-years from Earth. The cosmic collision, dubbed GW190814, was particularly noteworthy due to the size of the merging singularities, which weighed 23 and 2.6 solar masses, respectively.

Normally, merging black holes have a similar mass to one another because this creates the right type of gravitational friction for them to come together. At the time, GW190814 was the "most unequal mass ratio yet measured with gravitational waves," according to a 2020 study of the event. Scientists were particularly surprised by the size of the smaller singularity, which is only just massive enough to be considered a black hole.

In a new study, published July 21 in The Astrophysical Journal Letters, astronomers proposed that this uneven merger was caused by a hidden third object that provided the necessary gravitational kick for the two mismatched black holes to collide and transform into a single entity, despite their significant size difference.

Related: Accidental discovery of 1st-ever 'black hole triple' system challenges what we know about how singularities form

The team used simulations to predict how this interaction would influence the gravitational waves generated by the merger, and identified a unique "fingerprint" signal associated with the hidden object. They then reanalyzed the LIGO data from the initial discovery and found that this fingerprint signal was in fact present.

"This is the first international discovery of clear evidence for a third compact object in a binary black hole merger event," study co-author Wen-Biao Han, an astronomer at the Chinese Academy of Sciences, said in a statement. "It reveals that the binary black holes in GW190814 may not have formed in isolation but were part of a more complex gravitational system."

Based on the simulations, the team believes the most likely identity of the hidden compact object is a supermassive black hole. They don't yet know how large this behemoth may be, but the lower limit for supermassive black holes is around 100,000 solar masses, suggesting that it is at least that massive — and making it far larger than the other two objects initially identified in the system.

The smaller pair of merging black holes were likely part of a binary system that danced around the supermassive black hole as they spun around one another, similar to how Earth and the moon circle each other on their collective journey around the sun. This is the first time that this configuration has been seen in a black hole system.

The newly formed black hole from the merger will likely continue to dance around its supermassive partner for billions of years before eventually being swallowed by the larger object, the team added.

The new findings not only provide "significant insights into the formation pathways of binary black holes" but also provide a new way of identifying other hidden giants lurking in the background of other similarly uneven black hole mergers, Han said.

Since LIGO detected the first-ever gravitational waves in 2015, the observatory has spotted more than 100 additional gravitational wave events, most of which were caused by black hole mergers. Each new detection provides more data scientists can use to uncover new secrets about the universe's most massive objects, which are notoriously hard to study.

Black hole quiz: How supermassive is your knowledge of the universe?

Tiny black holes created in the aftermath of violent cosmic collisions could offer unprecedented insight into the quantum structure of space and time, a new theoretical study proposes.

What's more, signals from these "black hole morsels" could potentially be detected by current instruments, scientists reported in the study, which was published in the journal Nuclear Physics B.

"Our work shows that if these objects are formed, their radiation might already be detectable using existing gamma-ray observatories," Francesco Sannino, a theoretical physicist at the University of Southern Denmark and co-author of the study, told Live Science via email.

Hawking radiation and the smallest black holes

One of the deepest mysteries in modern physics is how gravity behaves at the quantum level. The new study offers a bold proposal to explore this regime by looking for the glow produced by tiny black holes created in the aftermath of giant black hole collisions.

The idea that black holes are not entirely black, and therefore could emit faint radiation, was first proposed by Stephen Hawking in the 1970s. His calculations revealed that quantum effects near a black hole's event horizon would cause it to emit radiation and lose mass — a process now known as Hawking radiation. The black hole temperature is predicted to be inversely proportional to its mass. So for massive astrophysical black holes, the effect is minuscule, with temperatures so low that the radiation is effectively undetectable. But for very small black holes, the situation is different.

"Black hole morsels are hypothetical micro-black holes that could be formed during the violent merger of two astrophysical black holes," Giacomo Cacciapaglia, a senior researcher at the French National Centre for Scientific Research (CNRS) and co-author of the study, said in an email. "Unlike the larger parent black hole, these morsels are much smaller — comparable in mass to asteroids — and thus much hotter due to the inverse relationship between black hole mass and Hawking temperature."

Related: Scientists detect most massive black hole merger ever — and it birthed a monster 225 times as massive as the sun

Because of this elevated temperature, these morsels would evaporate relatively quickly, releasing bursts of high-energy particles such as gamma-rays and neutrinos. The team's analysis suggests that this radiation could form a distinct signal that may already be within reach of present-day detectors.

A new handle on quantum gravity

Although no such morsels have been observed yet, the researchers argue that the formation of these tiny black holes is theoretically plausible. "The idea is inspired by analogous processes in neutron star mergers," Stefan Hohenegger, senior researcher at the Institut de Physique des Deux Infinis de Lyon and co-author of the study, explained in an email. "It's supported by estimates from beyond-General Relativity frameworks, including string theory and extra-dimensional models."

In such extreme environments, small-scale instabilities might pinch off tiny black holes during the merger process. These objects, in turn, could evaporate through Hawking radiation over timescales ranging from milliseconds to years, depending on their mass.

Crucially, if such radiation is detected, it could open a window into new physics. "Hawking radiation encodes information about the underlying quantum structure of spacetime," Sannino said. "Its spectral properties could reveal deviations from the Standard Model at extreme energy scale, potentially leading to discoveries of unknown particles or such phenomena as extra dimensions predicted by various theories."

Such energy scales lie far beyond the reach of even the most powerful particle colliders, like the Large Hadron Collider at CERN. The possibility that black hole morsels might provide a natural "accelerator" for probing these physics is what makes them so compelling.

According to the team, the signature of a black hole morsel would be a delayed burst of high-energy gamma-rays radiating in all directions — unlike typical gamma-ray bursts, which are usually beamed.

Instruments capable of detecting such high-energy signals include atmospheric Cherenkov telescopes, like the High Energy Stereoscopic System (HESS), in Namibia; the High-Altitude Water Cherenkov Observatory (HAWC), in Mexico; and the Large High Altitude Air Shower Observatory (LHAASO) in China, as well as satellite-based detectors, like the Fermi Gamma-ray Space Telescope. "Some of these instruments already have the sensitivity required," Hohenegger noted.

The researchers didn't stop at theorizing. They used existing data from HESS and HAWC to place upper bounds on how much mass could be emitted in the form of morsels during known black hole mergers. These limits represent the first observational constraints on such phenomena.

"We showed that if black hole morsels form during mergers, they would produce a burst of high-energy gamma rays, with the timing of the burst linked to their masses," Cacciapaglia said. "Our analysis demonstrates that this novel multimessenger signature can offer experimental access to quantum gravitational phenomena.”

What comes next

While the study provides a compelling case for morsels, many uncertainties remain. The exact conditions for their formation are still poorly understood, and no full simulations have been performed at the scales necessary to model them. But the researchers are optimistic.

"Future work will involve refining the theoretical models for morsel formation and extending the analysis to include more realistic mass and spin distributions," Sannino said. The team also hopes to collaborate with observational astronomers to perform dedicated searches in both archived and upcoming datasets.

"We hope this line of research will open a new window into understanding the quantum nature of gravity and the structure of spacetime," Hohenegger said.

If black hole morsels exist, they may not only illuminate the sky with exotic radiation but could also shed light on some of the deepest unsolved questions in physics.

Scientists have found evidence of a previously-undetected dwarf planet at the edge of the solar system.

The object, dubbed 2017 OF201, follows an extreme, oblong orbit, taking some 25,000 Earth years to circle the sun. The findings, which were confirmed by the International Astronomical Union's Minor Planet Center but have not yet been peer reviewed, were published May 21 on the preprint server arXiv.

2017 OF201 is a roughly spherical body about 435 miles (700 kilometers) in diameter, lurking beyond Neptune's orbit. A team of scientists spotted it while poring through archival data from the Blanco telescope in Chile and the Canada-France-Hawaii telescope based in Hawaii. The researchers tracked the object's motion across 19 sets of images spanning seven years.

At its closest, the dwarf planet orbits at nearly 45 AU, or 45 times the distance from Earth to the sun — a similar distance as its fellow dwarf planet Pluto. Based on the newfound object's trajectory, the scientists estimate its last close pass to the sun was in 1930, the same year Pluto was discovered. It's now twice as far away and rocketing off even further into space. At its farthest point, 2017 OF201 will be a whopping 1,600 AU before starting its journey back inward.

This oblong orbit hints at complex gravitational interactions, both with Neptune and with the pull of the Milky Way's gravity.

"There may have been more than one step in its migration," study co-author Sihao Cheng, an astrophysicist at the Institute for Advanced Study in Princeton, New Jersey, said in a statement. "It's possible that this object was first ejected to the Oort cloud, the most distant region in our solar system, which is home to many comets, and then sent back."

Related: Planet Nine: Is the search for this elusive world nearly over?

Because it's so difficult to spot solar system objects this far away, it's possible 2017 OF201 isn't the only dwarf planet waiting to be discovered.

"2017 OF201 spends only 1% of its orbital time close enough to us to be detectable," Cheng said. "The presence of this single object suggests that there could be another hundred or so other objects with similar orbit and size; they are just too far away to be detectable now."

The newly-discovered object could also challenge theories of Planet 9, a proposed but unobserved large planet orbiting billions of miles beyond Neptune. Some scientists have proposed the influence of Planet 9's gravity to explain the clustered orbits of some trans-Neptunian objects. But 2017 OF201 doesn't fit neatly into this observed pattern, and the researchers suggest that the gravitational pull of Planet 9 — if it exists — would knock 2017 OF201 out of the solar system fairly quickly. Further observations will be needed to better understand these possible interactions, the team wrote in the study.

"Even though advances in telescopes have enabled us to explore distant parts of the universe, there is still a great deal to discover about our own solar system," Cheng said.

Physicists have developed a novel approach to solving one of the most persistent problems in theoretical physics: uniting gravity with the quantum world.

In a recent paper published in the journal Reports on Progress in Physics, the scientists outline a reformulation of gravity that could lead to a fully quantum-compatible description — without invoking the extra dimensions or exotic features required by more speculative models, like string theory.

At the heart of the proposal is a rethinking of how gravity behaves at a fundamental level. While the electromagnetic, weak and strong forces are all described using quantum field theory — a mathematical framework that incorporates uncertainty and wave-particle duality — gravity remains the outlier. General relativity, Einstein's theory of gravity, is a purely classical theory that describes gravity as the warping of space-time geometry by mass and energy. But attempts to blend quantum theory with general relativity often run into fatal mathematical inconsistencies, such as infinite probabilities.

The new approach reinterprets the gravitational field in a way that mirrors the structure of known quantum field theories. "The key finding is that our theory provides a new approach to quantum gravity in a way that resembles the formulation of the other fundamental interactions of the Standard Model," study co-author Mikko Partanen, a physicist at Aalto University in Finland, told Live Science in an email.

Instead of curving space-time, gravity in their model is mediated by four interrelated fields, with each one similar to the field that governs electromagnetism. These fields respond to mass in much the same way that electric and magnetic fields respond to charge and current. They also interact with each other and with the fields of the Standard Model in a way that reproduces general relativity at the classical level while also allowing quantum effects to be consistently incorporated.

Related: 'Einstein's equations need to be refined': Tweaks to general relativity could finally explain what lies at the heart of a black hole

Because the new model mirrors the structure of well-established quantum theories, it sidesteps the mathematical problems that have historically hindered efforts to quantize general relativity. According to the authors, their framework produces a well-defined quantum theory that avoids common problems — such as unphysical infinities in observable quantities and negative probabilities for physical processes — that typically arise when general relativity is quantized using conventional, straightforward methods.

A key advantage of the approach is its simplicity. Unlike many models of quantum gravity that require undetected particles and additional forces, this theory sticks to familiar terrain.

"The main advantages or differences in comparison with many other quantum gravity theories are that our theory does not need extra dimensions that do not yet have direct experimental support," Jukka Tulkki, a professor at Aalto University and co-author of the paper, told Live Science in an email. "Furthermore, the theory does not need any free parameters beyond the known physical constants."

This means the theory can be tested without waiting for the discovery of new particles or revising existing physical laws. "Any future quantum gravity experiments can be directly used to test any (forthcoming) predictions of the theory," Tulkki added.

Looking ahead

Despite the promising features, the model is still in its early stages. Although preliminary calculations indicate that the theory behaves well under the usual consistency checks, a complete proof of its consistency remains to be worked out.

Moreover, the framework has yet to be applied to some of the deepest questions in gravitational physics, such as the true nature of black hole singularities or the physics of the Big Bang. "The theory is not yet capable of addressing those major challenges, but it has potential to do so in the future," Partanen said.

Experimental verification may prove even more elusive. Gravity is the weakest of the known forces, and its quantum aspects are incredibly subtle. Direct tests of quantum gravity effects are beyond the reach of current instruments.

"Testing quantum gravity effects is challenging due to the weakness of gravitational interaction," Tulkki said. Still, because the theory includes no adjustable parameters, any future experiment that probes quantum gravitational behavior could potentially confirm — or rule out — the new proposal.

"Given the current pace of theoretical and observational advancements, it could take a few decades to make the first experimental breakthroughs that give us direct evidence of quantum gravity effects," Partanen said. "Indirect evidence through advanced observations could be obtained earlier."

For now, Partanen and Tulkki's work opens up a fresh direction for theorists searching for a quantum theory of gravity — one that stays grounded in the successful frameworks of particle physics while potentially unlocking some of the most profound mysteries of the universe.

Digestive gas gets the best of everyone sooner or later, often in the form of a burp. Burping is how the body clears excess gas from the upper digestive tract, which would otherwise result in extremely uncomfortable pressure in your stomach and esophagus.

Or at least that's how it works on Earth. In space, everything works a little differently without gravity to help. So is it true that you can't burp in space? The answer is messier than you might expect.

You can't burp in microgravity the way you would on Earth, experts told Live Science. That's because, unlike vomiting, which uses the muscles of your digestive tract to force food back up, the mechanics of burping depend completely on gravity. First, gravity helps separate the gassy ingredients of a burp from the liquid and solid remnants of food in the stomach; gas is lighter and thus floats to the top. So, before you burp, the stomach contains a layer of hot, sometimes smelly gas hovering above a swampy mix of partially digested food.

When enough gas builds up, it puts pressure on the sphincter (a ring of muscle that acts as a barrier between parts of the digestive system) between the esophagus and the stomach; the sphincter opens and lets the gas rise into the lower part of the esophagus. A second sphincter, farther up, allows the rising gas into the upper esophagus, where it can escape as a burp.

"In space, air and liquids in the stomach can't separate like on Earth," Raffi Kuyumjian, chief medical officer of Operational Space Medicine at the Canadian Space Agency (CSA), told Live Science. Without gravity to sort out the contents of your stomach, it's all just one big chunky, gassy mess. In space, as on Earth, you can force yourself to burp by chugging a carbonated drink, gulping air and holding it in, and moving your abdominal muscles — but if you do it while floating in microgravity, it's going to be, as astronaut Chris Hadfield described it in a 2018 post on Twitter (now X), "chunky bubbles."

Related: What happens when you hold in a fart?

Gravity also helps the burp escape, thanks to a process called convection. When a gas or liquid is heated, its molecules spread out, making it less dense (and therefore lighter) than the surrounding gas or liquid. That's why you see columns of bubbles rising in a pot of boiling water, why the sun's surface is covered with convection cells, and why the hot gas of a burp inevitably rises from your stomach, back up your esophagus, and out through your (hopefully politely covered) mouth.

However, without gravity, convection doesn't work. If there's no gravity pulling on the contents of your stomach, it doesn't matter if some components of an astronaut's lunch are heavier or lighter than others.

"There's no up or down in weightlessness, so gas can't 'rise' from the stomach for burping," Kuyumjian said.

The good news is that astronauts don't have to worry about a badly timed belch sneaking out in the middle of an otherwise quiet workday. Adrianos Golemis, human spaceflight surgeon for the European Space Agency and the French space agency CNES, when asked about the physiology of burping in space, said "it's never come up in a post-flight debrief." (The pun may or may not have been intended.)

Some may face a different potential problem, though: "Instead of burping, astronauts may experience a reflux of stomach liquids and gas," Kuyumjian said. Reflux happens when the sphincter between the stomach and the esophagus relaxes, allowing stomach acid (and sometimes partially-digested food) back into the esophagus. It can be painful, and some people respond by swallowing more to try to clear the irritating acid from their throat — which, ironically, makes burping more likely.

In space, because liquids, solids and gases are all mixed up in the astronaut's stomach, liquid is as likely to come back up as gas. Floating in microgravity erases the line between reflux and burping.

If an astronaut absolutely has to try burping in space but doesn't want to belch out a mix of stomach fluids and partially chewed food, there's another way to do it: creating your own artificial gravity, astronaut Jim Newman told author Ariel Waldman in the book "What's It Like in Space?: Stories from Astronauts Who've Been There" (Chronicle Books, 2016). Give yourself a good shove away from a nearby wall, and the acceleration should mimic the effects of gravity, temporarily sorting out your stomach contents. But timing is everything, because you need to make yourself burp while you're still accelerating away from the wall — otherwise you'll get chunky bubbles.

Gas that makes it past the stomach and into the intestinal tract can come out the other end as a fart, which is an entirely different challenge.

Scientists have discovered a planet that is literally falling apart as it orbits its star. Located about 140 light-years from Earth in the Pegasus constellation , this doomed world named BD+05 4868 Ab whips around its star once every 30.5 hours — so close that its surface is being scorched into magma and vaporizing into space.

With each orbit, BD+05 4868 Ab leaves a blazing trail of molten rock behind it like a comet made of lava, offering a rare glimpse of an exoplanet in the final stages of its destruction. What's even more astonishing: with every blistering 30-hour orbit — which heats the planet to close to 3,000 degrees Fahrenheit (1,600 degrees Celsius) — the planet sheds as much mass of molten rock as an entire Mount Everest.

"The extent of the tail is gargantuan, stretching up to 9 million kilometers long, or roughly half of the planet's entire orbit," said Marc Hon, a postdoc in MIT's Kavli Institute for Astrophysics and Space Research in a statement.

This is an epic disintegration unfolding in real time, and the team predicts that it might take 1 to 2 million years for the entire planet to fully disintegrate. "We got lucky with catching it exactly when it's really going away," said Avi Shporer, a collaborator on the discovery who is also at the TESS Science Office. "It's like on its last breath."

Only three other disintegrating worlds have been identified among the more than 6,000 discovered exoplanets — each leaving a distinctive, comet-like tail of debris behind it. But BD+05 4868 Ab stands out: its tail is the longest of them all.

"That implies that its evaporation is the most catastrophic, and it will disappear much faster than the other planets," Hon said.

Because BD+05 4868 Ab orbits so perilously close to its star, its transit — the dip in starlight created as the planet passes in front of its star — appears especially bright and distinct. The planet was discovered with NASA's Transiting Exoplanet Survey Satellite (TESS) observatory. TESS, which scans nearby stars for periodic dips in brightness, revealed a strange, fluctuating transit that stood out from the usual planetary candidates.

This makes it an ideal target for NASA's James Webb Space Telescope, whose sensitive instruments can capture subtle changes in starlight to identify the chemical makeup of the vaporized rock trailing behind the planet. The result is a rare opportunity to watch a planet disintegrate in real time, and to study the composition of a world being stripped down to its core.

Hon says the discovery was a lucky break. "We weren't looking for this kind of planet," he explained. "We were doing the typical planet vetting, and I happened to spot this signal that appeared very unusual."

Though BD+05 4868 Ab's transit appears every 30.5 hours, the star's brightness took much longer than in other instances to return to normal. Even more bizarre was the depth the starlight's dip changed with every transit.

"The shape of the transit is typical of a comet with a long tail," Hon explained. "Except that it's unlikely that this tail contains volatile gases and ice as expected from a real comet — these would not survive long at such close proximity to the host star. Mineral grains evaporated from the planetary surface, however, can linger long enough to present such a distinctive tail."

Shporer explains that the planet is likely falling apart due to its low mass. "This is a very tiny object [between the size of Mercury and the moon], with very weak gravity, so it easily loses a lot of mass, which then further weakens its gravity, so it loses even more mass," Shporer stated. "It's a runaway process, and it's only getting worse and worse for the planet."

The team plans to carry out follow up observations this summer using the JWST. "This will be a unique opportunity to directly measure the interior composition of a rocky planet, which may tell us a lot about the diversity and potential habitability of terrestrial planets outside our solar system," Hon said.

And in the meantime, the researchers said they're looking for more examples in TESS data. "Sometimes with the food comes the appetite, and we are now trying to initiate the search for exactly these kinds of objects," Shporer said. "These are weird objects, and the shape of the signal changes over time, which is something that's difficult for us to find. But it's something we're actively working on."

Originally posted on Space.com.

A team of scientists has developed a recipe for black holes that eliminates one of the most troubling aspects of physics: the central singularity, the point at which all our theories, laws and models shatter.

If you were going to design an object to preserve mystery while being utterly troubling, you couldn't do much better than a black hole.

First, the outer boundary of these cosmic titans is a one-way light-trapping surface called an event horizon, the point at which a black hole's gravity is so powerful that not even light can escape. This means no information can escape from within a black hole, so we can never directly observe or measure what lies at its heart.

Using the mathematics of Einstein's 1915 theory of gravity, called general relativity, scientists can model the interior of a black hole. The problem is that, when they do this, general relativity tells us that all mathematical values go to infinity at the "singularity" at the heart of a black hole.

This new research suggests that "ordinary black holes" without a central singularity — the physics equivalent of having your cake and eating it — may be more than just the fever dream of hopeful physicists.

"The singularity is the most mysterious and problematic part of a black hole. It's where our concepts of space and time literally no longer make sense," study team member Robie Hennigar, a researcher at Durham University in England, told Space.com. "If black holes do not have singularities, then they are much more ordinary."

Related: What are black holes? Everything you need to know about the darkest objects in the universe

Singularity-minded: Physicists want one thing

Einstein's theory of general relativity states that objects with mass curve the very fabric of space-time (the three dimensions of space united with the one dimension of time), and gravity arises from this curvature. The greater the mass, the more extreme the curvature of space-time, and the stronger the influence of gravity. All of this is calculated with the equations that underpin general relativity: Einstein's field equations.

"The way that the space-time curves is determined by the Einstein field equations, which are the cornerstone of general relativity," team member Pablo Antonio Cano Molina-Niñirola, of the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) in Spain, told Space.com.

"These equations are extremely successful, as they predict a plethora of observable phenomena in the cosmos, from the motion of planets to the evolution of the universe and the existence of black holes," he added. "But they also predict the existence of singularities, and this is problematic."

Black holes — regions of space-time with extreme curvature — first arose as a concept from solutions to Einstein's field equations suggested by German physicist and astronomer Karl Schwartzchild as he served on the front line during the First World War in 1915. These solutions go to infinity at the center of that region. Physicists don't like infinities, as they indicate the breakdown or incompleteness of their models, or suggest something entirely unphysical. That means something really troubling and undesirable for physicists.

"In general relativity, the interior of a black hole is like a contracting universe, where the singularity represents the moment when space itself collapses," Molina-Niñirola said.

Molina-Niñirola added that many physicists believe that, when gravity becomes exceptionally strong and space-time is highly warped, general relativity must be replaced by a more fundamental theory. It has been presumed that this would be a theory of quantum gravity leading to a "theory of everything" that would unite the so-far incompatible theories of general relativity and quantum physics.

"The hope is that, in this complete theory, black hole singularities will be removed," Molina-Niñirola said. "Now, our recipe for regular black holes goes precisely in this direction, but instead of using a complete theory of quantum gravity, we use something called an 'effective theory.' This is a classical theory of gravity that is supposed to capture the effects of an assumed theory of quantum gravity."

This amounts to the team modifying the Einstein field equations so that gravity behaves differently when space-time is highly curved. Ultimately, this leads to the removal of black holes' central singularities.

Related: Albert Einstein: Biography, facts and impact on science

Quantum gravity and other problems

This newly modified theory suggests there is no singularity at the heart of a black hole. So what does exist in this extreme, exotic realm?

"In our model, the space-time collapse stops, and the singularity is replaced by a highly warped static region that lies at the core of the black hole," Molina-Niñirola said. "This region is static because it does not contract. That means an observer could hypothetically stay there, assuming they were able to survive the huge, but finite, gravitational forces in this region."

Apart from curved space-time, what else dwells at the heart of black holes, if this theory is correct? According to Hennigar, strictly speaking, nothing.

"These black holes are pure vacuum everywhere; there need not be matter present, but one can easily include it if desired," the University of Durham researcher continued. "It might sound weird to have a black hole in the absence of matter, but the same thing can happen even in general relativity."

Even if the team's black hole concept were verified, it likely wouldn't halt the search for a valid model of quantum gravity and a theory of everything.

"In some sense, this is a problem that cannot be avoided. Stars are collapsing all the time in our universe; it is an unavoidable physical process. But this commonplace occurrence is something that pushes us past everything we know," Hennigar continued. "In the final stages of collapse, just before one would reach the singularity, both gravity and quantum effects will be important.

"So we already know that the conclusions one would draw from general relativity alone are insufficient to describe such an extreme place/moment."

Does losing the singularity mean losing the mystery? Not quite...

If correct, this research may have somewhat demystified black holes, but it opens up many questions that will still have to be answered.

"Our work provides answers to some mysteries, but it opens others," Molina-Niñirola said. "For instance, according to our model — and other proposals in scientific literature — the matter that falls inside a regular black hole would ultimately exit the black hole through a white hole located in a different universe or in a disconnected region of the same universe.

"This looks very exotic, but it is the only possibility if singularities do not exist: all that goes into a black hole must eventually come out of it."

The researcher added that this process entails problems of its own, which must also be investigated to assess the robustness of the team's idea.

The big question is whether scientists could ever find evidence for this theory from actual observations of black holes; after all, we know we can't simply peer into their interiors.

"It’s difficult to say, since the effects that lead to singularity resolution might only become observable in regimes of extremely strong gravity, probably far stronger than what we can hope to observe," Molina-Niñirola said. "However, there are some experiments that can offer us some possibilities."

Molina-Niñirola explained that the observation of ripples in space-time called gravitational waves allows astronomers to observe much stronger gravitational fields than ever before. This gives scientists a unique chance at trying to spot effects beyond general relativity, including those that may lead to singularity resolution.

Additionally, if the team's theory is correct, there should be a tell-tale imprint in the very early universe, during the era of cosmic inflation right after the Big Bang.

"In this regard, the detection of a primordial, gravitational wave background — which has not been detected yet — could provide hints on possible modifications of gravity," Molina-Niñirola said. "Finally, a consequence of the absence of singularities is that the end-product of black hole evaporation via Hawking radiation would be a microscopic black hole.

"These microscopic black holes provide a possible dark matter candidate. Thus, if dark matter turned out to be composed of tiny black holes, this would be an indirect proof in favor of the absence of singularities."

The team's research was published in the journal Physics Letters B in February 2025.

Originally posted on Space.com.

Black hole quiz: How supermassive is your knowledge of the universe?

Astronomers may have just identified a rare example of a "three-body problem" hiding in plain sight beyond the solar system's most distant planet. If the observation can be confirmed, it suggests that many more cosmic triplets could be hiding in the outer reaches of our cosmic neighborhood, researchers say.

Back in 2001, astronomers discovered what they thought was a binary system made up of two large bodies orbiting each other approximately 3.7 billion miles (6 billion kilometers) from Earth in the Kuiper Belt — a ring of asteroids, comets and dwarf planets, including Pluto, that lies beyond the orbit of Neptune.

The two icy rocks, collectively named 148780 Altjira, are separated by around 4,700 miles (7,600 km), or roughly one-fiftieth the distance between Earth and the moon. (The system is named after the creation deity of the Aboriginal Arrernte people from Australia.)

But in a new study published March 4 in The Planetary Science Journal, researchers propose that the inner body in the Altjira system is actually a pair of smaller objects circling extremely close to one another, making this a triple system.

The study team came to this conclusion after combining images from the Hubble Space Telescope with 17 years' worth of data collected by the W. M. Keck Observatory on Hawaii's Mauna Kea volcano. This revealed subtle shifts in the trajectory of the outer body, suggesting that it is being gravitationally tugged on by two objects instead of one. However, the system is too far away to get visual confirmation of the separation of the inner bodies.

Related: 8 strange objects that could be hiding in the outer solar system

"A triple system was the best fit [when comparing different modeling scenarios]," study lead author Maia Nelsen, an astronomer at Brigham Young University in Utah, said in a NASA statement. However, it is also possible that the inner body could be a "contact binary" — two objects that touch each other and act as a single entity — or "something that actually is oddly flat, like a pancake," she added.

Over the next 10 years, the Altjira system will be in its "eclipsing season," where the outer body will be frequently positioned between the inner body and the sun, which could allow for more detailed observations of its orbital trajectory, the researchers wrote.

During this time, the James Webb Space Telescope is also scheduled to survey the Altjira system, which could conclusively settle the debate with its unmatched image-resolving powers, according to NASA.

The three-body problem

When three objects with similar mass orbit one another, the mathematics involved in calculating their trajectories becomes extremely challenging and leaves very little room for error. That means the slightest change to one of the objects' trajectories can throw the whole system out of balance.

"The puzzle of predicting how three gravitationally bound bodies move in space has challenged mathematicians for centuries," NASA representatives wrote in the statement. This is often referred to as the three-body problem, which has also inspired a popular science fiction novel and a recent TV series of the same name.

Several types of triple systems have been found throughout the cosmos, including triple-star systems like our nearest stellar neighbors — the Alpha Centauri system — and "Tatooine" exoplanets with two suns, providing several solutions to this problem. However, there is no single generally accepted solution to working out the orbital mechanics of a triple system, so the "problem" is often considered unsolved.

If confirmed as a triple system, Altjira will be the second of its kind discovered in the Kuiper Belt; 47171 Lempo, which has a nearly identical configuration as the one proposed for Altjira, was also previously classified as a binary system.

There are around 40 other known binary systems in the Kuiper Belt, some of which could be unrecognized triple systems. However, there are likely many more three-body systems there.

So far, scientists have found around 3,000 Kuiper Belt objects, but they estimate that there could be "several hundred thousand more" smaller objects there, each wider than 10 miles (16 km) across, researchers wrote. Therefore, this part of the solar system could be a great place to hunt for many more of these systems.

"The universe is filled with a range of three-body systems," Nelson said. "And we're finding that the Kuiper Belt may be no exception."

The Euclid space telescope has captured a stunning and rare "Einstein ring" magnifying light from the depths of the universe.

The image shows a faint halo surrounding the nearby galaxy NGC 6505, created as the galaxy warps and magnifies light from an even more distant galaxy behind it.

This type of magnification is called gravitational lensing and was first predicted by Albert Einstein in 1915. The powerful magnification means that the resulting image shows us light from an unnamed and undiscovered galaxy 4.42 billion light-years into the universe's past — even though NGC 6505 is only 590 million light-years away, in Earth's "cosmic backyard." The researchers published their findings Feb. 10 in the journal Astronomy and Astrophysics.

"An Einstein ring is an example of strong gravitational lensing," study lead author Conor O'Riordan, a researcher at the Max Planck Institute for Astrophysics in Germany, said in a statement. "All strong lenses are special, because they're so rare, and they're incredibly useful scientifically. This one is particularly special, because it's so close to Earth and the alignment makes it very beautiful."

Magnifying the stars

Einstein's theory of general relativity describes the way massive objects warp the fabric of the universe, called space-time. Gravity, Einstein discovered, isn't produced by an unseen force but by space-time curving and distorting in the presence of matter and energy.

Related: Mysterious 'Green Monster' lurking in James Webb photo of supernova remnant is finally explained

This curved space, in turn, sets the rules for how energy and matter move. Even though light travels in a straight line, light traveling through a highly curved region of space-time, such as the region around a massive galaxy, also travels in a curve — bending around the galaxy and splaying out into a halo.

The new image was retrieved from data collected by the Euclid space telescope during its early testing phase in September 2023. Launched on July 1, 2023, Euclid was designed to compile wide-lens images to help scientists hunt for two of the universe's most mysterious components: dark matter and dark energy.Researchers think dark matter and dark energy together make up about 95% of the universe, but they do not interact with light so can't be detected directly.

Instead, scientists study these mysterious components by observing the way they interact with the visible universe around them: Dark matter can be seen by observing its gravitational warping effects on galaxies; and dark energy can be spotted in the force propelling the universe's runaway expansion.

Astronomers have identified hundreds of Einstein rings. But these phenomena aren't sought after just because they make pretty pictures. Because the rings magnify light, scientists can reconstruct this light into its original, pre-bent, form, which can enhance the details astronomers are able to spot in very distant galaxies.

Also, because the extent to which light bends depends on the strength of the gravitational field of the object that bends it, Einstein rings can act as a cosmic scale for gauging the masses of galaxies and black holes, including how much dark matter they contain.

"I find it very intriguing that this ring was observed within a well-known galaxy, which was first discovered in 1884," study co-author Valeria Pettorino, a project scientist working on Euclid, said in the statement. "The galaxy has been known to astronomers for a very long time. And yet this ring was never observed before. This demonstrates how powerful Euclid is, finding new things even in places we thought we knew well. This discovery is very encouraging for the future of the Euclid mission and demonstrates its fantastic capabilities."

Euclid is partway through its six-year mission to catalog a third of the entire night sky by capturing thousands of wide-angle images. All told, Euclid will capture light from more than a billion galaxies that are up to 10 billion years old, according to the European Space Agency.

Once this is done, astronomers will use Euclid's images to create two maps: one of the gravitational lensing of galaxies that should reveal concentrations of dark matter, and the other of shock waves called baryon acoustic oscillations that can trace dark energy.

The Hula-Hoop is one of the most famous toys in history, but the science behind it has gotten little attention. Some of us are master twirlers, while others can't manage more than one spin. So how do Hula-Hoops keep from falling down as they are whirling, and why are some of us better at it than others?

Modern Hula-Hoops are plastic rings you twirl around your body by moving your hips. This swiveling motion is similar to that seen in the Hawaiian dance known as the hula (hence the name).

There is evidence of humans doing Hula-Hoop-like twirling "as far back as 500 B.C.," said Olivia Pomerenk, a doctoral candidate in math at New York University. It shows up again and again "in a myriad of cultures as a form of recreation, religious ceremony, or exercise."

Considering the activity's long history, you might think it "has been studied to death at this point, but it actually has not," Pomerenk told Live Science. Until recently, research into Hula-Hoop twirling was generally limited to two-dimensional models of a twirling hoop, "rather than the full 3D system," she noted. As such, this prior work could not answer how Hula-Hoops can keep from falling.

In a 2024 study published in the journal PNAS, Pomerenk and colleagues decided to investigate this head-spinning question.

"Our lab tends to gravitate towards these quirky, seemingly simple systems," Pomerenk said. "Many problems we study, when described, elicit the reaction, 'Wait, how has no one solved that already?' This Hula-Hoop problem is no different."

Related: Can we stop time?

To shed light on the question, Pomerenk and her colleagues created miniature robot Hula-Hoopers. They 3D-printed plastic items that were about 6.7 inches (17 centimeters) tall and came in a variety of shapes, such as cylinders, cones and hourglasses. Then, they made the shapes gyrate with a motor to whirl around hoops about 6 inches (15 cm) wide and used computer software to analyze high-speed video recordings of the resulting movements.

The researchers found that stable twirling of the hoop around these robots was possible given a range of gyration motions or bodies. For stable twirling to occur, you must start off by throwing the hoop with a sufficient amount of speed in the same direction as your body's gyration. After that point, centrifugal force and the friction from rolling can keep the hoop twirling stably.

However, keeping the hoop up against gravity for a significant amount of time is more difficult. Ideally, "the body must have 'hips' to provide the proper angle for pushing up the hoop, but also a 'waist' with curves to hold it in place," said study senior author Leif Ristroph, an applied mathematician and experimental physicist at New York University.

These findings suggested that people with hourglass shapes may be natural hoopers. However, "we hope no one takes our results to mean that they cannot Hula-Hoop because of their body shape," Ristroph told Live Science. "We think everyone can, and perhaps different shapes might take a little extra effort or a strategy different from what we investigated in our experiments."

The findings not only help to explain a familiar but poorly understood activity but may also point to a variety of applications involving "transforming one type of motion to another, or suspending and positioning objects without the need to grip or grasp them," Ristroph said.

For instance, with just a slight twitch of their body, a good Hula-Hoop twirler can send a hoop flying around in big orbits, Ristroph noted. This could inspire novel ways of "harvesting or recovering energy from vibrations," he explained.

Another possible application might involve controlling objects without actually holding them, Pomerenk said. For instance, the study presented a relatively simple way to control the vertical position of a twirling hoop along a body without grasping it.

"If you can hoist something up or move something down in a controlled manner without ever actually holding it in a traditional sense, this could be useful in robotic gripping — for example, holding one or several items, or even perhaps in efficiently transporting items vertically in a factory or construction setting," Pomerenk said.

On Earth, you can look up at night and see the Moon shining bright from hundreds of thousands of miles away. But if you went to Venus, that wouldn't be the case. Not every planet has a moon — so why do some planets have several moons, while others have none?

I'm a physics instructor who has followed the current theories that describe why some planets have moons and some don't.

First, a moon is called a natural satellite. Astronomers refer to satellites as objects in space that orbit larger bodies. Since a moon isn't human-made, it's a natural satellite.

Currently, there are two main theories for why some planets have moons. Moons are either gravitationally captured if they are within what's called a planet's Hill sphere radius, or they're formed along with a solar system.

Related: How far away is the moon?

The Hill sphere radius

Objects exert a gravitational force of attraction on other nearby objects. The larger the object is, the greater the force of attraction.

This gravitational force is the reason we all stay grounded to Earth instead of floating away.

The solar system is dominated by the Sun's large gravitational force, which keeps all of the planets in orbit. The Sun is the most massive object in our solar system, which means it has the most gravitational influence on objects such as planets.

In order for a satellite to orbit a planet, it has to be close enough for the planet to exert enough force to keep it in orbit. The minimum distance for a planet to keep a satellite in orbit is called the Hill sphere radius.

The Hill sphere radius is based on the mass of both the larger object and the smaller object. The Moon orbiting Earth is a good example of how the Hill sphere radius works. The Earth orbits around the Sun, but the Moon is close enough to Earth that Earth's gravitational pull captures it. The moon orbits around the Earth, rather than the Sun, because it is within Earth's Hill sphere radius.

Smaller planets like Mercury have a tiny Hill sphere radius, since they can't exert a large gravitational pull. Any potential moons would likely get pulled in by the Sun instead.

Many scientists are still looking to see whether these planets may have had small moons in the past. Back during the formation of the solar system, they may have had moons that got knocked away by collisions with other space objects.

Mars has two moons, Phobos and Deimos. Scientists still debate whether these came from asteroids that passed close into Mars' Hill sphere radius and got captured by the planet, or if they were formed at the same time as the solar system. More evidence supports the first theory, because Mars is close to the asteroid belt.

Jupiter, Saturn, Uranus and Neptune have larger Hill sphere radii, because they are much larger than Earth, Mars, Mercury and Venus and they're farther from the Sun. Their gravitational pulls can attract and keep more natural satellites such as moons in orbit. For example, Jupiter has 95 moons, while Saturn has 146.

Moons forming with a solar system

Another theory suggests that some moons formed at the same time as their solar system.

Solar systems start out with a big disk of gas rotating around a sun. As the gas rotates around the sun, it condenses into planets and moons that rotate around them. The planets and moons then all rotate in the same direction.

But only a few moons in our solar system were likely created this way. Scientists predict that Jupiter's and Saturn's inner moons formed during the emergence of our solar system because they're so old. The rest of the moons in our solar system, including Jupiter's and Saturn's outer moons, were probably gravitationally captured by their planets.

Earth's Moon is special because it likely formed in a different way. Scientists believe that long ago, a large, Mars-sized object collided with the Earth. During that collision, a big chunk flew off the Earth and into its orbit and became the Moon.

Scientists guess that the Moon formed this way because they’ve found a type of rock called basalt in soil on the Moon’s surface. The Moon’s basalt looks the same as basalt found inside the Earth.

Ultimately, the question of why some planets have moons is still widely debated, but factors such as a planet’s size, gravitational pull, Hill sphere radius and how its solar system formed may play a role.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Ever since the ancient Greeks first made observations of the circular Moon and the skies, scientists have known that the Earth is a sphere. We've all seen beautiful images of the Earth from space, some photographed by astronauts and others collected remotely by orbiting satellites. So why doesn't our planet look round when we're standing in a park or looking out a window?

The answer is all about perspective. Humans are pretty tiny creatures living on a really large sphere.

An average adult is between 5 feet and 6 feet 6 inches tall (1.5 to 2 meters), and kids are smaller. Imagine you're a circus acrobat standing on a ball that's about 3 feet (1 meter) wide. From on top of the ball, you would see it curving away from your feet in all directions.

Now picture a tiny fly on that circus ball. Its viewpoint would probably be a millimeter or less above the surface. Since the fly is much smaller than the ball, and its view is close to the surface, it can't see the whole ball.

The Earth is about 42 million feet (12.8 million meters) wide, and even a tall adult's viewpoint is just 6 feet (about 2 meters) above its surface. There is no way our eyes can take in the size of the spherical Earth when we are standing on it. You couldn't tell the Earth was a sphere even if you hiked to the top of Mount Everest, which is 29,035 feet (8,850 meters) above sea level.

Related: Flat Earth 'theory': Why do some people think the Earth is flat?

The only way to see the curve of the Earth is to fly more than 6 miles (10 kilometers) above its surface. This is because the length of the horizon that we see depends on how high we are above Earth's surface.

Standing on the ground with nothing blocking our vision, our eyes can see about 3 miles (4.8 kilometers) of the horizon. That's not enough of the planet's circumference to see the horizon line begin to show off its curve. Like a fly on a circus ball, we just can't see enough of the edge where the Earth meets the sky.

To see the whole spherical planet, you would need to hitch a ride with an astronaut or on a satellite. This would give you a full view of Earth from a much greater distance.

Big commercial airliners also can fly high enough to give glimpses of Earth's curvature, although pilots have a much better view from the front of the plane than passengers get from side windows.

Not quite a sphere

Even from space, you wouldn't detect something important about Earth's shape: It's not perfectly round. It's actually a slightly oblate spheroid, or an ellipsoid. This means it is a little bit wider around the equator than it is tall, like a sphere that someone sat on and squashed a little bit.

This is caused by Earth's rotation, which creates centrifugal force — the same force that would cause you to fly off a spinning merry-go-round if you didn't hold on. This force produces a slight bulge at the planet's waistline.

Topographic features on Earth's surface, such as mountains and deep-sea trenches, also distort its shape slightly. They cause small variations in the strength of Earth's gravitational field — the force that pulls all objects on Earth downward, toward the planet's center.

Earth science, the field that I study, has a branch called geodesy that's devoted to studying Earth's shape and how it's positioned in space. Geodesy informs everything from building sewers and making accurate maps of sea level rise to launching and tracking spacecraft. It's an important area of current scientific research and a reminder that we are still learning about this amazing planet we call home.

This question was submitted by Zayden, age 11, from Corona, California as part of The Conversation's Curious Kids series.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Traveling to the moon is no easy feat. Our natural satellite orbits Earth at an average distance of 238,855 miles (384,400 kilometers). So how long does it take to reach our closest neighbor from the moment a spaceship lifts off?

Based on lunar missions from the past few decades, the answer ranges from about eight hours to 4.5 months. The fastest human-made craft to whiz by the moon — meaning it didn't stop there — was the New Horizon probe launched by NASA in 2006 to study Pluto; this spacecraft passed by the moon 8 hours and 35 minutes after launch.

But for missions whose destination is the moon, the journey takes a bit longer. In 1959, in humanity's first ever moon mission, the Soviet Union's Luna 1 took 34 hours to reach the moon. This uncrewed mission was intended to impact the moon's surface, but the spacecraft went off course, passing 3,725 miles (5,995 kilometers) away from the moon. It eventually stopped transmitting when its batteries died, and it's still floating through space to this day.

In 1969, when astronauts actually landed on the moon, it took the Apollo 11 crew 109 hours and 42 minutes from liftoff to Neil Armstrong's first step on the moon.

The reasons for these variable travel times to the moon depend on many factors, but one of the most important reasons is the amount of fuel used. Engineers have found that using less fuel in a lunar mission can take longer, but it still gets the job done. This can be pulled off by using the natural gravitational forces of celestial bodies, such as Earth and the moon, to help guide the spacecraft along a longer route.

For instance, in 2019, Israel sent an uncrewed spacecraft named Beresheet to land on the moon. After liftoff, Beresheet looped around Earth for about six weeks in ever-widening orbits before gaining enough momentum to zip off toward the moon. It got there, but not the way the Israeli organization SpaceIL wanted: The team lost contact and Beresheet crashed into the lunar surface 48 days after launch, spilling thousands of microscopic tardigrades onto the moon in the process.

Related: Could a spaceship fly through a gas giant like Jupiter?

The spacecraft that holds the record for longest journey to the moon is NASA's CAPSTONE probe, a 55-pound (25 kilograms) cubesat that took 4.5 months to leave Earth, circle it several times, and finally enter the moon's orbit in 2022. CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment) was sent to the moon to test out an orbit that NASA plans to use for its planned Gateway space outpost.

No matter which route a spacecraft takes, every lunar mission goes through several essential steps. Between 60% and 90% of the launch weight of any space mission is the fuel that enables it to escape Earth's gravity and enter space. Once the spacecraft arrives in orbit, it needs to use as little fuel as possible to achieve the optimal trajectory to its target, as including more fuel makes the spacecraft heavier and more expensive.

Finally, the craft needs to execute a further fuel burn to escape Earth's orbit and be on its way. The velocities of spacecraft in transit tend to be similar, but where Luna 1 had a direct trajectory, Apollo 11 needed a more precise lunar orbit trajectory, which accounted for the longer travel time. That meant directing the craft not at the moon but just beside it so it enters orbit, and at a speed safe enough to launch a lander and receive it again.

Apollo 11 took around 4.5 days to reach the moon for other reasons, too. For example, it needed to complete a battery of maneuvers and checks to the guidance and navigation systems before it left Earth's gravity.

"Once outside of the main Earth gravity influence, only minor orbit corrections are needed, so less fuel is required," Gretchen Benedix, a founding member and professor at the Space Science and Technology Centre at Curtin University in Australia, told Live Science. "Gravity does all the work — the moon's gravitation will pull on whatever mass was launched."

But the travel time also depends on other factors. One of the biggest, according to Mark Blanton, who leads NASA's Moon to Mars mission analysis and integrated assessments, is the purpose of the mission.

"Missions or agencies will evaluate the type of rockets available and their capabilities to carry spacecraft," he told Live Science. "The rocket capabilities and mission objectives will set the sizing of the spacecraft — for instance, if it's a science instrument versus a crewed mission.

"When you put all those constraints together, it will let you design an optimal trajectory, and that will inform on the number of Earth orbits to set up a particular geometry or trajectory," Blanton said.

That means that, like with everything related to spacecraft and spaceflight, precise calculations about craft size, crew size, fuel allocation and every other possible detail can have an impact on total travel time to the moon.

The Indian Ocean "gravity hole" is the site of the deepest dent in Earth's gravitational field. It's a circular ocean region with a gravitational pull that's so weak, sea levels are 348 feet (106 meters) lower there than elsewhere on Earth. Discovered in 1948, the origins of this giant gravity hole — or geoid low, as it is technically called — remained a mystery until recently.

Related: 'A force more powerful than gravity within the Earth': How magnetism locked itself inside our planet

The hole spans 1.2 million square miles (3.1 million square kilometers) and sits 746 miles (1,200 km) southwest of India. Various theories have tried to explain its existence since geophysicists first detected its trace, but the answer only came in 2023 with a study published in the journal Geophysical Research Letters. Researchers used 19 computer models to simulate the motion of Earth's mantle and tectonic plates over the past 140 million years, and then teased out the scenarios giving rise to a geoid low similar to the real-life one.

The study indicated that the Indian Ocean gravity hole formed after the death of an ancient ocean called Tethys, which existed between the supercontinents Laurasia and Gondwana. Tethys sat on a chunk of Earth's crust that slipped beneath the Eurasian plate during the breakup of Gondwana 180 million years ago. As this happened, shattered fragments of the crust sank deep into the mantle.

Around 20 million years ago, as these fragments landed in the lowermost regions of the mantle, they displaced high-density material originating from the "African blob" — a compact bubble of crystallized magma, 100 times taller than Mount Everest, that is trapped beneath Africa. Plumes of low-density magma rose to replace the dense material, diminishing the overall mass of the region and weakening its gravity.

Scientists are yet to confirm these model predictions with earthquake data, which could help to verify the existence of low-density plumes beneath the hole. Meanwhile, researchers are realizing more and more that Earth’s magma is full of strange blobs, including some that were thought to be missing and have turned up in unexpected places.

And it's not just Earth — explorations of Mars, too, have revealed blobs of all shapes and sizes lurking below the planet’s surface.

Discover more incredible places, where we highlight the fantastic history and science behind some of the most dramatic landscapes on Earth.

For the first time, researchers have used data from the James Webb Space Telescope (JWST) to uncover an example of a previously hypothetical phenomenon known as an "Einstein zig-zag" — where light from an object in the distant cosmos passes through two different regions of warped space-time. The newly confirmed effect, which was discovered among six identical copies of a luminous quasar, could shed light on an issue that is beginning to plague cosmology, experts say.

In 2018, astronomers discovered a quartet of identical bright points billions of light-years from Earth, later named J1721+8842. Initially, the scientists assumed that the four lights were mirror images of a single quasar — a luminous galactic core powered by a feeding black hole — that had been duplicated through a phenomenon known as "gravitational lensing."

Gravitational lensing happens when light from a distant object appears to get bent as it passes through warped space-time that has been pulled out of shape by the immense gravity of a lensing object — usually a massive galaxy or cluster of galaxies — located between the distant object and the observer. This warping effect can either duplicate the initial light source, as the light takes different routes around the lensing object, or stretch out the light into luminous halos, known as Einstein rings after Albert Einstein, who first predicted gravitational lensing with his theory of general relativity in 1915.

But in a 2022 study, researchers discovered that J1721+8842 had two additional points of light alongside the original quartet, as well as a faint red Einstein ring. The newly discovered points were slightly fainter than the other four points, which led researchers to suspect that the light show showed a pair of adjacent quasars, known as a binary quasar, that had been duplicated three times (rather than a single quasar that had been copied six times).

Related: Researchers solve mystery of inexplicably dense galaxy at the heart of perfect 'Einstein ring' snapped by James Webb telescope

However, in a new study, uploaded Nov. 8 to the preprint server arXiv, researchers reanalyzed J1721+8842 using new data from JWST and found that all six points of light are actually from a single quasar after all. The team also found that newly unveiled bright spots have been lensed around a second massive object farther away from the first, which is also responsible for the faint Einstein ring seen in more recent images. (The study has not yet been peer-reviewed but has been submitted for publication in the journal Astronomy & Astrophysics.)

After observing the light curves of each bright spot over two years, researchers showed that there is a slight delay in the time it takes the two faintest duplicate images to reach us, which suggests that the light in these copies has to travel farther than the other four bright spots. This is likely because the light in these images passes around the opposite sides of each lensing object (i.e. around the left side of the first lens and right side of the second lens).

The study team has dubbed this "extremely rare lensing configuration" an Einstein zig-zag because light from some of the double-lensed bright spots has swerved back and forth as it passed around both lensing galaxies, the researchers wrote.

Saving cosmology

Gravitationally lensed objects, such as Einstein rings, are treasured by astronomers and cosmologists because the warped light can help reveal the mass of the galaxies that lensed them. This, in turn, can help reveal secrets of the universe such as the secret identity of dark matter and how dark energy drives cosmic expansion.

JWST has been exceptionally good at finding these objects in parts of the universe where we have never been able to see them before. But unfortunately, the state-of-the-art telescope has also highlighted discrepancies we cannot currently explain.

For example, measurements from the telescope have confirmed that different parts of the universe are expanding at different rates, which threatens to "break" our understanding of cosmology. Researchers refer to this problem as the Hubble tension.

However, researchers believe that the newly confirmed Einstein zig-zag could help to smooth out this tension because its unique configuration will allow astronomers to precisely measure both the Hubble constant — the rate at which cosmic expansion is accelerating — and the amount of dark energy — the invisible force driving the universe's expansion — in this region of space. Normally, scientists can only determine exact figures for one or the other but detailed knowledge of both is needed to truly understand cosmic expansion, the researchers wrote.

Thomas Collett, an astrophysicist at the University of Portsmouth in the U.K. who was not involved in the study, told Science magazine that studying the zig-zag will "shine a light on whether the expansion rate of the universe is consistent with the cosmological model or not." However, it could take researchers more than a year to resolve the figures they need from the tangled images, he added. "So we might have to wait a while [for an answer]."

Atmospheric ripples from Hurricane Helene spread far north of Florida as the devastating storm made landfall, new NASA images show.

The agency's Atmospheric Waves Experiment (AWE) captured concentric bands of atmospheric gravity waves stretching across the Southeast as the hurricane progressed miles away.

"Like rings of water spreading from a drop in a pond, circular waves from Helene are seen billowing westward from Florida's northwest coast," AWE principal investigator Ludger Scherliess, a physicist at Utah State University, said in a statement.

Atmospheric gravity waves are vertical ripples that move through quiet areas of the atmosphere, dividing the air into peaks and troughs. According to NASA, these waves can be created by large thunderstorms, wind bursts, hurricanes, tornadoes and even tsunamis. (They are different from gravitational waves, which are ripples in the fabric of space-time that result from violent cosmic events, such as black hole collisions.)

Related: Hurricane season 2024: How long it lasts and what to expect

The AWE instrument is mounted on the International Space Station and detects these waves by measuring airglow — a faint light given off by gasses in the mesosphere, the third layer of Earth's atmosphere. The mesosphere ranges from 31 to 53 miles (50 to 85 kilometers) above Earth's surface. Most weather occurs in the first layer of Earth's atmosphere, the troposphere, though cloud tops can rise into the second layer, the stratosphere, in very strong storms. (These are called "overshooting cloud tops.")

AWE started observing in November 2023, and the Helene gravity-wave images are among the first AWE images that NASA has released publicly. One of the project's goals is to help scientists understand how weather on Earth's surface can affect space weather, the disturbances in the upper atmosphere caused by interactions with charged cosmic particles.

Hurricane Helene was a Category 4 storm with winds up to 140 mph (225 km/h) when it made landfall near Perry, Florida. The storm subsequently moved inland, stalling over eastern Tennessee and western North Carolina, where it caused massive flooding. More than 230 people were killed, according to the Associated Press.

Landslides and tremors may transform the asteroid Apophis during its 2029 brush with Earth, according to a new study.

Named after Apep, the ancient Egyptian god of chaos, Apophis is a 1,100-foot-long (340 meters), peanut-shaped asteroid. Although an impact with a space rock that size wouldn't annihilate our planet, it could easily destroy a city.

When Apophis was discovered in 2004, astronomers calculated that it could pass extremely close to Earth in 2029. More detailed observations in 2021 allowed scientists to determine Apophis' path with greater accuracy, revealing that it had a smaller chance of hitting Earth than researchers initially estimated. At present, Apophis is predicted to sail as close as 20,000 miles (32,000 kilometers) to Earth on April 13, 2029, bringing it closer than some artificial satellites.

Given that distance, Apophis probably won't affect Earth much in 2029. But how will the asteroid itself fare after this close encounter?

That question intrigued Ronald-Louis Ballouz, an asteroid scientist at the Johns Hopkins University Applied Physics Laboratory. Tiny meteoroids constantly bombard asteroid surfaces in a process called space weathering, Ballouz told Live Science in an email. However, Ballouz added that astronomers have long seen that asteroids that pass close to planets like Earth often lack weathered surfaces.

The exact physical mechanism that removes the evidence of weathering isn't well known, Ballouz said. One possibility is that a planet's gravity pulls on rocks on an asteroid's surface, tossing them away and revealing the underlying layer.

To test this hypothesis, Ballouz and a group of international researchers created computational models of Apophis. Few of the asteroid's physical features are known, so the researchers based their models on a similar two-lobed asteroid, Itokawa, which has been studied in greater detail. The researchers then simulated each model's movement toward Earth, tracking both large- and small-scale physical changes.

Related: NASA's most wanted: The 5 most dangerous asteroids to Earth

The researchers discovered that two physical processes — triggered by Earth's gravitational tugs — will likely sculpt Apophis' surface during its 2029 encounter. One is tremors that will probably begin an hour before Apophis reaches its closest point to Earth and continue for a short while after.

The tremors' strength is difficult to estimate, Ballouz said. However, "Apophis' gravity is about 250,000 times smaller than Earth's," he added. "So, we think that events of much smaller magnitude could plausibly shake things up on its surface."

This means the quakes may be intense enough to loft boulders from Apophis' surface. While some rocks may escape, most will fall back onto Apophis, creating distinct surface patterns that a passing spacecraft could identify.

The other process that could "refresh" Apophis' surface is a change in its tumbling. Tumbling occurs because the asteroid doesn't rotate on a fixed axis or time period; instead, it tumbles through space like a badly thrown football.

An unrelated 2023 study showed that Earth's gravity would cause the asteroid to rotate either more quickly or more slowly depending on its orientation during the 2029 approach. The new simulations confirmed this finding. They also revealed that the changes in Apophis' tumbling will cause the sloping faces of surface rocks to destabilize, potentially triggering landslides in extreme cases. Unlike the seismic shaking, these changes will occur gradually, over tens of thousands of years.

The results don't just predict how Apophis will fare following the 2029 encounter. According to Ballouz, "they introduce a novel mechanism for asteroid surface refreshing that may provide an answer to a decades-long problem of how close planetary encounters can modify small body surfaces."

Ballouz and his colleagues hope that NASA's OSIRIS-APEX mission will confirm their hypotheses. Repurposed from OSIRIS-REx — the spacecraft that picked up samples from the asteroid Bennu — OSIRIS-APEX is scheduled to study Apophis during its 2029 encounter. It will rendezvous with the asteroid for 18 months to study its chemical composition and chart its surface.

The study, currently available on the arXiv preprint database, has been accepted for publication in The Planetary Science Journal.

Astronomers have accidentally discovered the first-known "black hole triple" system, containing a dark void orbited by two stars. The unique configuration of this triad hints that the black hole was not born via a supernova, which blows away what we thought we knew about how these cosmic entities form.

Until now, most discovered black holes — excluding the supermassive variety at the center of most galaxies — exist in binary systems, in which they are orbited by another large object, such as a star, neutron star or a smaller black hole. This is because the invisible space-time voids are easier to spot when they are gravitationally tugging on other objects.

But in a new study, published Wednesday (Oct. 23) in the journal Nature, researchers discovered that one of these known binary systems, which contains the black hole V404 Cygni feasting on a nearby star, actually has a second star circling the pair at a much greater distance.

Gravitational calculations show that the newfound star could not have remained in this delicate system if the black hole was birthed by an exploding star, or supernova, as most other black holes are believed to form. If it had, the distant star would have been blown out of the system by the resulting shockwave. Instead, the team suggests that the black hole formed via the gradual collapse of a massive third star that was once orbited by the other two stars.

This possibility is "super exciting for black hole evolution," study lead author Kevin Burdge, an astrophysicist at MIT, said in a statement. "We think most black holes form from violent explosions of stars, but this discovery helps call that into question," Burdge added.

Related: Epic NASA video takes you to the heart of a black hole — and destroys you in seconds

The black hole in the newly realized triad, V404 Cygni, is about nine times more massive than the sun and located in the Milky Way around 8,000 light-years from Earth. It was one of the first black holes ever discovered when it was spotted in 1992 and has been studied extensively since. Scientists have also long known about its nearby star, which circles the black hole every 6.5 days and is slowly being devoured by its massive partner.

As a result, the study authors were shocked when they reexamined images of V404 Cygni, as part of a wider survey of the Milky Way's black holes, and spotted the second star circling the black hole at a distance of around 3,500 astronomical units — or around 90 times farther away than Pluto orbits the sun. At this distance, it likely takes the newfound star more than 70,000 years to orbit V404 Cygni.

The team then compared the movements of the two stars over the last decade and found that they "moved exactly in tandem," proving that they were gravitationally bound to one another as well as to V404 Cygni. Researchers say the odds that the stars moved in this way without being linked are around 1 in 10 million.

"It's almost certainly not a coincidence or accident," Burdge said. "We're seeing two stars that are following each other because they're attached by this weak string of gravity. So this has to be a triple system."

This is not the first time that researchers thought they had found a black hole triple. In 2020, researchers spotted what they believed to be a black hole being orbited by two stars around 1,000 light-years from Earth, which would have made it the closest black hole to us. However, subsequent observations revealed that this system was actually a binary system containing a "vampire star" instead — that is, a star that slowly steals gas from a smaller partner star.

If V404 Cygni formed through gradual collapse, as the researchers suspect, then the team believes the peculiar black hole was birthed at some point in the last 4 billion years, after the two stars were born.

Over the last few years, researchers have begun to suspect that gradual collapse could be a more common origin for black holes than previously realized. And in March, researchers proposed this mechanism could be behind the disappearance of "vanishing stars" that astronomers have recently lost track of. The new findings suggest that this could be the case.

The region of the universe we live in may be significantly bigger than we thought. A new study reveals that the intergalactic supercluster holding the Milky Way may be part of an even bigger "basin of attraction" that's up to 10 times larger than the one we currently call home.

The universe is full of basins of attraction (BOAs) — regions within which everything is being pulled inward by the gravity of a massive object. BOAs can stack inside one another like nesting dolls. For example, the moon circles Earth, which in turn orbits the sun along with the rest of the solar system, which is itself spiraling around the supermassive black hole at the heart of our galaxy.

But the story doesn't end there. The next layer of the BOA doll is the Local Group, which includes the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy, along with their smaller satellite galaxies such as the Large and Small Magellanic Clouds. After that, the next layers are the Virgo Cluster, which holds around 2,000 galaxies, and the larger Virgo Supercluster. The final known layer is Laniākea (meaning "immense heaven" in the Hawaiian language) — a supercluster first discovered in 2014, which holds around 100,000 galaxies and spans roughly 500 million light-years across.

But in the new study, published Sept. 27 in the journal Nature Astronomy, researchers analyzed the relative movements of more than 56,000 galaxies to create a 3D "probabilistic" map of all the BOAs surrounding the Milky Way. This revealed that there is a decent chance our home galaxy is part of an even larger BOA — the Shapley Concentration — that has a volume up to 10 times greater than Laniākea. (Scientists already knew the Shapley Concentration existed but did not previously believe it impacted the Milky Way.)

Related: Cosmic record holders: The 12 biggest objects in the universe

"It is perhaps unsurprising that the further into the cosmos we look, we find that our home supercluster is more connected and more extensive than we thought," study co-author Noam Libeskind, a cosmologist at the Leibniz Institute for Astrophysics Potsdam in Germany, said in a statement. "Discovering that there is a good chance that we are part of a much larger structure is exciting."

At the moment, the researchers — most of whom were involved in the discovery of Laniākea — believe there is a 60% chance that the Milky Way resides in the Shapley Concentration. The uncertainty is largely caused by high error rates in measuring the speeds of distant galaxies, as well as the presence of dark matter between galaxies, which can exert massive gravitational effects across large regions of space without being visible.

If true, the new findings could also mean that the Milky Way is not part of Laniākea and that the heavenly supercluster might not even exist. Instead, it could just be an outer section of the Shapley Concentration, the researchers wrote in the statement.

The map shows dozens of objects spread out across billions of light-years around the Shapley Concentration, such as the South Pole Wall, the Boötes Void and the Perseus-Pisces Supercluster. The largest BOA on the map is the Sloan Great Wall, which stretches around 1.4 billion light-years across.

While the new map helps us to better pinpoint our place in the wider universe, it also raises the possibility that we could be missing even more information, the researchers wrote.

"This discovery presents a challenge: our cosmic surveys may not yet be large enough to map the full extent of these immense basins," study co-author Ehsan Kourkchi, an astronomer at the University of Hawai'i, said in another statement. "We are still gazing through giant eyes, but even these eyes may not be big enough to capture the full picture of our universe."

You can check out an interactive version of the new map below. The differently colored blobs represent different basins of attraction (light yellow is the Shapley Concentration, blue is Laniākea and red is the Sloan Great Wall). The Milky Way is located at the intersection of the red, blue and green arrows.

Every planet in our solar system is essentially round. But out in the universe, are there any planets that aren't spherical?

Technically, planets are round, by definition; they need to have enough mass to produce the gravity required to pull themselves into a spherical shape.

"Actually, one of the specifications for being a planet is, they have enough mass that makes them round," Susana Barros, a senior researcher at the Institute of Astrophysics and Space Sciences in Portugal, told Live Science.

But that doesn't necessarily mean planets are perfect spheres. "We call them round, but they're not really perfectly round, including our own Earth," Amirhossein Bagheri, a planetary science and geophysics researcher at the California Institute of Technology, told Live Science.

Earth and planets like it often have a bulge around the equator caused by centrifugal force, the outward force experienced by an object that's spinning. On Earth, the bulge is slight but significant: Due to differences in centrifugal force and the distance from Earth's center, things weigh about 0.5% less at the equator than they do at the poles.

Related: How many galaxies are in the universe?

But this effect can be dramatic in the right circumstances. "If the planet is rotating very fast, the poles will flatten," Barros said, leading to a squished, football-like shape.

Centrifugal force isn't the only force that can alter the shape of a planet. "If the body is close enough to the host star, then these gravitational forces that are acting on the body become so large that the planet gets elongated," Bagheri said.

One such body is the exoplanet WASP-103 b, a gas giant twice the size of Jupiter and 1.5 times its mass that orbits a star nearly twice as large as the sun.

WASP-103 b is also "really, really close to the star," Barros said. That changes its shape. "There's a balance between the force of the gas that's called the hydrostatic equilibrium, that wants to expand the planet … and the strength of the gravitational attraction." This pull from the host star leads to a planet that's "tear-shaped," Barros said.

This deformation can even change the way the planet rotates. If a planet starts out with a pronounced bulge toward the home star and continues rotating normally, "then this bulge would not always be in the same place," Barros said.

Moving that bulge around the planet as it rotates uses a lot of energy. "So they start like this, but then, very fast, they will align," Barros said. The planet becomes tidally locked to its host star, with the same side of the planet facing the star at all times.

On top of that, WASP-103 b is orbiting its star extremely quickly, leading to a flattening of its poles, Barros said. The result is a very squished planet.

But even a squished sphere is still mostly spherical. Some scientists have posed the possibility of a toroidal — or doughnut-shaped — planet. This could hypothetically happen if a planet were rotating fast enough for the outward centrifugal force to outweigh the force of gravity pulling the planet's mass toward its center.

But a toroidal planet has never been observed, and it's not likely to be in the near future. "It's more science fiction than science," Bagheri said.

Dozens of mysteriously dense blobs — including one surprisingly pooch-shaped structure — are lurking beneath an ancient seabed surrounding Mars' north pole, a new "gravity map" of the Red Planet reveals. The first-of-its-kind atlas also confirms a recent finding about Mars' tallest mountain that could help reveal secrets about the planet's volcanic past and present.

In a new study, researchers created the first true global density map of Mars by combining data about the planet's crust from NASA's InSight lander with records of fluctuations in the orbits of satellites, such as NASA's Mars Reconnaissance Orbiter and the European Space Agency's Mars Express, as they were pulled out of place by hidden gravitational anomalies.

The standout features in the new map were 20 underground blobs in the Borealis Basin in Mars' northern hemisphere, which was an ancient seabed more than 3 billion years ago. These blobs come in a range of shapes and sizes, including one that "resembles the shape of a dog," and have densities between 300 and 400 kilograms per cubic meter higher than the surrounding ground, the researchers wrote in a statement.

However, it is currently unclear what these blobs are, why they are so dense or how they were created.

"These dense structures could be volcanic in origin or could be compacted material due to ancient [meteor] impacts," study lead author Bart Root, a planetary scientist at Delft University of Technology in the Netherlands, said in the statement. And "there seems to be no trace of them at the surface," he added.

The team presented their findings at the Europlanet Science Congress, which was held in Berlin Sept. 8-13.

Related: 15 Martian objects that aren't what they seem

The new map also confirmed the presence of a massive 1,100-mile-wide (1,750 kilometers) blob under Olympus Mons, a dormant volcano near Mars' equator that, at more than 16 miles (25 km) above the planet's surface, is the solar system's tallest mountain.

This gigantic blob, which was first detected in 2022, is less dense than its surroundings and is most likely a massive plume of cooled lava, hinting that Mars' volcanism may have ended only recently or could be ongoing.

The new map of the Olympus Mons blob could also shed light on the surrounding Tharsis plateau, an elevated region of Mars that is home to the planet's tallest volcanoes, as well as other intriguing geological structures, such as the recently mapped "Labyrinth of Night" canyon.

The newly mapped blobs are not the only interesting objects found under the Red Planet's surface recently.

In January, NASA satellites revealed a 2-mile-thick (3.2 km) layer of ice buried beneath Mars' equator. And in August, scientists announced the major discovery of an enormous hidden ocean — with enough water to cover the planet with 1 mile (1.6 km) of water — trapped deep below the Red Planet's surface.

The study authors are among a group of scientists proposing a Mars mission known as the Martian Quantum Gravity (MaQuls), which would fly twin spacecraft around the Red Planet, allowing researchers to more accurately measure concealed gravitational anomalies. The MaQuls mission was also recently discussed at the Europlanet Science Congress but has not been approved by any agency.

NASA has detected a planet-wide electric field surrounding Earth for the first time ever.

The field, known as the ambipolar electric field, was discovered by NASA's suborbital Endurance rocket more than 60 years after it was first hypothesized, and is thought to be as fundamental to our planet as its better known magnetic and gravitational fields.

By studying it, scientists hope to get a better understanding of how our planet's atmosphere evolved and how it behaves today. The researchers published their findings Aug. 28 in the journal Nature.

"Any planet with an atmosphere should have an ambipolar field," study lead author Glyn Collinson, principal investigator of the Endurance mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "Now that we've finally measured it, we can begin learning how it's shaped our planet as well as others over time."

In a layer of Earth’s atmosphere known as the ionosphere (located between 37 to 190 miles (60 to 300 kilometers) above the Earth's surface), ultraviolet radiation from the sun bombards atoms, stripping them of electrons to transform them into ions. In theory, this should create a slight electric field around our planet, as well as others like it.

Hints of the electric field's existence were first detected in 1968 by spacecraft flying over our planet's North and South Poles. They came in the form of a "polar wind," or a stream of particles that were streaming from Earth's atmosphere into space.

Related: Eerie sounds triggered by plasma waves hitting Earth's magnetic field captured in new NASA sound clip

Some of Earth's atmosphere is expected to escape into space, especially after it's heated by sunlight. But the polar wind was altogether more mysterious; the particles in it were cold, meaning they had not been heated up, but were somehow still moving at speeds that broke the sound barrier.

"Something had to be drawing these particles out of the atmosphere," Collinson said. Yet detecting a possible electric field proved difficult — the field was very weak, with detectable fluctuations only taking place over distances of hundreds of miles.

To investigate the origins of the polar wind, the researchers launched the Endurance rocket from a rocket range in Svalbard near the North Pole, sending it to an altitude of 477.23 miles (768.03 km) above the ground before it splash-landed in the Greenland Sea 19 minutes later.

Over the 322 mile (518 km) range across which Endurance collected data, it detected a miniscule 0.55 volt change, about the strength of a watch battery. Nonetheless, this voltage difference pushes hydrogen ions, the most abundant particles in the solar wind, with a force 10.6 times stronger than gravity.

"That's more than enough to counter gravity — in fact, it's enough to launch [atmospheric particles] upwards into space at supersonic speeds," co-author Alex Glocer, Endurance project scientist at NASA Goddard, said in the statement.

"It's like this conveyor belt, lifting the atmosphere up into space," Collinson added.

Now that the field has been detected, the scientists say that studying the field should help us learn how it changed Earth's atmosphere across our planet's lifetime. They also expect to find similar electric fields in the atmospheres of planets such as Venus and Mars.

Microscopic wormholes may be driving the accelerated expansion of the universe, scientists say. These tiny wormholes are constantly being born from the vacuum of space due to subtle quantum effects.

If confirmed through experiments and observations, the wormholes could become a valuable source of information on quantum gravity — a theoretical unification of the fundamental forces of the universe, often considered to be the Holy Grail of theoretical physics.

Numerous astronomical observations show that our universe is expanding at an ever-increasing rate. However, Einstein's general theory of relativity states that if the universe contains only the species of particles and radiation we know, such behavior of the fabric of space is impossible.

To reconcile the observations of universe expansion with this theory, scientists have proposed that space is filled with an enigmatic entity that can't be detected in ground or space-based experiments.

This mysterious substance, called dark energy, interacts very weakly with other types of matter and fields, so, there is currently no reliable information about its structure or origin.

In a recent study published April 5 in the journal Physical Review D, researchers proposed a bold new candidate for dark energy: subatomic-size wormholes — or tiny tunnels connecting disparate points in space.

Related: Wormholes might bend light like black holes do — and that could be the key to finding them

According to the authors, these wormholes are constantly being born and destroyed in the vacuum of space due to quantum effects. This is similar to how particles are produced near the event horizons of black holes, leading to Hawking radiation; or how electron-positron pairs are generated by a strong electric field — a phenomenon known as the Schwinger effect.

However, the creation of these wormholes is somewhat different from those other phenomena because their mathematical description requires quantum effects in gravity to be accounted for — a task that's much more complicated and poorly understood.

These difficulties in calculating quantum gravitational phenomena prevented the authors from accurately deriving the wormhole birth rate. However, using an approach known as Euclidean quantum gravity, they showed that if about 10 billion wormholes are spontaneously created per cubic centimeter per second, the energy they generate would be sufficient to explain the currently observed rate of the universe's expansion.

"Although our result was derived on the grounds of Euclidean quantum gravity… it is likely that our modification may hold for other quantum gravity theories as well," study co-author Stylianos Tsilioukas, a doctoral student at the University of Thessaly and National Observatory of Athens, told Live Science via email.

Moreover, the team's analysis showed that their model of dark energy is even better observationally than the most widely accepted theory, known as the Standard Cosmological Model, which posits that dark energy has a time-independent energy density.

"According to our proposal dark energy can change as time flows," Tsilioukas said. "This is a major advantage because recent observations suggest that the rate of expansion of the universe is different in recent times than it was in the early universe."

However, no matter how successful the researchers' model is at explaining the general properties of dark energy, the validity of any physical theory must be tested with experimental data. And for now, the theory remains untestable.

In the future, the ever-increasing accuracy of space experiments and observations should enable astronomers to deduce the universe expansion rate in more detail, as well as to measure other observable manifestations of dark energy. This could enable researchers to test whether this newly proposed model of dark energy is correct.

In the meantime, the authors plan to further improve their theoretical analysis. "We are working right now on a model which calculates the rate of wormhole formation. " Tsilioukas said. "The research seems promising and we hope to publish the results very soon."

Dead stars may produce intense flashes of light through the power of gravity itself, researchers have demonstrated. Understanding this phenomenon could reveal new insights about some of the largest, most mysterious explosions in the universe.

Neutron stars are among the strangest objects in the universe. These collapsed cores of massive stars are incredibly dense, hosting more mass than the entire sun compressed into the volume of a city. They are made almost entirely of neutrons bound together — essentially, neutron stars are the largest atomic nuclei in the cosmos. Because of this incredible density, they have gravitational pulls surpassed only by black holes. Their gravity is strong enough to pull light itself into orbits around the star and accelerate nearby objects to nearly the speed of light.

Despite their name, neutron stars aren't entirely neutral. They maintain some electric charge, and combined with the star's rapid rotation — the fastest ones spin more quickly than a kitchen blender — they can power some truly enormous magnetic fields — in some cases, the most powerful magnetic fields in the universe.

The combination of intense magnetic fields and the ultrastrong gravitational environment can lead to some strange new physics, researchers explained in a paper that was uploaded to the preprint database arXiv in June but has yet to be peer-reviewed. 

One interesting new possibility the researchers investigated is neutron stars' ability to give off short, immense bursts of light powered by gravity itself. The flashes take advantage of a phenomenon known as resonance, where a trigger mechanism keeps pumping energy into a system at just the right frequency to keep amplifying itself. Resonance appears throughout physics. Perhaps the most familiar example is a guitar string: When plucked, it sets up resonance with the body of the guitar to dramatically amplify its own sound.

Related: Physicists want to use gravitational waves to 'see' the beginning of time

In the case of neutron stars, the strong magnetic fields around them generate an enormous number of photons, the fundamental bits of light. Usually, these photons scatter and dissipate, adding to the general glow of the neutron star.

But the rapidly rotating neutron star can generate gravitational waves, which are ripples in the fabric of space-time. Astronomers have already detected gravitational waves from black hole and neutron star collisions, but these rotation-powered waves would be much higher-frequency. They would be far too weak to be detectable from Earth, but they could transfer energy from the neutron star to the region where the magnetic fields are generating photons and, if conditions are right, trigger resonance.

If the waves had just the right frequency, they could amplify photons, which would cascade through a complex series of channels to produce even more photons directly from the gravitational field. This process would build on itself until it fell apart, releasing a burst of radiation.

The researchers think some strange astrophysical explosions, like gamma-ray bursts and fast radio bursts, may be driven by this gravity-to-light resonance. It depends on how well gravity can directly connect to light and produce photons — something we know is incredibly rare, but not impossible.

The researchers used known neutron star flashes to put limits on the connection between gravity and light, demonstrating how these powerful explosions serve as nature's own laboratory for testing some of the most unexpected interactions in the universe.

Black holes are massive, mostly invisible and so powerful not even light can escape them. So what would happen if one entered our solar system?

It depends on a lot of factors, including the size and distance of the black hole, experts told Live Science. But in many scenarios, not much would happen."They're not, per se, destructive," Karina Voggel, a postdoctoral researcher at the Strasbourg Astronomical Data Center in France, told Live Science. "It's just mass. Very compressed mass, but mass. It's not a cosmic vacuum cleaner."

If a black hole did wander into our solar system, the largest effects would be gravitational. And those effects would depend on the mass of the black hole.

"The black holes that we're confident exist are all much more massive than the sun," Robert McNees, an associate professor of physics at Loyola University Chicago, told Live Science. "And the sun's gravity dominates the behavior of bodies in the solar system all the way out to tremendous distances, and so anything more massive than the sun wandering into our neighborhood on scales much larger than the solar system would have noticeable effects."

The black holes we know of are either stellar-mass black holes — that is, black holes that are between a few to 100 times the mass of the sun — or supermassive black holes, which are 100,000 to billions of times the mass of the sun and are generally found at the centers of galaxies. But there are other possibilities, too.

Related: Could Earth be inside a black hole?

For example, it may be possible to create mini black holes in a particle accelerator. These would range from a single gram to around the mass of a human, and they'd be microscopic in size. "Nothing would happen if that just passed by, even if it passed by right in front of my face, because they evaporate in less than a second," Voggel said.

Then, there are primordial black holes, which may have formed as a result of mass and density fluctuations in the very early universe.

"It's not completely ruled out that there might be primordial black holes floating around in the universe with masses comparable to a few Earth masses," McNees said. "And something that's a few Earth masses wouldn't have the really drastic gravitational effects that an astrophysical black hole would have if it entered into our solar system."

Some astronomers have even considered whether Planet Nine, a hypothetical planet far out in the solar system that may be responsible for irregularities seen in the orbits of our known planets, could be a "baby" black hole in the mass range of what could be a primordial black hole.

But considering that this object, if it does exist, causes only tiny irregularities in the solar system's planets, a primordial black hole far out in the solar system wouldn't have very big effects.

However, if a stellar-mass or larger black hole were to whiz through our solar system, it could spell disaster, depending on how close and how fast it was. If it passed through the Oort cloud — the most distant region of the solar system — it could disturb the comets and asteroids that orbit there and cause them to head toward our own planet, Voggel said.

If the black hole got a bit closer — say, 100 astronomical units, or just beyond the orbit of Pluto — it could change the orbits of Uranus, Neptune and Pluto. "But likely Earth wouldn't be affected much yet," Voggel said.

Only once the black hole crossed between the orbits of Uranus and Pluto would Earth start to feel its effects. "If Uranus and Pluto were passed by really close, they might get attracted by the black hole dynamically so much that they're now in orbit around the black hole," Voggel said. And now our orbit would be altered."

That could change our seasons, plunging us into an ice age or raising temperatures so much that life on Earth went extinct.

Related: How does a black hole form?

If the black hole passed within Saturn's orbit, it would probably move us outside the habitable zone where liquid water can exist. If it passed within Jupiter's orbit, we'd begin to feel tidal effects as Earth began to orbit the black hole.

"If you go even closer … between us and Mars or something, then you resurface the Earth," Voggel said. "The tidal effects would heat it. You would have magma; the oceans would evaporate; definitely no life possible anymore."

But both Voggel and McNees said all of these scenarios are extremely unlikely. "We worry about asteroids occasionally hitting the Earth, but that's because there's lots of those," McNees said. "So even though the chances for each individual one are pretty small, the solar system is just peppered with them."

But black holes are much rarer in the universe, and the chance of one traveling through the solar system even a single time is low — much less that it would collide with something. "Obviously, you could make a summer blockbuster about it." McNees said.

This 2023 astronaut photo captures the striking beauty of a pair of perfectly aligned, arrow-shaped wave clouds rippling above the Crozet Islands — an archipelago of uninhabited French islands in the Southern Ocean, located roughly halfway between South Africa and Antarctica. 

Wave clouds are repeating bands of cloud and non-cloud that look like wispy streaks floating in the sky when viewed from above or below. These bands are created when stable air moves over raised land. The topography pushes the air upward before gravity pulls it back down, causing the air to oscillate up and down, according to NASA's Earth Observatory.

When the air rises, it cools down, and water vapor condenses into clouds. But when gravity pulls the air downward, it is compressed by increased atmospheric pressure, which heats it up via a process called adiabatic heating and forces any clouds in the air to evaporate, according to the National Oceanic and Atmospheric Administration.  

Wave clouds are also known as gravity waves. This should not be confused with "gravitational waves," which are ripples in space-time created by extreme cosmic entities such as black holes and colliding galaxies.

Related: 12 amazing images of Earth from space 

The wave cloud on the left of the image is formed by air passing over Mount Marion-Dufresne, a 3,576-foot-tall (1,090 meters) peak on East Island; the pattern on the right is caused by Mascarin Peak, a mountain on Possession Island that stands 3,064 feet (933 m) tall. The two islands are separated by around 10 miles (16 kilometers) of water.

Wave clouds are rare but can be found more often over large bodies of water because the air there is more stable, which makes it more prone to oscillating if it is disturbed, according to NASA. 

Similar wave clouds were spotted over the Crozet Islands in 2012 and 2014 but did not look as perfectly aligned as the pair in this photo.

The Crozet Islands are uninhabited apart from a small research station on Possession Island, from where scientists study the islands' more than 5 million penguins and other marine wildlife, according to NASA's Earth Observatory.

In the past, these islands have also temporarily been home to several groups of marooned sailors who survived being shipwrecked by the jagged rocks along the islands' steep cliffs, according to Encyclopedia Britannica. In 1876, half of the 88 passengers and crew onboard the shipwrecked vessel Strathmore survived on Possession Island for seven months before being rescued, according to The New Zealand Herald.

Astronomers may have found a rare "missing link" black hole in the Milky Way after spotting a group of improbably fast-moving stars at the heart of a nearby stellar cluster. If confirmed, the cosmic juggernaut, known as an intermediate-mass black hole (IMBH), would be the second-largest black hole ever found in our galaxy. 

IMBHs are an extremely rare subset of black holes that are larger than stellar-mass black holes but smaller than supermassive black holes. This means they can be anywhere between 100 and 100,000 times the mass of the sun, according to NASA.

In theory, IMBHs should be just as common as other black hole types. However, astronomers have struggled to locate potential IMBHs or confirm their existence — and they aren't sure why. As a result, IMBHs are often referred to as missing link black holes. While several promising candidates have been detected, none have been proven to be the real deal.

Now, in a recent study uploaded April 4 to the preprint server arXiv, researchers may have uncovered evidence of a large IMBH in the globular cluster Omega Centauri — a compact group of around 10 million stars in the Milky Way located around 17,000 light-years from Earth.

The team compared 500 photographs of Omega Centauri taken by the Hubble Space Telescope and mapped the movements of around 1.4 million stars at the cluster's center. This revealed at least seven stars that "should not be there," researchers wrote in a statement.

This is because these stars were spotted whizzing around fast enough to escape the cluster's gravity and fly off into intergalactic space. But despite this, the stars continue to orbit at breakneck speed near the cluster's center.

"The most likely explanation [for this] is that a very massive object is gravitationally pulling on these stars and keeping them close to the [cluster's] centre," study lead author Maximilian Häberle, a doctoral candidate at the Max Planck Institute for Astronomy (MPIA) in Germany, said in the statement. "The only object that can be so massive is a black hole, with a mass at least 8,200 times that of our sun."

Related: The most elusive black holes in the universe could lurk at the Milky Way's center

The Milky Way's second-biggest black hole?

Omega Centauri is an unusual entity: It is around 10 times larger than most other globular clusters and is surprisingly flat. It is so massive that you can even see it with the naked eye on dark, clear nights, when it takes up almost as much of the night sky as the moon when viewed from Earth.

Researchers suspect that the cluster likely used to be a dwarf galaxy that orbited the Milky Way, before being pulled into the galaxy's middle. As a result, scientists have often wondered if there could have been a sizable black hole at its heart.

Researchers first proposed the idea of an IMBH in Omega Centauri in 2008, when Hubble revealed how tightly the cluster's stars are bunched at its center. However, at the time, other researchers argued that this could be caused by a swarm of several smaller, stellar-mass black holes.

But the superfast stars highlighted in the new study indicate the existence of an IMBH, the study authors argue.

"This discovery is the most direct evidence so far of an IMBH in Omega Centauri," study co-author Nadine Neumayer, an astronomer at MPIA, said in the statement. If confirmed, it would be the Milky Way's second-largest known black hole behind Sagittarius A* — the supermassive black hole at the heart of our galaxy, she added. "This is exciting because there are only very few other black holes known with a similar mass."

However, the presence of an IMBH in Omega Centauri is not confirmed, and more data is needed to know for certain if it is really there. It is also unclear exactly how large the cosmic entity might be and where it is.

The researchers have been granted time in the future to use the powerful James Webb Space Telescope to peer deeper into the cluster, which means we may not have to wait too long for more evidence of the black hole's existence.

New research suggests that extreme objects known as "kugelblitze" — black holes formed solely from light — are impossible in our universe, challenging Einstein's theory of general relativity. The discovery places significant constraints on cosmological models and demonstrates how quantum mechanics and general relativity can be reconciled to address complex scientific questions.

Black holes — massive objects with such a strong gravitational pull that not even light can escape their grasp — are among the most intriguing and bizarre objects in the universe. Typically, they form from the collapse of massive stars at the ends of their life cycles, when the pressure from thermonuclear reactions in their cores can no longer counteract the force of gravity.

However, more exotic hypotheses exist regarding black hole formation. One such theory involves the creation of a "kugelblitz," German for "ball lightning." (The plural form is "kugelblitze.")

"A kugelblitz is a hypothetical black hole that, instead of forming from the collapse of 'ordinary matter' (whose main constituents are protons, neutrons, and electrons), is formed from concentrating humongous amounts of electromagnetic radiation, such as light," study co-author José Polo-Gómez, a physicist at the University of Waterloo and the Perimeter Institute for Theoretical Physics in Canada, told Live Science in an email.

"Even though light does not have mass, it does carry energy," Polo-Gómez said,  adding that, in Einstein's theory of general relativity, energy is responsible for creating curvatures in space-time that result in gravitational attractions. "Because of that, it is in principle possible for light to form black holes — if we concentrate enough of it in a small enough volume," he said.

Related: Tweak to Schrödinger's cat equation could unite Einstein's relativity and quantum mechanics, study hints

These principles hold true under classical general relativity, which does not account for quantum phenomena. To explore the potential impact of quantum effects on kugelblitz formation, Polo-Gómez and his colleagues examined the influence of the Schwinger effect.

"When there is an incredibly intense electromagnetic energy — for example, due to huge concentrations of light — part of this energy transforms into matter in the form of electron-positron pairs," lead study author Álvaro Álvarez-Domínguez of the Institute of Particle and Cosmos Physics (IPARCOS) at the Universidad Complutense de Madrid, told Live Science in an email. "This is a quantum effect called the Schwinger effect. It is also known as vacuum polarization."

In their study, which has been accepted for publication in the journal Physical Review Letters but has not been published yet, the team calculated the rate at which electron-positron pairs produced in an electromagnetic field would deplete energy. If this rate surpasses the replenishment rate of the electromagnetic field's energy in a given region, a kugelblitz cannot form.

The team found that, even under the most extreme circumstances, pure light could never reach the required energy threshold to form a black hole.

"What we prove is that kugelblitze are impossible to form by concentrating light, either artificially in the laboratory or in naturally occurring astrophysical scenarios," study co-author Luis J. Garay, also of IPARCOS, told Live Science. "For instance, even if we used the most intense lasers on Earth, we would still be more than 50 orders of magnitude away from the intensity required to create a kugelblitz."

This finding has profound theoretical implications, significantly constraining previously considered astrophysical and cosmological models that assume the existence of kugelblitze. It also dashes any hopes of experimentally studying black holes in laboratory settings by creating them through electromagnetic radiation.

Nonetheless, the study's positive outcome shows that quantum effects can be efficiently integrated into problems involving gravity, thus providing clear answers to actual scientific questions.

"From a theoretical viewpoint, this work showcases how quantum effects can play an important role in the understanding of the formation mechanisms and appearance of astrophysical objects," Polo-Gómez said.

Inspired by their findings, the researchers plan to continue exploring the influence of quantum effects on various gravitational phenomena, which have both practical and fundamental significance.

"Several of us are very interested in continuing the study of the gravitational properties of quantum matter, particularly in scenarios where this quantum matter violates traditional energy conditions," said Eduardo Martín-Martínez, also of the University of Waterloo and the Perimeter Institute. "This type of quantum matter can, in principle, give rise to exotic space-times, resulting in effects such as repulsive gravity or producing exotic solutions like the Alcubierre warp drive or traversable wormholes."

Deep in the outer reaches of the solar system — so far away from the known planets that the sun would barely be distinguishable from a nearby star — a massive, icy world may be lurking in the shadows, waiting to be discovered by humanity.

And the day that we finally find this elusive planet may be coming soon, thanks to a state-of-the-art telescope that will begin scanning the sky next year.

The solar system has eight official planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. But in recent years, astronomers have proposed that a ninth world, imaginatively nicknamed "Planet Nine," could be hiding in the far reaches of our cosmic neighborhood.

And no, we're not talking about Pluto, which was demoted from full planetary status to "dwarf planet" in 2006. Instead, scientists believe Planet Nine is a gas or ice giant billions of miles farther out than the rest of the planets. If it exists, it could also rewrite our understanding of the solar system's origins and evolution.

Related: How long would it take to reach Planet 9, if we ever find it?

Astronomers have predicted how big this hypothetical world could be, how far away it could lie and even where it should be in its orbit around the sun. Yet actually finding Planet Nine, sometimes called Planet X, has eluded scientists for nearly a decade.

But the hunt for the solar system's potential ninth planet may soon be coming to a close. With the opening of the groundbreaking Vera C. Rubin Observatory in 2025, we may either finally find Planet Nine within the next few years — or rule out the idea for good, experts told Live Science.

"It's really difficult to explain the solar system without Planet Nine," Mike Brown, an astronomer at Caltech who proposed the Planet Nine hypothesis along with a colleague, told Live Science. "But there's no way to be 100% sure [it exists] until you see it."

The Planet Nine hypothesis

The idea of a ninth planet in the solar system was first seeded by the discoveries of Uranus in 1781 and Neptune in 1846, more than 3,000 years after the other planets were first spotted by the Babylonians. These discoveries proved that the solar system was much larger than humanity had once thought and raised the possibility that other worlds were waiting to be discovered.

But other than the now-demoted Pluto, no full-fledged planets beyond Neptune or the Kuiper Belt — a massive ring of asteroids, comets and dwarf planets that orbit the sun beyond Neptune — have shown up since. And as astronomers mapped more of the outer solar system, it seemed increasingly unlikely that they were missing something as large as a planet.

However, a 2004 discovery changed that. Scientists found that Sedna — a potential dwarf planet located beyond the Kuiper Belt — had a weird orbit around the sun. Its unusual trajectory hinted that another large mass in the outer solar system was gravitationally pulling on the miniworld. But without more information, this hypothesis was hard to prove.

Then, in a 2014 study, astronomers announced that they had detected a smaller object in the Kuiper Belt, named 2012 VP113, with an eccentric orbit similar to Sedna's. The findings also hinted that more eccentric trans-Neptunian objects (TNOs) were waiting to be found.

These findings caught the attention of Brown and fellow Caltech astronomer Konstantin Batygin, who noticed that both Sedna and 2012 VP113 had the same "kink" in their orbits. This shared irregularity, which causes the objects to briefly dip below the known planets' plane of orbit, suggested that something — such as an asteroid clump, a dwarf planet or even a full-fledged world — was tugging on these objects.

Related: 8 strange objects that could be hiding in the outer solar system

"At the beginning, we didn't say there was a planet because we thought that was a ridiculous thing for there to be," Brown, who also co-discovered Sedna and was instrumental in Pluto's planetary demotion, told Live Science. "But we tried a lot of different things to explain what we were seeing, and nothing else worked."

Even after the pair realized a ninth planet was possible, they decided to sit on their findings until they could come up with another, less-controversial explanation. However, they then found four more TNOs with matching, misshaped orbits, which suddenly made a missing planet look like the most logical explanation.

At the time, the duo calculated that there was just a 2% chance that all six TNOs they had studied shared their orbital oddities thanks to random chance. "And as soon as you see that, you're like, 'Oh crap, there's a planet there,'" Brown said.

So, in 2016, Brown and Batygin published their "Planet Nine hypothesis," which has captured the public's imagination ever since.

Filling in the gaps

Since 2016, Brown, Batygin and others have continued the hunt for Planet Nine. Although they have not found it yet, they have discovered even more eccentric TNOs — bringing the total to 13 and further strengthening the case for Planet Nine.

These discoveries also constrain Planet Nine's potential size, its distance from the sun and its orbital trajectory through the solar system.

"Our best estimates are that it's about seven times more massive than Earth," or anywhere between five and 10 times the mass of our planet, Brown said. This would make it the fifth-most-massive planet in the solar system, behind Jupiter, Saturn, Neptune and Uranus, he added.

Planet Nine's composition is probably "most like Neptune," due to its distance from the sun, Brown said. "That would put its diameter at something like two times the width of Earth," he added. Some scientists have also suggested Planet Nine could be surrounded by moons, just as the hefty gas giants are.

Related: Where does the solar system end?

If it exists, Planet Nine is likely around 500 astronomical units away from the sun, on average — meaning it's 500 times farther from the sun than Earth is. This may sound distant, but similarly sized exoplanets have been discovered orbiting alien stars at equally massive distances, showing that this is possible.

This far out, it could take between 5,000 and 10,000 years for Planet Nine to complete a single trip around the sun. Its orbit is probably highly elliptical, so its distance from the sun would vary widely over time. It also likely does not orbit on the same plane as the rest of the planets, which makes it even trickier to find.

Planet Nine's unusual orbit and extreme distance from the sun also raise the possibility that it could be a rogue planet — an interstellar world that was captured by the sun after being ejected from its star system. However, Brown and Batygin believe Planet Nine likely formed alongside the other planets in the solar system.

Is it really out there?

Many astronomers are cautiously optimistic about Planet Nine's existence.

It is "quite likely" that Planet Nine exists, Alessandro Morbidelli, an astronomer at the Côte d'Azur Observatory in France, told Live Science in an email. "There are several indirect lines of evidence in favor of its existence," added Morbidelli, who reviewed Brown and Batygin's 2016 paper before its publication.

David Rabinowitz, an astrophysicist at Yale University, agreed that something is likely out there, and Planet Nine is "the best explanation so far," he told Live Science. The discoveries of additional eccentric TNOs since Planet Nine was first proposed have maintained confidence in this theory, he said.

But not everyone is convinced that Planet Nine is real.

"It's been a roller-coaster! I've gone from thinking it was 90% there to 10% and all around," Sean Raymond, a researcher at the Bordeaux Astrophysics Laboratory in France, told Live Science in an email. "I'm rooting for it to be there, but I'm still agnostic on whether I believe it's there."

Related: What's the maximum number of planets that could orbit the sun?

Doubts about Planet Nine are rooted in alternative potential explanations for the strange behavior seen among TNOs. For instance, the gravitational anomalies Brown and Batygin flagged might be caused by a baby black hole, an invisible giant disk of dust or a historic close encounter with a rogue planet. Alternatively, the TNOs may be evidence that our model of gravity needs to be tweaked.

Others believe the apparent TNO kinks are simply an "observational bias," because it is easier to spot TNOs that are closer to Earth than more distant ones, Samantha Lawler, an astronomer at the University of Regina in Canada and a prominent critic of the Planet Nine hypothesis, told Live Science in an email.

"I believe that there are a lot of really interesting bodies left to discover in the outer solar system," Lawler said. But Planet Nine is not one of them, she added.

However, Brown and Batygin discount the notion that observational bias is creating the illusion of a ninth planet.

"I am as confident as you can possibly be [that Planet Nine exists] until you actually find it," Brown said.

Why haven't we found it?

So, if Planet Nine does exist, why haven't we spotted it yet?

The short answer is that the hidden planet is "very, very far away," Brown said. Light reflected off the planet would be very dim by the time it traveled across most of the solar system (twice), which makes it all but impossible to see.

Initially, the researchers also had no idea where the planet was along its predicted orbital path. That means they've had to study a huge region of the sky to look for this faint body — akin to trying "to find a single white whale in an ocean," Brown said.

Related: How many times has Earth (and the other planets) orbited the sun?

So far, researchers have analyzed thousands of images from multiple sky surveys along Planet Nine's proposed orbital pathway, looking for objects that move over time, Brown said.

Unfortunately, the night sky is teeming with bright, moving objects, such as comets, so the researchers have to sort through "a lot of garbage" to find the planet, Brown added.

In their most recent work, Brown and Batygin analyzed data from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) at Haleakala Observatory in Hawaii and confidently ruled out around 78% of the suspected orbital pathway as possible hiding places for the planet.

This narrowed down Planet Nine's location to somewhere in the most distant 22% of its orbital pathway. Unfortunately, telescopes like Pan-STARRS are not powerful enough to properly search this space.

When will we find it?

If Planet Nine is hiding in the most distant reaches of its orbit, we'll need a telescope powerful enough to spot it.

Brown and Batygin have already begun analyzing data from Japan's Subaru Telescope in Hawaii, which has a better chance of finding the planet than Pan-STARRS does. But if this survey fails to complete the job, they will turn to the forthcoming Vera C. Rubin Observatory, which is currently under construction in Chile.

This ground-based telescope, which will be equipped with the world's largest digital camera, will let researchers peer farther into the solar system than any of its predecessors allowed, similar to how the James Webb Space Telescope has enabled researchers to look farther across the observable universe than ever before.

The observatory is currently slated to open in late 2025.

Related: 5 space discoveries that scientists are struggling to explain

With the help of the state-of-the-art telescope, Planet Nine could be found within the next two years, Brown said. However, he also joked that he has been saying the same thing every year since 2016.

Raymond and Rabinowitz both agreed that Planet Nine could be found within a year after the Rubin Observatory comes online. If the telescope cannot find the planet within the first few years, however, "then the hypothesis is pretty much dead," Raymond said.

But Morbidelli and Rabinowitz pointed out that even if the telescope does not find the planet immediately, it could still identify more TNOs, which would help show if the theory is viable.

Why does Planet Nine matter?

Although scientists remain divided over Planet Nine's existence, one thing they all agree on is that actually finding the elusive world would likely be the biggest solar system discovery of the century.

It would be a "remarkable" find, Raymond said. It would also be "huge" for our understanding of the solar system's origins and evolution, he added.

Observing the planet could also teach us more about the formation and evolution of giant planets, Morbidelli said. This would not only help us learn more about planets in the solar system but also shed light on thousands of giant exoplanets around distant stars.

If space agencies like NASA send probes to fly close to the planet, they could also reveal more clues about the solar system's past.

"It'll have many secrets that will be unlocked by studying it in detail," Brown said.

A rocket launch. Our nearest stellar neighbor. A Netflix show. All of these things have something in common: They must contend with the "three-body problem." But exactly what is this thorny physics conundrum?

The three-body problem describes a system containing three bodies that exert gravitational forces on one another. While it may sound simple, it's a notoriously tricky problem and "the first real worry of Newton," Billy Quarles, a planetary dynamicist at Valdosta State University in Georgia, told Live Science.

In a system of only two bodies, like a planet and a star, calculating how they'll move around each other is fairly straightforward: Most of the time, those two objects will orbit roughly in a circle around their center of mass, and they'll come back to where they started each time. But add a third body, like another star, and things get a lot more complicated. The third body attracts the two orbiting each other, pulling them out of their predictable paths.

The motion of the three bodies depends on their starting state — their positions, velocities and masses. If even one of those variables changes, the resulting motion could be completely different. 

"I think of it as if you're walking on a mountain ridge," Shane Ross, an applied mathematician at Virginia Tech, told Live Science. "With one small change, you could either fall to the right or you could fall to the left. Those are two very close initial positions, and they could lead to very different states."  

There aren't enough constraints on the motions of the bodies to solve the three-body problem with equations, Ross said. 

Related: Cosmic 'superbubbles' might be throwing entire galaxies into chaos, theoretical study hints

But some solutions to the three-body problem have been found. For example, if the starting conditions are just right, three bodies of equal mass could chase one another in a figure-eight pattern. Such tidy solutions are the exception, however, when it comes to real systems in space.

Certain conditions can make the three-body problem easier to parse. Consider Tatooine, Luke Skywalker's fictional home world from "Star Wars" — a single planet orbiting two suns. Those two stars and the planet make up a three-body system. But if the planet is far enough away and orbiting both stars together, it's possible to simplify the problem. 

"When it's the Tatooine case, as long as you're far enough away from the central binary, then you think of this object as just being a really fat star," Quarles said. The planet doesn't exert much force on the stars because it's so much less massive, so the system becomes similar to the more easily solvable two-body problem. So far, scientists have found more than a dozen Tatooine-like exoplanets, Quarles told Live Science.

But often, the orbits of the three bodies never truly stabilize, and the three-body problem gets "solved" with a bang. The gravitational forces could cause two of the three bodies to collide, or they could fling one of the bodies out of the system forever — a possible source of "rogue planets" that don't orbit any star, Quarles said. In fact, three-body chaos may be so common in space that scientists estimate there may be 20 times as many rogue planets as there are stars in our galaxy.

When all else fails, scientists can use computers to approximate the motions of bodies in an individual three-body system. That makes it possible to predict the motion of a rocket launched into orbit around Earth, or to predict the fate of a planet in a system with multiple stars.

With all this tumult, you might wonder if anything could survive on a planet like the one featured in Netflix's "3 Body Problem," which — spoiler alert — is trapped in a chaotic orbit around three stars in the Alpha Centauri system, our solar system's nearest neighbor. 

"I don't think in that type of situation, that's a stable environment for life to evolve," Ross said. That's one aspect of the show that remains firmly in the realm of science fiction.

A variation on the theory of quantum gravity — the unification of quantum mechanics and Einstein's general relativity — could help solve one of the biggest puzzles in cosmology, new research suggests.

For nearly a century, scientists have known that the universe is expanding. But in recent decades, physicists have found that different types of measurements of the expansion rate — called the Hubble parameter — produce puzzling inconsistencies.

To resolve this paradox, a new study suggests incorporating quantum effects into one prominent theory used to determine the expansion rate.

"We tried to resolve and explain the mismatch between the values of the Hubble parameter from two different prominent types of observations," study co-author P.K. Suresh, a professor of physics at the University of Hyderabad in India, told Live Science via email.

An expanding problem

The universe's expansion was first identified by Edwin Hubble in 1929. His observations with the largest telescope of that time revealed that galaxies farther from us appear to move away at faster speeds. Although Hubble initially overestimated the expansion rate, subsequent measurements have refined our understanding, establishing the current Hubble parameter as highly reliable.

Later in the 20th century, astrophysicists introduced a novel technique to gauge the expansion rate by examining the cosmic microwave background, the pervasive "afterglow" of the Big Bang.

However, a serious problem arose with these two types of measurements. Specifically, the newer method produced a Hubble parameter value almost 10% lower than the one deduced from the astronomical observations of distant cosmic objects. Such discrepancies between different measurements, called the Hubble tension, signal potential flaws in our understanding of the universe's evolution.

Related: Newfound 'glitch' in Einstein's relativity could rewrite the rules of the universe, study suggests

In a study published in the journal Classical and Quantum Gravity, Suresh and his colleague from the University of Hyderabad, B. Anupama, proposed a solution to align these disparate results. They underscored that physicists infer the Hubble parameter indirectly, employing our universe's evolutionary model based on Einstein's theory of general relativity.

The team argued for revising this theory to incorporate quantum effects. These effects, intrinsic to fundamental interactions, encompass random field fluctuations and the spontaneous creation of particles from the vacuum of space.

Despite scientists' ability to integrate quantum effects into theories of other fields, quantum gravity remains elusive, making detailed calculations extremely difficult or even impossible. To make matters worse, experimental studies of these effects require reaching temperatures or energies many orders of magnitude higher than those achievable in a lab.

Acknowledging these challenges, Suresh and Anupama focused on broad quantum-gravity effects common to many proposed theories.

"Our equation doesn't need to account for everything, but that does not prevent us from testing quantum gravity or its effects experimentally," Suresh said.

Their theoretical exploration revealed that accounting for quantum effects when describing the gravitational interactions in the earliest stage of the universe's expansion, called cosmic inflation, could indeed alter the theory's predictions regarding the properties of the microwave background at present, making the two types of Hubble parameter measurements consistent.

Of course, final conclusions can be drawn only when a full-fledged theory of quantum gravity is known, but even the preliminary findings are encouraging. Moreover, the link between the cosmic microwave background and quantum gravitational effects opens the way to experimentally studying these effects in the near future, the team said.

"Quantum gravity is supposed to play a role in the dynamics of the early universe; thus its effect can be observed through measurements of the properties of the cosmic microwave background," Suresh said.

"Some of the future missions devoted to studying this electromagnetic background are highly probable and promising to test quantum gravity. … It provides a promising suggestion to resolve and validate the inflationary models of cosmology in conjunction with quantum gravity."

Additionally, the authors posit that quantum gravitational phenomena in the early universe might have shaped the properties of gravitational waves emitted during that period. Detecting these waves with future gravitational-wave observatories could further illuminate quantum gravitational characteristics.

"Gravitational waves from various astrophysical sources have only been observed so far, but gravitational waves from the early universe have not yet been detected," Suresh said. "Hopefully, our work will help in identifying the correct inflationary model and detecting the primordial gravitational waves with quantum gravity features."

It seems like everything orbits something in space. Moons orbit planets. Planets orbit stars. Stars orbit the centers of galaxies. But beyond that, things get a little harder to visualize. Do galaxies — and, specifically, the Milky Way — orbit anything?

To answer that, we first need to know how orbits work. Consider two objects orbiting each other. Those two bodies exert a gravitational pull on each other, keeping them bound together. The objects orbit their common center of mass — if you could shrink the system, the center of mass would be the point where you could balance it on your finger. But in the case of the solar system, or Earth and the moon, one of the objects is much larger than the other. The center of mass ends up lying inside the larger body, so the larger object doesn't move much and the smaller object moves on a roughly circular path around the bigger one.

At larger scales, things get a little more complicated. Our galaxy is part of a collection of galaxies called the Local Group, which includes the Milky Way; the Andromeda galaxy; a smaller spiral galaxy called Triangulum; and several dwarf galaxies, including the Large and Small Magellanic Clouds. The Milky Way and Andromeda are the two largest objects in the Local Group. Because their masses are comparable, the center of mass lies between the two galaxies, said Sangmo Tony Sohn, an astronomer at the Space Telescope Science Institute in Maryland. There's nothing significantly larger than either galaxy nearby, so the two end up orbiting each other.

But the Milky Way's orbit isn't circular or elliptical like the orbits of planets around the sun. "It's going to be weird to say if the Milky Way is orbiting around something, because that kind of implies that there's a bigger object," Sohn told Live Science. "But that's not the concept here."

Related: Why aren't all orbits circular? 

Instead, both the Milky Way and Andromeda are on mostly radial orbits. "Imagine the gravity of two things pulling on each other, and they're not moving in any way other than the gravitational pull. They will just move directly on the line towards each other. That's a purely radial orbit," said Chris Mihos, an astronomer at Case Western Reserve University in Ohio. The Milky Way's orbit isn't perfectly radial because there's a bit of sideways motion between the two galaxies, Mihos told Live Science.

Their mostly radial orbits toward each other mean that the Milky Way and Andromeda will eventually collide, some 4.5 billion years from now. Individual stars likely won't crash into each other because of the huge distances between them, so the galaxies will pass through each other and separate again — but not for long. 

"The galaxies [will] then turn around and come back together. And, over the course of hundreds of millions or billions of years, they'll actually merge together into one big galaxy," Mihos said.

The gravitational interactions will likely jostle the stars in both galaxies enough to make the combined galaxy an elliptical galaxy rather than a spiral one like the Milky Way and Andromeda. The merger could also heat the gas along each galaxy's spiral arms enough to form new stars, Sohn said.

Orbits on scales larger than galaxy groups are even less defined, but "we certainly know the Local Group is moving," Mihos said. The Local Group is being pulled toward the Virgo Cluster, which contains several hundred galaxies and lies about 65 million light-years away. But the Local Group will never make it there, Mihos said, because the expansion of the universe is pulling the Milky Way away faster than the gravitational pull of the Virgo Cluster is drawing it in.

Some people see a face in the moon; others see a rabbit or a toad. But regardless of what you see on the lunar surface, we all view the same side of our natural satellite. So why don't we ever see the far side of the moon?

From Earth, it appears as if the moon doesn't rotate at all, but it does spin on its axis, just like Earth does. However, the moon is tidally locked to our planet. That means it takes just as long for the moon to rotate about its axis as it does to orbit Earth — roughly one month.

Tidal locking occurs thanks to gravitational attraction between two celestial bodies. The attraction between the moon and Earth distorts both bodies and stretches them slightly toward each other, into a shape resembling an American football, said Robert Tyler, a physical oceanographer at NASA's Goddard Space Flight Center. "That would be the shape if all the fluids and solids could respond instantaneously," Tyler told Live Science.

But the fluids and solids that make up both the moon and Earth can't respond instantaneously. When the two bodies pull on each other, they create friction that slows the rotation of both objects. 

Related: Will Earth ever lose its moon?

For example, "the moon is pulling on the ocean, so part of the ocean is trying to propagate in a way that would, ideally, create a bulge that was staying right under the moon," Tyler said. But "the tides are dragging across the seafloor and trying to get around continents." It takes time and energy to move the tidal bulge — the end of the football — in response to the moon's motion around our planet.

The same thing happens as rocks on the moon shift in response to Earth's pull. "Rocks are not elastic. When they get flexed, the energy is going to be used up," Matija Ćuk, an orbital dynamicist at the SETI Institute, told Live Science. "Energy has to come from somewhere, so it comes from the rotation of the body." The moon's rotation relative to Earth slowed down, until it eventually reached zero.

The moon is also slowing Earth's rotation. Half a billion years ago, Earth might have had a 21-hour day, Tyler said. If given enough time, the moon could slow our planet's rotation enough that it could become tidally locked to the moon, and only one side of our planet would ever see the moon. But that wouldn't happen for another 50 billion years — long after the death of the sun about 5 billion years from now.

Although we'll never see the far side of the moon directly from Earth, spacecraft have photographed it. Soviet spacecraft Luna 3 captured the first images of the far side in 1959. Since then, several other spacecraft have snapped photos of the lunar far side, including NASA's Lunar Reconnaissance Orbiter and China's Chang'e 4, the first spacecraft to land on the far side of the moon.

The images show that the moon's far side is covered in craters and has fewer large, dark spots — called maria — than the near side. With fewer maria, it's harder to pick out shapes like a face or a rabbit in the lunar far side, but there's still plenty to see.

When cats fall, they flip themselves rightside-up with seemingly little effort — which has perplexed scientists for decades. Our feline friends appear to defy the laws of physics by reconfiguring themselves mid-air without intervention from another force. So how do they do it?

Several factors affect how a cat is able to land on all fours, but simply put, there are two main forces at play: physics and neurology.

"One of the reasons that physicists were surprised that cats could rotate to always land on their feet is the conservation of angular momentum," Greg Gbur, a physicist at the University of North Carolina at Charlotte, told Live Science.

Essentially, this means that if something twists clockwise, something else has to twist counterclockwise. Imagine a cat falling from a stationary upside-down position. By bending at the waist, the cat can twist the front half of its body in one direction and the back half in the opposite direction. By the time it unfolds at the waist, the cat is rightside-up. Gbur dubbed this the "bend and twist" model in his book, "Falling Felines and Fundamental Physics" (Yale University Press, 2019). 

But there are other techniques that can help cats right themselves mid-air, and it's likely they employ more than one. In the "tuck and turn" method, a cat extends its front legs and tucks in its back legs, giving the backside a lower moment of inertia, meaning a lower amount of resistance to changes in rotational motion. Then, it does the opposite, tucking in its front legs and extending the hind legs. It has an effect similar to a spinning figure skater: Extending the arms out wide increases the moment of inertia, while drawing the arms close to the body decreases it. This inversely correlates with speed. As inertia goes up, speed goes down, and vice-versa. Cats can also use what Gbur calls "the propeller tail," which works similarly. As the tail spins in one direction, the body can rotate in the other. 

Related: Can cats really see in the dark?

Such contortions are possible thanks to cats' flexible lumbar region — the space between their pelvis and ribcage — John Hutchinson, a professor of evolutionary biomechanics at the Royal Veterinary College at the University of London, told Live Science. Cats have skinny vertebrae, which makes them more flexible than other four-legged vertebrates. 

This ability to land on all fours also has a neuroanatomical explanation: the righting reflex or response. Unlike a simple reflex, like a knee jerk, righting in animals is a complex reflex, meaning it's tied to the conscious brain, Gbur said. 

Righting is a behavioral response to gravity that relies on the vestibular system, which controls balance. Semicircular canals and sensors called otoliths in a cat's inner ear detect changes in its acceleration and position relative to the ground, prompting its muscles to move in a way that helps it land on its paws. Interestingly, experiments in the 1950s showed that this righting response isn't simply ingrained in cats from birth. When adult cats and newborn kittens were flown on jets in zero-gravity conditions, the kittens weren't able to right themselves but the adult cats did. Because otoliths are made of dense bone, it's possible that they need to develop fully before an animal can properly right itself, Hutchinson noted, but scientists aren't completely sure. 

However, the height of the fall also matters. Studies have shown that cats land with less impact when they fall from higher than the seventh floor, for example, than lower heights. This is mostly because of air resistance, which slows the cat's body enough to allow it to turn. Further, cats are unlikely to turn properly from less than 1 or 2 feet (0.3 to 0.6 meters), according to research published in Annals of Improbable Research. (It is not safe for cat owners to purposely drop their cats to test their righting response; they can get hurt, especially if they have a vestibular disease.)

Righting responses aren't unique to domestic cats. Many wild cats exhibit the same behavior, as do rats and rabbits. For cats, the most plausible evolutionary explanation is their tendency to climb trees and other spots high off the ground. For rabbits, predation is a likely evolutionary force. As a hawk swoops toward a rabbit horizontally, for example, the rabbit will jump up vertically, causing the hawk to fumble for the rabbit, knocking it off its linear path. Thus, the rabbit evolved a way to land upright and unharmed.

For a behavior scientists once thought of only as an instinct for many years, Gbur said that multiple techniques can be the answer.

"We have it sort of built into our DNA to look for the simplest solution to a problem, but nature just cares about the most effective solution," he said. "Anything that gets it on its feet faster is better."

The solar system is an enormous place. Our cosmic neighborhood includes eight planets, around half a dozen dwarf planets, several hundred moons and millions of asteroids and comets, all spinning around the sun — and in many cases each other —at speeds of thousands of miles per hour, like a giant top.

But where does it end? Well, the answer may depend on whom you ask and how they define the solar system.

There are not one, but three potential boundaries to the solar system, according to NASA: the Kuiper Belt, the ring of rocky bodies beyond the orbit of Neptune; the heliopause, the edge of the sun's magnetic field; and the Oort Cloud, a distant reservoir of comets that are barely visible from Earth. 

The arguments for each boundary "all have merit," which makes choosing between them complicated, Dan Reisenfeld, a researcher at Los Alamos National Laboratory in New Mexico, told Live Science in an email. 

But there is one that most astronomers most commonly agree upon.

Related: Have all 8 planets ever aligned? 

Kuiper Belt

The Kuiper Belt stretches between 30 and 50 astronomical units (AU) away from the sun, according to NASA. (One astronomical unit is equal to the distance between Earth and the sun.) 

This region is filled with asteroids and dwarf planets, such as Pluto, that have been ejected from the inner solar system by one-sided gravitational tugs-of-war with the planets.

Some astronomers argue that the Kuiper Belt should be considered the edge of the solar system because it loosely represents the edge of where the sun's protoplanetary disk — the swirling ring of gas and dust that later became the planets, moons and asteroids — would have been.

"If one narrowly defines the solar system as just the sun and its planetary bodies, then the edge of the Kuiper Belt can be considered to be the edge of the solar system," Reisenfeld said. 

But this definition of the solar system is considered to be far too simple by some astronomers, such as Caltech's Mike Brown. 

"It's not really true," Brown told Live Science in an email. "Things have moved around a lot — mostly outward — since the planets were formed." This means the Kuiper Belt does not contain all of the solar system's "stuff," such as the elusive, hypothetical Planet Nine, which (if it exists) likely lies far beyond the Kuiper Belt.

In October 2023, the discovery of a dozen new objects beyond the Kuiper Belt also hinted that there may be a "second Kuiper Belt" lurking even further out. 

The uncertainty around this region's own outer edge therefore makes it an unreliable boundary for the solar system as a whole, some researchers argue.

Heliopause

The heliopause is the outer edge of the sun's magnetic influence, known as the heliosphere. At this point, the stream of charged particles emitted by the sun, known as the solar wind, becomes too weak to repel the oncoming stream of radiation from stars and other cosmic entities in the Milky Way. 

"Because the plasma inside the heliopause is of solar origin, and the plasma outside the heliopause is of interstellar origin, some people consider the heliopause to be the boundary of the solar system," Reisenfeld said. As a result, the space beyond the heliopause is also often referred to as "interstellar space," or the space between stars, he added.

Two spacecraft have traveled beyond the heliopause: Voyager 1, which made the crossing in 2012, and Voyager 2, which crossed over in 2018. As the Voyager probes crossed the heliopause, they quickly detected changes in the types and levels of magnetism and radiation hitting them, signifying that they had crossed some kind of border, Brown said.

However, despite its name, the heliosphere is not a perfect sphere. Instead, it is more of an oblong blob because most of the interstellar plasma bombarding the solar system hits us from one direction, which creates a bow shock — a rounded shock wave that deflects incoming radiation around the rest of the solar system. The bow shock is located around 120 AU from the sun, and creates a long tail that stretches at least 350 AU from the sun in the opposite direction.

Using the heliopause to delineate the solar system therefore leaves us with a lopsided neighborhood, which goes against some researchers perceptions of planetary systems.

Oort Cloud

The Oort Cloud is the furthest and most expansive potential solar system boundary, extending up to around 100,000 AU from the sun, according to NASA. 

"People who define the solar system as everything that is gravitationally bound to the Sun consider the edge of the Oort cloud to be the edge of the solar system," Reisenfeld said.

For some researchers, this is the clear choice for a solar system boundary because in theory, a planetary system consists of all objects orbiting a star.

"I don't understand how anyone considers anything other than the Oort Cloud to be the edge of the solar system," Sean Raymond, an astronomer at the Bordeaux Astrophysics Laboratory in France, told Live Science in an email. "Any other definition seems ludicrous. It is literally the edge of where something can orbit the Sun."

However, other researchers believe that because the Oort Cloud is located in interstellar space, it lies beyond the solar system even if it is bound to our home star. 

There is also a large amount of uncertainty about where the Oort Cloud actually ends, which some would argue makes it just as unreliable a border as the Kuiper Belt.

Which boundary is best? 

Out of the three possible boundaries, the heliopause is the one that is most often used by researchers, and by NASA, to define the solar system's edge. This is because it is the easiest to pin down and because the magnetic properties on either side of it are significantly different.

"I would argue for the heliopause to be the boundary because it really is a boundary," Reisenfeld said. "Once you've passed it, you know it."

But that doesn't mean that everything beyond the heliopause should be considered an interstellar object, such as the enormous space rock 'Oumuamua, Reisenfeld added. "The Oort Cloud was originally part of the same stuff that the planets were formed from, so it is composed of solar system material, not interstellar material," he said.

But while some researchers are happy to pick a side in this argument, others see no reason why the solar system cannot have multiple boundaries.

"I would say that there is no actual debate," Brown said. "There are just different ways to define it depending on what is important for the question you are trying to answer."

In 2009, theoretical physicist Erik Verlinde proposed a radical reformulation of gravity. In his theory, gravity is not a fundamental force but rather a manifestation of deeper hidden processes. But in the 15 years since then, there hasn't been much experimental support for the idea. So where do we go next?

Emergence is common throughout physics. The property of temperature, for example, isn't an intrinsic property of gases. Instead, it's the emergent result of countless microscopic collisions. We have the tools to match those microscopic collisions to temperature; indeed, there is an entire branch of physics, known as statistical mechanics, that makes these connections known.

In other areas, the connections between microscopic behaviors and emergent properties aren't so clear. For example, while we understand the simple mechanisms behind superconductivity, we do not know how microscopic interactions lead to the emergence of high-temperature superconductors.

Verlinde's theory is based on what Stephen Hawking and Jacob Bekenstein observed in the 1970s: Many properties of black holes can be expressed in terms of the laws of thermodynamics. However, the laws of thermodynamics are themselves emergent from microscopic processes. To Verlinde, this was more than a mere coincidence and indicated that what we perceive as gravity may be emerging from some deeper physical process.

Related: There may be a 'dark mirror' universe within ours where atoms failed to form, new study suggests

In 2009, he published the first version of his theory. Crucially, we do not need to know what those deeper processes are, since we already have the tool kit — statistical mechanics — for describing emergent properties. So Verlinde applied these techniques to gravity and arrived at an alternate formulation of gravity. And because gravity is also tied to our concepts of motion, inertia, space and time, this means our entire universe is also emergent from those same deeper processes. 

At first, not much came of this; rewriting a known law of physics, while interesting, doesn't necessarily provide deeper insights. But in 2016, Verlinde expanded his theory by discovering that a universe containing dark energy naturally leads to a new emergent property of space, thus allowing it to push inward on itself in regions of low density.

This discovery led to a flurry of excitement, as it provided an alternative explanation for dark matter. Currently, astronomers believe that dark matter is a mysterious, invisible substance that makes up the bulk of all the mass of every galaxy. While that hypothesis has been able to explain a vast wealth of observations, from the rotation rates of stars within galaxies to the evolution of the largest structures in the cosmos, we have yet to identify the mysterious particle.

In Verlinde's picture of emergent gravity, as soon as you enter low-density regions — basically, anything outside the solar system — gravity behaves differently than we would expect from Einstein's theory of general relativity. At large scales, there is a natural inward pull to space itself, which forces matter to clump up more tightly than it otherwise would.

This idea was exciting because it allowed astronomers to find a way to test this new theory. Observers could take this new theory of gravity and put it in models of galaxy structure and evolution to find differences between it and models of dark matter.

Over the years, however, the experimental results have been mixed. Some early tests favored emergent gravity over dark matter when it came to the rotation rates of stars. But more recent observations haven't found an advantage. And dark matter can also explain much more than galaxy rotation rates; tests within galaxy clusters have found emergent gravity coming up short.

This isn't the end of emergent gravity. The idea is still new and requires a lot of assumptions in its calculations to make it work. Without a fully realized theory, it's hard to tell if the predictions it makes for the behavior of galaxies and clusters accurately represent what emergent gravity would tell us. And astronomers are still trying to develop more stringent tests, like using data from the cosmic microwave background, to really put the theory through its paces.

Emergent gravity remains an intriguing idea. If it's correct, we would have to radically reshape our understanding of the natural world and see gravity and motion — and even more fundamental concepts, like time and space — through a lens of emergence from deeper, more complicated interactions. But for right now, it remains simply an intriguing idea. Only time and extensive observational testing will tell us if we're on the right track.

Originally posted on Space.com.