A mysterious wavelength of light is missing from the dwarf planet Pluto and Saturn's supersized moon Titan, new James Webb Space Telescope (JWST) observations show. The surprising discovery hints that these worlds harbor an unknown molecule that has not been seen in any other solar system world or exoplanet so far.

Every element or molecule in the universe absorbs unique wavelengths of electromagnetic radiation. Therefore, one of the main ways astronomers study distant worlds — both inside and outside the solar system — is by closely examining the light that reflects off them and searching for dark "absorption lines" that correspond to the wavelengths of known chemical compounds.

For example, molecular oxygen absorbs light at 230 nanometers, so if the electromagnetic spectrum of a faraway exoplanet has an absorption line at this frequency, researchers can be confident that its atmosphere contains oxygen, according to a 2021 study.

JWST has proved to be exceptionally good at capturing electromagnetic spectra and identifying specific chemicals in exoplanet atmospheres, around distant stars and within primitive galaxies. It has even identified a molecule on one alien world that could point to extraterrestrial life.

In a new study, uploaded June 11 to the preprint server arXiv, researchers analyzed JWST data from Pluto and Titan, focusing on very small wavelengths that have been relatively unexplored until now. This revealed a specific absorption line at around 5.11 micrometers in both worlds' spectra. (These findings have not been published in a peer-reviewed journal yet.)

The team pored through similar studies on other planetary spectra but "did not find any band referenced in these publications that corresponds to the location of the observed absorption in Titan and Pluto," the researchers wrote in the paper.

A molecular mystery

The discovery is even stranger because there are very few similarities between Pluto and Titan that could explain why they share a molecule not found anywhere else.

Titan is the largest of Saturn's many moons and is even larger than Mercury. It is also the only solar system world, other than Earth, that is known to have liquid rivers and oceans on its surface. Pluto, on the other hand, is a completely frozen world that's around half the size of Titan and roughly four times farther from the sun than Saturn's satellite is.

Both worlds do have similar atmospheres that are rich in methane and nitrogen. However, the researchers are confident that the molecule responsible for the 5.11-micrometer absorption line is located on both worlds' surfaces, not in their atmospheres.

Pluto's absorption line is around three times thicker than Titan's, meaning the mystery molecule is likely much more abundant on the dwarf planet. But on Titan, the molecule seems to be unevenly distributed, with a stronger absorption line on its trailing side — the hemisphere opposite to its forward momentum around Saturn — than on its leading side.

The researchers proposed that it could be benzene — a ring-shaped hydrocarbon — mixed with an unknown molecule, or some form of acetylene or ketene ice. However, much more work is needed to prove that any of these potential candidates are responsible for absorbing this specific wavelength, they wrote.

NASA's Dragonfly spacecraft, which is set to launch no earlier than 2028 and fly through Titan's atmosphere in 2034, could eventually shed more light on the situation. The helicopter-like craft's onboard spectrograph could identify the mystery molecule on Saturn's moon, which would also help reveal if it is viable on Pluto, the researchers suggested. But in the meantime, we'll have to wait to unravel this curious cosmic conundrum.

See how well you know our planetary neighborhood with our solar system quiz!

Astronomers have detected an atmosphere that shouldn't exist on an icy object beyond the orbit of Pluto — sparking calls for follow-up observations.

Japanese astronomers found evidence for a thin atmosphere surrounding the body, which is located within the Kuiper Belt in the cold outer reaches of the solar system, according to a new study published May 4 in the journal Nature Astronomy.

The object, known as (612533) 2002 XV₉₃, is supposed to be too small and too cold to sustain an atmosphere. At about 311 miles (500 kilometers) across — a little wider than the Grand Canyon is long — the object is more than four times smaller than Pluto, which was thought to be the only body beyond Neptune with an atmosphere in our solar system.

The new observations challenge assumptions about which objects can sustain atmospheres in our solar system. However, these initial findings must be verified by outside researchers, with some experts keen to make follow-up observations with the James Webb Space Telescope (JWST) to confirm the atmosphere exists.

"This is an amazing development, but it sorely needs independent verification," Alan Stern, a planetary scientist and principal investigator for NASA's New Horizons mission to explore Pluto and the Kuiper Belt, who was not involved in the new study, told the Associated Press. "The implications are profound if verified."

Oddball ice ball

Researchers observed (612533) 2002 XV₉₃ as it passed directly in front of a distant star in January 2024. The observations were made by a team of professional and amateur astronomers from three sites in Japan. Their data hinted at attenuation — or gradual loss of starlight caused by an atmosphere, according to the study.

"The observation data showed a smooth change of the star's brightness near the edge of the shadow, lasting about 1.5 seconds," study first author Ko Arimatsu, an associate professor and senior lecturer at the National Astronomical Observatory of Japan, told CNN. "This kind of smooth brightness change is naturally explained if the starlight was bent by a very thin atmosphere around the object."

The team's observations suggested that the atmosphere is extremely thin, around 5 million to 10 million times thinner than Earth's atmosphere — too thin to support life. The potential atmosphere is also not permanent, with calculations suggesting that it will last less than 1,000 years unless it's restocked with gas.

The discovery raises questions about how an atmosphere could have formed around this small, cold world. Previous JWST observations revealed that the object's surface doesn't appear to have any frozen gas that could have sublimated to produce an atmosphere. The researchers speculated that the atmosphere could be sustained by cryovolcanoes (ice volcanoes), with some unknown event forcing gases to the object's surface.

Another possibility is that the object was hit by another icy body, such as a comet, which then produced gases to form an atmosphere. Future observations, particularly by JWST — an infrared telescope designed to probe atmospheres around alien worlds — could help solve the mystery, according to Arimatsu.

"That is why future monitoring is so important," Arimatsu told the Associated Press. "If the atmosphere fades over the next several years, that would support an impact origin. If it persists, or varies seasonally, that would point more toward ongoing internal gas supply."

See how well you know our planetary neighborhood with our solar system quiz!

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?

Look into the night sky, and it might seem like space is a vast expanse of darkness. But are any regions darker than others? What's the darkest place in the solar system and, on a grander scale, the universe?

In short, the answer isn't straightforward, and it depends on whom you ask, experts told Live Science.

True darkness, the blackest black, is surprisingly rare and hard to pinpoint. This is because there is a lot of dust in the cosmos: Dust scatters light, making space glow far beyond stars, Marc Postman, an astronomer at the Space Telescope Science Institute (STScI) in Baltimore, told Live Science. As a result, there is a background glow that permeates much of the universe. (The color of the universe is actually "cosmic latte," a beige shade not too far off white.)

Darkness also "depends on how you define it," Andreas Burkert, a theoretical astrophysicist at the University of Munich, told Live Science. The visible light spectrum illuminates some places in the universe with light. However, other wavelengths on the electromagnetic spectrum, like gamma rays and ultraviolet light, touch almost everything. This means that space, when viewed in the full electromagnetic spectrum, is quite luminous.

Low albedos

If you consider only visible light, there are some exceedingly dark places in space. A number of factors contribute to this darkness.

Firstly, cosmic objects can be made of light-absorbing material, making them appear very dark. Scientifically, this is known as albedo, or the amount of light reflected off a surface. A perfect mirror, for instance, would reflect 100% of the light directed at it and have an albedo of 1, while charcoal has an albedo of 4%.

The nucleus of comet Borrelly (also called 19P/Borrelly) is one of the darkest spots in our solar system, according to the Guinness Book of World Records. The 5-mile-long (8 kilometers) comet is made up of dust and ice that reflects less than 3% of sunlight, based on an image taken in 2001.

Similarly, the darkest known exoplanet in the universe, TrES-2 b, reflects less than 1% of light, thought to be because of high amounts of sodium vapors and gaseous titanium oxide in the atmosphere. In contrast, Earth reflects about 30% of the sun's light.

Black holes, too, are dark because they capture light that crosses the event horizon. But interestingly, "that doesn't mean that there is no light," Burkert said. "It simply is trapped." As a result, "when you enter the black hole, it's actually extremely bright," he explained.

Blocking light and distant objects

Darkness can also exist if something is blocking light from nearby stars. For example, some craters on our moon, located at the poles, never see the sun's light. These places are very dark because they are in "permanent shadow," Postman said. The shadowed craters on Pluto can be quite dark as well, because of their distance away from the sun.

Far beyond that, dense dust clouds, called molecular cores or Bok globules, are also thought to be pitch black. They look like "a hole in the sky," Burkert said. This is because the globule, made up of a mix of molecular hydrogen, carbon oxides, helium and silicate dust, blocks nearly all visible light from surrounding stars, making them disappear. However, this light-blocking is not as profound in the infrared spectrum, Burkert noted, who has studied Barnard 68, a globule around 500 light-years away from Earth.

Finally, there are pockets of the sky that are dark simply because they are far away from any light source. These places were documented by NASA's New Horizons telescope, an instrument sent to take photos of the galaxy's outer corners.

Based on a 2021 paper published in The Astrophysical Journal, these faraway regions are very dark. "On average, the sky out there is 10 times darker than it is near the Earth," said Postman, who was a co-author on the study. However, these regions still had light from the background glow of the cosmos.

Burkert noted the Earth sits in a relatively dark cavity in the Milky Way, which uniquely allows us to have an unobstructed view far into space.

"We sit in the middle of this big bubble, and so we can see a lot of stars," Burkert said. "If we would not be in the bubble, maybe astronomy would not have developed. So [darkness] is a very relevant, I think, important idea and question."

Scientists armed with the immense observing power of the James Webb Space Telescope (JWST) have detected faint traces of fluorescent flatulence leaking out from the dwarf planet Makemake, which lurks in the outer reaches of the solar system. This is only the second time that a gas has been detected on an object this far from Earth, and hints that this wee world is far more active than we once thought.

Makemake is a roughly spherical object measuring around 890 miles (1,430 kilometers) across, which is less than half the diameter of the moon. It is located around 45 times further from the sun than Earth on average, in a region known as the Kuiper Belt — a ring of asteroids, comets and larger icy objects, such as Pluto, beyond the orbit of Neptune. It was discovered in 2005 and has a small moon, dubbed MK2, which is around 110 miles (175 km) across.

Like the dozens of other dwarf planets and dwarf planet candidates within the solar system, Makemake is not considered a true planet because it has not cleared away all the debris from its orbital pathway around the sun. This is the same reason that Pluto lost its full planetary status in 2006.

Scientists have long known that Makemake's reddish-brown surface is covered with layers of methane and ethane ice, and some even think that small pellets of these chemicals, measuring around half an inch (1 centimeter) in diameter, may rest on its cold surface, according to NASA.

But in a new study, uploaded Sept. 8 to the preprint server arXiv and accepted for future publication in The Astrophysical Journal Letters, researchers used JWST to take a closer look at Makemake and found that the planetary dwarf also has faint concentrations of methane gas surrounding its icy surface.

"The Webb telescope has now revealed that methane is also present in the gas phase above the surface, a finding that makes Makemake even more fascinating," study lead author Silvia Protopapa, a researcher at the University of Maryland and the Southwest Research Institute in Texas, said in a statement. "It shows that Makemake is not an inactive remnant of the outer solar system, but a dynamic body where methane ice is still evolving."

Until now, the only other trans-Neptunian object (an object beyond Neptune) known to produce its own gas is Pluto, which has a very thin atmosphere composed mainly of nitrogen, with smaller amounts of methane and carbon monoxide, Live Science's sister site Space.com reported. The dwarf planet Ceres, which is located in the asteroid belt between Mars and Jupiter, also has a wispy atmosphere made up of water vapor.

JWST spotted the methane thanks to a phenomenon dubbed "solar-excited fluorescence," which causes the gas to faintly glow as it soaks up solar radiation. Using the telescope's advanced infrared instruments, the team also measured the temperature of the gas, which is a frigid minus 387 degrees Fahrenheit (minus 233 degrees Celsius).

However, the researchers are still unsure exactly where the methane comes from, or whether it is permanently trapped within Makemake.

One possible explanation is that the methane ice on the dwarf planet's surface sublimates, or transitions from solid to gas, and is contained within a fragile, superthin atmosphere, like on Pluto and Ceres. But if this is the case, the new data suggests that the pressure in this layer of gas must be more than 1 billion times weaker than Earth's atmosphere and more than 1 million times weaker than Pluto's, making it hard to confirm if it exists.

Another possible explanation is that the gas is being ejected from beneath the object's icy surface in plumes, like on Saturn's frozen ocean moon Enceladus. This potential process, known as outgassing, could be happening at various spots across the dwarf planet, with methane shooting outward at up to "a few hundred kilograms per second," Protopapa said. But it is also very hard to tell if this is the case with the data available.

The researchers hope that follow-up observations by JWST could help solve this puzzle. But regardless of where the methane comes from, the new findings "challenge the traditional view of Makemake as a quiescent, frozen body," the researchers wrote in the paper.

Earlier in the year, another group of researchers using JWST also revealed that Makemake and another distant dwarf planet, named Eris, could be geologically active, potentially to the point where they might support life. (There are currently no planned missions to study either of the distant dwarf planets up-close.)

Pluto is a bit of a loner. The dwarf planet is no longer considered a regular planet; it does not orbit on the same plane as the solar system's eight planets; and its orbit is both highly elliptical and extremely tilted.

In fact, its orbit is much more similar to that of its neighbors in the Kuiper Belt, a doughnut-shaped region beyond Neptune's orbit that's also home to other dwarf planets like Makemake and Eris, as well as millions of icy objects. But even compared with the rest of the objects in the Kuiper Belt, Pluto's orbit is peculiar.

So, how did Pluto's orbit end up this way?

First, let's compare Pluto to Earth and other planets in the solar system. The dwarf planet's orbit has an eccentricity — how much it deviates from a perfect circle — of 0.25. For comparison, Earth's orbit has an eccentricity of 0.0167, meaning it's nearly circular. Saturn and Mars have eccentricities of 0.054 and 0.093, respectively.

Pluto's orbit is tilted 17.4 degrees, compared with Earth's 1.5 degrees and Mercury's roughly 2 degrees.

Pluto's unusual eccentricity and tilt is likely due to its interactions with neighboring Neptune and other giant planets, said Renu Malhotra, a planetary scientist at the University of Arizona who has studied Pluto's orbit extensively.

Related: Why aren't all orbits circular?

Scientists believe Neptune's migration due to gravitational interactions with its giant neighbors plays a part in explaining Pluto's orbit. At some point, the planets migrated, and Pluto "was in this kind of orbit because Neptune's orbit had migrated outward … and swept Pluto up into this resonance," or when the gravities of orbiting bodies periodically influence each other as they pass nearby, Malhotra told Live Science.

Say you have a flat surface, Malhotra said. If you randomly toss rocks on it, they'll end up anywhere. But if you have dips in the surface, the rocks will end up in those dips. In the case of the planets, Neptune's migration created a gravitational well for resonant objects like Pluto.

Neptune keeps Pluto in check despite its strange orbit. For 20 years of 248-year orbit, Pluto is inside Neptune's orbit. The two planets are in a 3:2 orbital resonance, which means that Pluto completes two orbits every time Neptune completes three.

"That configuration is very stable and it protects Pluto from getting jostled further," said Will Grundy, an astronomer at the Lowell Observatory in Arizona and co-investigator on NASA's New Horizons mission to Pluto.

There's another odd aspect of Pluto's orbit. When the dwarf planet reaches perihelion — its closest point to the sun — it's always above the plane of the planets. That's "really peculiar," Malhotra said. Generally, planets and other dwarf planets dip above and below the plane over time — something that simulations suggest Pluto has never done. This is caused by a dance between Pluto and the planets Jupiter and Uranus that works to keep Pluto from launching into chaos.

"We used to think it was just the fact of Neptune and Pluto, but it turns out that the other planets are also very important in this other characteristic of Pluto," Malhotra said.

Pluto isn't alone in having a strange orbit, though. For instance, the dwarf planet Eris has an eccentricity of 0.45 and an inclination of about 43 degrees. "It's a much more extreme orbit," Grundy told Live Science.

There's still a lot to learn about how the planets migrated and by how much. And Pluto's cosmic neighborhood, the Kuiper Belt, is certain to offer even more mysteries.

"There's more landscape on objects [100 kilometers, or about 60 miles, and bigger] in the Kuiper Belt than there is in the entire rest of the solar system combined on solid surfaces," Grundy said. "It really is a ripe area for exploration."

Solar system quiz: How well do you know our cosmic neighborhood?

When NASA's New Horizons spacecraft sped by Pluto in 2015, it revealed an incredible world of ice and haze carved by various geological processes — hinting that an ocean may have played a role in the dwarf planet's recent history. The bounty of scientific riches has left researchers working to solve some of the tiny world's mysteries a decade after the spacecraft's flyby.

"There's still a lot of questions that are open," Carly Howett, a planetary scientist at the University of Oxford and a New Horizons team member, said last month at the Progress in Understanding the Pluto Mission: 10 Years after Flyby conference in Laurel, Maryland. With such questions lingering, Howett and her colleagues designed a follow-up mission in hopes of finally solving some of Pluto's mysteries.

Such a mission, sent to investigate the outskirts of the solar system, would span several decades. But it's far from being approved. "This mission could operate for over 50 years, challenging engineering, mission operations, and data analysis in ways that have never been done before," Howett wrote in a 2021 study published in the Planetary Science Journal detailing the mission concept.

A subsurface ocean?

In Roman mythology, Pluto is the god and ruler of the dead. For a return mission to the dwarf planet, Howett and her colleagues opted for the name Persephone, after Pluto's wife and "queen of the underworld" in Greek mythology.

"Given that Pluto is named after the Roman god of the underworld, and that we wanted a female name to reflect our diverse team with many women in leadership roles, this name seemed apt," Howett wrote.

Persephone would carry 11 instruments, all based on tools flown on previous missions but with some alterations. The primary question it would seek to answer would be whether Pluto has a subsurface ocean today.

If that question had been asked before New Horizons sped by, most scientists would have said it was unlikely. While many icy worlds may start off with a watery layer, it freezes over time. To remain liquid for the 4.5 billion-year life of the solar system, that ocean must stay warm.

Related: Skyscraper-size spikes of methane ice may surround Pluto's equator

Some moons constantly flex as they gravitationally interact with their host planet and neighboring moons, keeping their ocean from freezing. Charon, Pluto's largest moon, is nearly as massive as Pluto; they are often called a "double planet" (though neither meets the criteria for a planet). Charon could potentially keep Pluto warm, if the ocean had a high enough nonwater component to lower its freezing point, scientists think.

It wasn't until New Horizons revealed the remarkably young, lightly cratered surface of Pluto that most scientists began to consider the possibility of an ocean (although some did before the spacecraft's arrival). New Horizons studied Pluto in depth for only a few hours, although it continued to observe the dwarf planet for months before and after its closest approach.

Persephone, by contrast, would aim to enter orbit around the tiny world for just over three years, allowing much longer close-up views. "There's no substitute for proximity," New Horizons principal investigator Alan Stern said at the same conference.

What Persephone would study

Persephone would study the shape of Pluto to hunt for signs of a telltale fossil bulge — a "beer belly" of sorts caused when gravity pulls on the mass of a world. Bulges form more easily when the layers are liquid, and they can be frozen into place. New Horizons didn't observe such a pileup, but Persephone would send a more sensitive instrument that could make a more detailed examination.

"This mission should be able to image the whole of Pluto," Howett said at the conference. "It should be phenomenal."

Persephone would also seek to determine the composition of Pluto and Charon, using gravity and topography measurements similar to those taken of Saturn's moon Enceladus, and potentially calculate the thickness of the internal ice layers.

Pluto suffers from a century-long winter, and Persephone would arrive in the thick of it. Much of the dwarf planet would be shielded in darkness, so the mission would require instruments capable of peering through the veil. Cameras would map the surface of the entire world in greater depth and in varying wavelengths, including the half that was shrouded when New Horizons sailed past. They would search for hotspots, signs of ongoing activity and eruptions that might indicate a warm interior, as well as look for indications that the surface has changed since the 2015 observations. They would also provide a more detailed crater count on both Pluto and Charon, which would help scientists better understand how active the Kuiper Belt has been over the years.

Although icy worlds abound in the solar system, both Pluto and Charon have unusual surface features. In Pluto's Tartarus Dorsa region, blades of methane ice cover the surface. Scientists suspect that these spikes formed by sublimation as methane skipped instantly from solid to gas, but that's not definitive. And Charon has a strange ice mountain submerged in a frozen moat — a unique feature among icy peaks. Both surfaces are covered with exotic ices, and an understanding of their properties at the frigid temperatures on Pluto would be a key component of the mission.

Related: James Webb Space Telescope deciphers the origins of Pluto's icy moon Charon

In 1988, astronomers spotted a wispy atmosphere around Pluto, but its composition eluded them. New Horizons answered questions even as it raised more. Curiously, it appears that Pluto is slowly shedding part of its atmosphere onto Charon, creating a distinctive red pole that may shift hemispheres over time. One of the key objectives of Persephone would be to perform a direct detection of the atmosphere's composition through mass spectrometry.

Persephone would also study the space around the tiny world. Pluto is so far away that light from the sun takes just over 5.5 hours to reach it. From Pluto, the sun is a point-like spot in the sky.

Charon wouldn't be the only moon targeted by the mission. Persephone's launch time would determine how many of the smaller moons it could observe, but it would likely get a good look at the four other satellites as well.

New Horizons revealed bands of water ice on Nix, Hydra and Kerberos, as well as hints of ammonia on Nix and Hydra. Persephone would take a more detailed spectra for these three, as well as Styx, and try to determine how much of their surfaces, along with Charon's, is littered with debris from the collision that likely formed them. Although the best observations of the smaller satellites would most likely be of the close-orbiting moon Styx and the worst of Pluto's outermost moon, Hydra, a flyby of either Kerberos or Hydra might be possible, depending on when the mission launched and arrived at Pluto.

Persephone would remain at Pluto for just over three Earth years. During that time, the spacecraft could use orbits around the binary system to ultimately fling it from the pair.

Extending the mission by one year would allow the spacecraft to visit another Kuiper Belt object, much as New Horizons visited Arrokoth after the Pluto flyby. Such an extension would depend on when the mission launched and arrived, but it could provide significant insight into the debris left over from the solar system's formation. This would provide a big scientific return, because the distance of Kuiper Belt objects makes them challenging to study from Earth.

Pluto: The next generation

A return to Pluto would not be a casual undertaking. While it took New Horizons less than a decade to make the original trip, the changes in planetary alignment would make the next visit's flight alone take just over 27.5 years and would require five Next-Generation Radioisotope Thermoelectric Generators (NGRTGs), nuclear batteries that use the decay of radioactive material to power the spacecraft. The trip's extensive lifetime would require several NGRTGs to keep things warm in the freezing temperatures of space. That's a big ask at a time when plutonium for spaceflight is still at a premium. Currently, NASA's goal is to create 1.5 kilograms of plutonium per year; current RTGs use 4.8 kilograms.

That would require a significant investment in plutonium for outer-planet exploration. It would also take a financial investment. The estimated price tag for the Persephone mission is $3 billion, Howett said in her presentation.

With the potential of a half-century mission, the spacecraft carries several backup systems. But Stern pointed out that, although the timescale is long, it's not unheard of. NASA's Hubble Space Telescope is still functioning after 35 years, and so are NASA's Voyager 1 and 2 spacecraft, which launched 48 years ago.

Another challenge for the Persephone mission relates to the people working on it. With the potential for a 50-year mission, the researchers estimate that Persephone would be a three-generation mission that would cycle through three sets of scientists over their career lifetimes. Information and training would need to be passed from one generation to the next. In fact, Stern said delayed engagement was one of the hardest parts of planning the New Horizons mission.

"We know how to do these things," Stern said. "You have to be patient, and you have to plan for it."

Launch opportunities for the spacecraft are available every year from 2029 to 2032. After that, Jupiter's orbit prevents subsequent opportunities for a full decade.

Persephone was part of NASA's Planetary Mission Concept Study, which funds a range of projects to permit the decadal study to make informed decisions about potential future missions.

"While I think any mission needing more than one RTG is going to struggle to get selected at the moment, I do think the process of doing the mission proposal was useful," Howett said. Not only did the proposal show that such a mission is valid, parts of it, such as the orbital tour, could be used by other missions.

But don't look for Persephone to fly in the near future: the power request alone may keep it off the official books. Howett's mission proposal was one of several requested to help inform NASA's decadal survey about priorities and viabilities of future missions.

But the space agency is continuing to work to improve RTGs and their supporting technology. Howett is hopeful that ongoing technology developments will improve the mission's odds.

"One of the things that Persephone showed was that returning to Pluto to orbit was possible, but it wasn't cheap," she said. "It would have to be a Flagship-level mission."

Skyscraper-size spires of mehtane ice may cover around 60% of Pluto's equatorial region — a larger area than scientists previously estimated, new research finds.

The study, published July 5 in the Journal of Geophysical Research: Planets, was based on data collected by NASA's New Horizons spacecraft, which captured the first close-up images of the tiny world a decade ago, on July 14, 2015.

During that flyby, the spacecraft spotted spires of methane ice, each about 1,000 feet (300 meters) tall — about as tall as the Eiffel Tower. They're separated by up to 4.4 miles (7 kilometers) in somewhat parallel rows to form a geological feature astronomers call "bladed terrain."

The spires were spotted in high-altitude regions along the dwarf planet's equator in the Tartarus Dorsa region, a mountainous stretch just east of Pluto's famous heart-shaped Tombaugh Regio.

The features appear to be a larger but more spaced-out version of Earth's penitentes — structures of water ice that form in high altitude regions, such as the Andes, and reach a maximum of 9 feet (3 m). Similar structures have also been seen on Jupiter's moon Europa and may exist on Mars.

Related: Pluto may have an ice-spewing 'supervolcano' the size of Yellowstone, New Horizons data reveals

New Horizons was only able to snap high-resolution images of the bladed terrain on the side of Pluto that faced the probe — the encounter hemisphere — during its flyby. But additional data collected at infrared frequencies hinted that most of the dwarf planet's equatorial region, even on the non-encounter hemisphere, was methane-rich. This suggested that the spires are there, too.

However, photos of Pluto's non-encounter hemisphere are too fuzzy to directly spot spires. One way to detect them, though, is to use "indirect clues in the images," Ishan Mishra, a postdoctoral fellow at the Jet Propulsion Laboratory in Pasadena, California, and the new study's first author, told Live Science in an email.

These indirect clues, Mishra said, include surface roughness — irregularities, including slopes or ridges such as Pluto's spires — that were detected on scales too small for spacecraft cameras to resolve. He noted that rougher surfaces appear darker than smoother ones under the same lighting conditions because irregularities create shadows. That means rough, blade-covered surfaces would produce a detectable "darkening" trend, even if it were impossible to identify the icy spikes directly.

Following this reasoning, the study's authors analyzed photos of Pluto in which light had been reflected from the surface at many different angles. Using this reflectance data, the researchers studied how Pluto's surface brightness varied depending on the viewing angle. They focused on six specific regions, including the bladed terrain that the spacecraft had spotted on the encounter hemisphere and the hypothesized bladed terrain on the dwarf planet's other side. Using a mathematical model, the team then calculated how the surface brightness varied with roughness.

The astronomers found that, despite great variation in each region, the dark side's methane-rich regions were very rough — on average, twice as rough as the bladed terrain in the encounter hemisphere.

The results imply that bladed terrain of ice spires exists in a band spanning about 60% of the planet's circumference — equivalent to five times the width of the continental United States — with a majority located on the non-encounter hemisphere. But it isn't clear if the band is continuous or patchy, Mishra told Live Science.

The band extends between 30 degrees north and south of Pluto's equator, where the climatic conditions seem just right for the spikes to form, Mishra explained. "The formation of bladed terrain depends on long-term cycles of methane condensation and sublimation, which are governed by Pluto's seasons and orbital variations," he said.

Direct evidence will be needed to confirm the new observations. The most definitive way to confirm the bladed terrain's extension into Pluto's dark side is a future spacecraft mission, Mishra said. "Until then, studies like ours offer the best indirect evidence using the available data."

In 2006, Pluto was famously demoted from a planet to a dwarf planet. It remains the most famous dwarf planet today, but there are others in our solar system, including potentially hundreds that haven't been discovered yet.

But what, exactly, is a dwarf planet? And how many dwarf planets are there?

A dwarf planet is a celestial body that is smaller than a planet but bigger than an asteroid or other small rock in the solar system. They’re not planets because they only meet two of the three criteria for something in space to be considered a true planet.

A planet is defined by three qualities: It orbits the sun, it's mostly round, and it's massive enough for its gravity to have mostly kicked large objects from its orbit. Pluto and some other known dwarf planets meet the first two criteria but not the third because they orbit in the Kuiper Belt, a region beyond Neptune filled with small icy debris.

When Pluto was first discovered in 1930, it (and other similar objects in the Kuiper Belt) were so far away and faint that they were really hard to spot, even with the most powerful telescopes at the time. Indeed, the very qualities that make something a dwarf planet make those objects particularly hard to detect.

Related: James Webb telescope discovers 'a new kind of climate' on Pluto, unlike anything else in our solar system

"There are a few things that work against us spotting them," Mathew Yu, an astronomer at UCLA, told Live Science in an email. "They are far away from the sun. The reflected light from them is faint as a result. Some have low reflectivity, making light reflecting off them fainter."

Plus, "they move slowly across the sky due to their large distance from the sun," he added. "They go around the sun once every hundreds of Earth years. So their relative motions to the background stars are small. For astronomers with a limited observing time, these objects become hard to detect."

Astronomers got a lot better at finding objects in the Kuiper Belt in the 1990s and 2000s as telescope technology improved. And once they could see a bit better, they noticed there were a lot more rocks out there with orbits similar to Pluto's. Between 2002 and 2005, astronomers found seven more particularly large rocks: Haumea, Makemake, Quaoar, Sedna, Orcus, Eris and Salacia.

The International Astronomical Union (IAU) — the group of astronomers that decides on official names and terms for things in space — decided in 2006 that a new category was needed to describe these objects that were pretty big but not big enough to clear their orbits as a true planet does. Thus, the name "dwarf planet" was born.

How many dwarf planets are there?

With eight named large rocks in the Kuiper Belt, you might think that means there are eight dwarf planets. However, not all astronomers agree on the number.

"Accepted numbers range between around 10 to a few hundred in the Kuiper Belt alone," Yu said. "Some estimate that there may be up to tens of thousands of them beyond the Kuiper Belt."

The IAU has officially recognized only five dwarf planets: Pluto, Haumea, Makemake and Eris in the Kuiper Belt, plus Ceres in the asteroid belt. We've sent space probes to only two of these: Pluto (visited by NASA's New Horizons) and Ceres (visited by NASA's Dawn), both of which flew by those bodies in 2015.

Astronomers who observe dwarf planets, however, tend to recognize nine: the previous five, plus Quaoar, Sedna, Orcus and Gonggong. Gonggong was discovered in 2007 after the dwarf planet name change, but it wasn't confirmed until some new observations from NASA telescopes in 2016. The IAU is still considering proposals on these dwarfish bodies.

There are at least a handful more objects that have been proposed but need more observations to confirm their size and, therefore, dwarf planet status.

And in June 2025, scientists proposed a new possible dwarf planet: 2017 OF201. Astronomers found this particularly faint object by searching through an old pile of data with new advanced algorithms.

If you count all of the currently proposed dwarf planets plus the newcomer, that makes eighteen or more dwarf planets in the solar system — and that's just the ones we've seen. There are likely more out there, perhaps even hundreds or thousands, someday to be revealed as our telescopes keep getting better.

Astronomers using the James Webb Space Telescope (JWST) have taken a fresh look at the distant edges of our solar system — and found that, once again, Pluto is defying expectations.

When NASA's New Horizons spacecraft flew past Pluto in 2015, it shattered the notion that the dwarf planet was a dormant ball of ice, instead revealing it to be rich with icy plains and jagged mountains. But one of the biggest surprises floated above it all: a bluish, multi-layered haze blanketing the world's sky, stretching more than 185 miles (300 kilometers) above the surface — far higher and more intricate than scientists had predicted.

Now, nearly a decade later, new data from JWST confirm that Pluto's haze isn't just a visual oddity, it also controls the dwarf planet's climate.

"This is unique in the solar system," Tanguy Bertrand, an astronomer at the Paris Observatory in France who led the analysis, told Live Science. "It's a new kind of climate, let's say."

The findings, described in a study published June 2 in the journal Nature Astronomy, suggest similar dynamics may be at play on other haze-shrouded worlds in our solar system, and even offer clues about our own planet's early climate.

Lifting the haze

Pluto's high-altitude haze is made of complex organic molecules from sunlight-driven reactions of methane and nitrogen. The idea that this haze could control Pluto's climate was first proposed in 2017. Computer models suggested these particles absorb sunlight during the day and release it back into space as infrared energy at night, cooling the atmosphere much more efficiently than gases alone. This could also explain why Pluto's upper atmosphere is roughly -333 degrees Fahrenheit (-203 degrees Celsius) — 30 degrees cooler than expected.

Related: Why is Pluto not considered a planet?

For years, however, testing that theory proved difficult. One major challenge was Pluto's large moon, Charon, which orbits the frigid planet so closely that their thermal signals often overlap in telescope data. "Basically, we couldn't know what part of the signal is due to Charon and what part is due to Pluto's haze," Bertrand said.

The researchers behind the 2017 study predicted that Pluto's haze would make the world unusually bright in mid-infrared wavelengths — a prediction that, at the time, could only be tested with future instruments. That opportunity arrived in 2022, when JWST's powerful infrared instruments were finally able to separate the two worlds' signals. Sure enough, the faint infrared glow of Pluto's haze matched the predictions.

"In planetary science, it's not common to have a hypothesis confirmed so quickly, within just a few years," Xi Zhang, a planetary scientist at the University of California, Santa Cruz who led the 2017 team, said in a statement. "So we feel pretty lucky and very excited."

These findings also open up the possibility that similar haze-driven climates might exist on other hazy worlds, such as Neptune's moon Triton or Saturn's moon Titan, Bertrand said.

Even Earth's distant past might bear a resemblance, the researchers said. Before oxygen transformed our planet's skies, it's possible that Earth was veiled in a haze of organic particles — a blanket that may have helped stabilize temperatures and foster early life.

"By studying Pluto's haze and chemistry, we might get new insights into the conditions that made early Earth habitable," Zhang said in the statement.

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.

New research suggests that billions of years ago, Pluto may have captured its largest moon, Charon, with a very brief icy "kiss." The theory could explain how the dwarf planet (yeah, we wish Pluto was still a planet, too) could snare a moon that is around half its size.

The team behind this research thinks that two frigid worlds located in the Kuiper Belt, a ring of icy bodies located far from the sun at the edge of the solar system, collided together billions of years ago. Rather than mutually obliterating each other, the two bodies were united as a spinning "cosmic snowman." These bodies separated relatively quickly but remained orbitally linked to create the Pluto/Charon system we see today.

This "kiss and capture" process represents a new theory of moon capture and cosmic collision. It could also help scientists better investigate the structural strength of frigid, icy worlds in the Kuiper Belt.

"We've found that if we assume that Pluto and Charon are bodies with material strength, Pluto can indeed capture Charon from a giant impact," team leader and University of Arizona lunar and planetary researcher Adeene Denton told Space.com. "The process of this collisional capture is called 'kiss-and-capture' because Pluto and Charon briefly merge, the 'kiss' element, before separating to form two independent bodies."

Most planetary collision scenarios are classified as "hit and run" or "graze and merge," meaning this "kiss and capture" scenario is something entirely new.

"We were definitely surprised by the 'kiss' part of kiss-and-capture," Denton continued. "There hasn't really been a kind of impact before where the two bodies only temporarily merge before re-separating!"

The team's research was published Jan. 6 in the journal Nature Geoscience.

Pluto won Charon over with a 10 hour kiss

The reason Pluto's relationship with Charon has been challenging to scientists is because of the relatively small difference in size and mass between the two icy bodies.

"Charon is HUGE relative to Pluto, to the point where they are actually a binary," Denton explained. "It's half Pluto's size and 12% of its mass, which makes it much more similar to the Earth's moon than any other moon in the solar system."

For comparison, our moon is just a quarter of the size of Earth, while the largest moon in the solar system, Ganymede, is around 1/28 the size of its parent planet, Jupiter.

The University of Arizona researcher, who is also a NASA postdoctoral fellow, added that it's hard to get such a relatively big moon in a "normal" way. ("Normal" being the gravitational capture of moons like Mars' moons Phobos and Deimos and the moons of the giant planets Jupiter and Saturn.)

That means that the prevailing theory of the formation of the Pluto and Charon system is based on the collisional capture idea, similar to how a massive body is believed to have slammed into Earth to launch out material that our planet captured to birth our moon.

"Something big hits Pluto, and you get Charon, but like with the Earth-moon system, we don't fully know how that works and the conditions under which that occurs," Denton said. "It's a pretty big question since a bunch of other large Kuiper Belt Objects also have large moons, so it seems like this is something that happens in the Kuiper Belt with some frequency, but we don't know how or why."

During a standard "collision capture," a massive collision occurs, and the two bodies stretch and deform in a fluid-like way. This process explains the creation of the Earth/moon system well because the intense heat generated in the clash and the greater mass of the bodies involved causes them to act in a fluid way.

When considering Pluto and Charon in a collision capture process, there is an extra factor to consider: the structural strength of the colder icy and rocky bodies. This is something that has been neglected in the past when researchers considered the collisional creation of Charon.

To factor this into simulations, the team turned to the University of Arizona's high-performance computing cluster. When Denton and colleagues accounted for the strength of these materials in their simulation, something completely unexpected emerged.

"Because both bodies have material strength, Charon did not penetrate deep enough into Pluto to merge with it; this isn't true when the bodies are fluids," Denton explained. "For the same impact conditions, if we assume Pluto and Charon to be strengthless, they do merge into one large body, and Charon is absorbed. With strength, however, Pluto and Charon remain structurally intact during their brief merger."

Because Charon couldn't sink into Pluto in this scenario, it remained beyond the so-called "co-rotation radius" of both bodies. As a result, it could not rotate as fast as Pluto, which meant the two bodies could not stay merged. As they separated and this icy kiss ended, the team thinks that Pluto would have torqued Charon into a close, higher circular orbit from which the moon would have migrated outward.

"The 'kiss' in this kiss-and-capture, the merger is very brief, geologically speaking, lasting for 10 to 15 hours before both bodies separate again," Denton said. "Charon then begins its slow outward migration towards its current position."

The team thinks the initial collision happened very early in solar system history, probably tens of millions of years after the solar system formed, which would be billions of years ago.

"Typical large collisions are straightforward mergers, where the bodies combine, or both bodies remain independent," Denton said. "So this was very new to us. It also raised a lot of interesting geological questions that we'd like to test, because whether kiss-and-capture works depends on the thermal state of Pluto, which we can then tie to Pluto's contemporary geology to test.

"I'd really like to determine how the initial Pluto-Charon impact can influence whether and how Pluto and Charon develop oceans."

Denton explained that there are two avenues the team can follow to build upon this development.

"The first is looking at how this applies to the other large Kuiper Belt Objects with large moons, like Eris and Dysnomia, Orcus and Vanth, and the others," Denton explained. "Our initial analysis suggests that kiss-and-capture can also be the source of these other systems, but since they're all different in their compositions and mass, it's critical to learn how kiss-and-capture may have operated across the Kuiper Belt."

The second avenue the team intends to follow involves looking at the long-term tidal evolution of Charon to confirm their formation theory.

"To really be sure that this is the process that formed Pluto and Charon, we need to make sure that Charon migrates to its current location at around 8 times the width of Pluto away," Denton said. "However, that's a process that occurs over much longer timescales than the initial collision, so our models aren't well-suited to track it.

"We're planning to have a much closer look at this in the future to determine which conditions not only reproduce Pluto and Charon as bodies but also put Charon in the right spot, where it is today."

Originally posted on Space.com.

When the International Astronomical Union (IAU) demoted Pluto from a planet to a dwarf planet in 2006, it surprised a lot of people, including some scientists. Even many years later, some astronomers want to revise the definition of a planet to clarify the parameters that set planets apart from other celestial objects.

But why isn't Pluto considered a planet anymore? It starts with the definition of a planet — or lack thereof. Before 2006, there weren't strict criteria for a planet. Instead, planets were loosely regarded as objects larger than asteroids that orbited the sun. In the mid-1800s, for example, more than a dozen objects that we now regard as asteroids were considered to be planets.

When did Pluto become a planet?

When Pluto was discovered by Clyde Tombaugh in 1930, scientists were searching far and wide for an unknown celestial body to explain some irregularities in Uranus' orbit. Tombaugh, a newly minted astronomer at Lowell Observatory in Arizona, was tasked with identifying the culprit. After several months, he successfully located a round, rocky object beyond Uranus that he believed might contribute to its orbital wobble. It would eventually be named Pluto after the Roman god of the underworld. Despite being smaller than several known moons, it was deemed large enough to be considered a planet.

However, it soon became apparent that Pluto was not big enough to exert the kind of gravitational pull necessary to influence Uranus' orbit. What's more, in the 1990s, astronomers discovered that Pluto was surrounded by a number of similarly sized objects; it belonged to a region of the solar system later named the Kuiper Belt.

This sparked debate about Pluto's status in the planetary canon, which came to a head at a 2006 meeting in Prague.

Why is Pluto not considered a planet?

That year, the IAU tasked a small committee with creating a working definition of a "planet." They landed on three criteria:

1. It must orbit around the sun.
2. It must have enough mass to draw itself into a round shape.
3. It must have cleared all other celestial bodies, except its own moons, from its orbit.

Based on the third requirement, the committee declared that Pluto no longer qualified as a planet because of its position in the cluttered Kuiper Belt, where thousands of objects sit beyond the orbit of Neptune. Pluto, therefore, is not the gravitationally dominant object in its neighborhood — and thus, not a planet, according to the new definition.

Related: When will Pluto complete its first orbit since its discovery?

But this framework drew immediate criticism. "That definition is clearly inadequate, because it excludes exoplanets," or planets discovered beyond our solar system, Jean-Luc Margot, a planetary scientist at UCLA, told Live Science. It is also extremely difficult to determine when a body has cleared its own orbit, he said — Pluto clearly hasn't, but by some definitions, neither has Mars.

Pluto's demotion remains controversial for some scientists, though, in part because of the way it was reclassified. Philip Metzger, a planetary physicist who worked on NASA's New Horizons mission to Pluto, has previously pointed out that the IAU did not put their definition of a planet up for a vote from the larger scientific community. In his view, that makes the new definition invalid. And for others, it's a matter of sentiment — many people grew up thinking of Pluto as a planet, and they are still emotionally invested in it.

Regardless of whether Pluto is a planet or a dwarf planet, it remains a fascinating part of the solar system, from its huge white "heart" of frozen nitrogen to the ice-spewing 'supervolcano' thought to lurk below its surface.

"Pluto hasn't changed," Margot said. "It's just as exciting."

Using the James Webb Space Telescope (JWST), astronomers have detected carbon dioxide and hydrogen peroxide on the frozen surface of Pluto's largest moon, Charon. Detecting these molecules could tell scientists how Charon and other icy bodies at the solar system's edge were born.

Since its discovery in 1978, Charon has been extensively studied — but previous research has been limited in terms of what wavelengths of light could be explored during these analyses. That left gaps in our understanding of the surface composition of this moon of Pluto. As a result, though scientists have detected water ice, ammonia-bearing species and organic compounds on Charon, carbon dioxide and hydrogen peroxide have evaded detection. Until now, that is.

The team, led by Silvia Protopapa of the Southwest Research Institute (SwRI), filled in these gaps by studying Charon with the JWST's Near-Infrared Spectrograph (NIRSpec) instrument.

"Our research reveals that Charon’s surface preserves evidence of its formation through the presence of carbon dioxide, as well as signs of irradiation processes, indicated by the presence of hydrogen peroxide," Protopapa told Space.com. "These discoveries expand Charon's known compositional inventory, which includes water ice, ammonia-bearing species and organic materials responsible for its gray and red coloration."

Related: Pluto's huge white 'heart' has a surprisingly violent origin, new study suggests

Charon is a midsized body roughly 750 miles (1,207 kilometers) wide and located in the Kuiper Belt, a ring of icy debris, comets and dwarf planets, also referred to as trans-Neptunian objects (TNOs), at the solar system's edge.

Unlike many of the larger objects in the Kuiper Belt, Charon's surface is not obscured by volatile ices like methane, meaning it offers scientists valuable insights into the effects of sunlight exposure and cratering on these distant bodies. Additionally, Charon is the only mid-sized TNO for which geologic mapping is available. This is thanks to data gathered by NASA's New Horizons spacecraft, which visited the Pluto system around a decade ago.

"Overall, these factors make Charon an invaluable target from which we can learn extensively," Protopapa said. "Our findings provide valuable insights into how processes such as sunlight exposure and cratering shape the surface of Charon and, by extension, other mid-sized icy bodies beyond Neptune's orbit."

Cool suprises on Charon

The composition of stars, planets and moons can be determined from the light they emit or reflect from their surface. This is possible because elements absorb and emit light at specific wavelengths. Thus, looking at a celestial body's spectra via a technique called "spectroscopy" reveals the "fingerprints" of elements and chemical compounds.

Protopapa and colleagues reached their findings by comparing JWST spectroscopic observations with lab-based measurements and detailed spectral models of the surface of Charon. This led them to conclude that carbon dioxide is primarily present as a surface veneer on a water-ice-rich subsurface.

"The surface of Charon, as revealed by the New Horizons mission, features numerous craters surrounded by bright ejecta blankets that are rich in water ice and ammonia-bearing compounds," Protopapa explained. "These geologic features suggest that materials from beneath the surface have been exposed by impact events, providing a window into the moon's subsurface composition.

"Our preferred interpretation is that the upper layer of carbon dioxide originates from the interior and has been exposed to the surface through cratering events."

She added that carbon dioxide was also expected because the compound is known to be present in regions of the protoplanetary disk from which the Pluto system formed. The fact that carbon dioxide wasn't spotted by NASA's New Horizons spacecraft when it visited Pluto and captured images of Charon in 2015 has been troubling scientists for some time.

"The detection of carbon dioxide was a satisfying confirmation of our expectations," Protopapa continued.

What was not expected by the team was the detection of hydrogen peroxide.

"The detection of hydrogen peroxide on Charon came as a surprise. I
honestly did not expect to find evidence of it on the surface," Protopapa said. "Hydrogen peroxide has been known to be present on Jupiter's moon Europa’s surface since the 2000s. I never imagined I would be writing a paper comparing
these icy satellites, Charon and Europa, given how different their
environments are."

The surprise presence of hydrogen peroxide on Charon suggested to the team that the water-ice-rich surface of Pluto's largest moon is being actively altered by ultraviolet light from the sun, energetic particles from the solar wind, and streams of charged particles from beyond the solar system called "galactic cosmic rays."

"Hydrogen peroxide forms from the combination of neighboring hydroxide ion radicals, which originate from the breakup of water molecules due to incoming ions, electrons, or photons," Protopapa continued. "Our team conducted new laboratory measurements to confirm that it is possible to generate hydrogen peroxide even when carbon dioxide is present."

The team hasn't finished with Pluto's largest moon yet. The JWST will continue to study Charon, and scientists will use the resulting data to better understand icy TNOs as a whole.

"Future JWST observations targeting the spectral gaps, not covered in the current data, could lead to new Charon discoveries and further expand its chemical inventory, possibly revealing other mechanisms at play," Protopapa said.

The team's research was published on Tuesday (Oct. 1) in the journal Nature Communications.

Originally posted on Space.com.

Astronomers are proposing a new definition of a planet, nearly two decades after a previous such change led to Pluto's demotion. The new definition would add measurable criteria, including considerations of the planet's mass — and it would still leave Pluto with its dwarf planet status.

Currently, the International Astronomical Union (IAU) defines a planet as a celestial body with three characteristics: It orbits the sun, it's big enough for gravity to mold it into a roundish shape, and it has cleared away any smaller objects, other than moons and other satellites, surrounding its orbit. 

"The problem in the past was, you had a word 'planet' but you didn't have a quantitative definition of it," Brett Gladman, an astronomer at the University of British Columbia who, along with colleagues, proposed the new definition, told Live Science. The proposal was outlined in a paper published July 17 in The Planetary Science Journal and is scheduled to be presented at the IAU General Assembly in August.

Without a quantitative point of reference, the current definition runs into many issues, Gladman said. Namely, the criteria are vague; it's unclear how large these bodies need to be, or "how clear" their orbits should be, to be considered planets. For example, Earth and Jupiter have asteroids that regularly cross their paths, raising the question of whether the planets truly have clear orbital paths. The current definition also fails to consider objects that orbit stars other than the sun, such as the more than 5,500 exoplanets that have been detected beyond our solar system.

But "the most problematic by far is the roundness criterion," Jean-Luc Margot, an astronomer at UCLA and lead author of the paper, told Live Science. "Roundness is simply not observable. We do not have the technology, and we will not have the technology anytime soon."

Related: Enormous planet discovered around tiny star could break our understanding of solar system formation

Instead of focusing on the planets' roundness and orbital paths, the new definition emphasizes a measurable quantity: the object's mass. This new definition describes a planet as a celestial object that meets the following criteria: 

To ensure that their classification framework would be logical and unbiased, the scientists used a method called unsupervised clustering, an algorithm that groups similar objects. This technique also successfully grouped the eight planets in the solar system.

"No matter how many criticisms you can lodge at the current IAU definition, you can at least be satisfied that the outcome, the eight planets, is the reasonable classification," he said.

The astronomers argued that the ability to clear the orbital paths, known as dynamical dominance, can be determined by the planet's mass. Only objects of a sufficiently high mass, including the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), are capable of dynamical dominance. 

The dynamical dominance also sets a lower mass limit on what qualifies as a planet. However, Pluto, which has a mass of only 2.88 x 10^22 pounds (1.31 x 10^22 kg), will still not qualify as a planet under the new definition. 

With regard to roundness, the mass of a celestial object larger than 10^21 kg is typically spherical due to gravitational pull. In this way, the new definition does not really contradict the current definition but rather adds specificity to it, Margot said. 

"Humans are very tied to language and names and classification, because that's how we think about the world," Gladman added. "That's how we sort out the complexity of the world. We want to give names to things and put them in classes. And scientists want to do that, too. We just want to do it precisely."

There is always a possibility of pushback. Margot will present the proposal at the IAU General Assembly in August, but he is not expecting a consensus to be made then. Instead, he hopes that through the presentation, the team will identify individuals who are interested in the idea and continue the discussion.

The proposal was written out of "conviction that we can do better as a community," Margot said. "We owe it to ourselves and to the public to come up with better definitions for these important astrophysical terms."

Editor's note: This article was updated on July 18 to correct information regarding the masses of the eight planets compared to dwarf planets. We have also added a link to the peer-reviewed version of the study, published July 17.

When NASA's New Horizons spacecraft flew by Pluto in 2015, it returned images with a sweet surprise: a heart-shaped formation dominating the surface of the dwarf planet.

Now, researchers believe they've uncovered the origin of this cosmic Valentine. The heart, they report today (April 15) in the journal Nature Astronomy, was formed in a slow-motion, glancing collision with an icy rock wider than Kansas is long.

The researchers determined this scenario by using computer models to simulate the impacts on Pluto's surface and the resulting formations. Pluto's heart, scientifically known as Tombaugh Regio, gets its light coloration from nitrogen ice. Impacts between icy bodies in the far reaches of the solar system aren't like those closer to the sun, said study co-author Erik Asphaug, a professor at the University of Arizona's Lunar and Planetary Laboratory.  

"We are used to thinking of planetary collisions as incredibly intense events where you can ignore the details except for things like energy, momentum and density," Asphaug said in a statement. But in the distant solar system, velocities are so much slower, and solid ice is strong, so you have to be much more precise in your calculations. That's where the fun starts."

Related: Uranus and Neptune aren't made of what we thought, new study hints

Led by Martin Jutzi, a senior researcher at the University of Bern in Switzerland, the team used a simulation method called smoothed particle hydrodynamics to test various angles of collision and sizes of impactors to learn which dynamics would lead to the formation of Sputnik Planitia, the western portion of Pluto's heart. This roughly 800-square-mile (2,000 square kilometers) region sits about 2.5 miles (4 km) lower than its surroundings.

The team found that the formation probably originated from an oblique collision, leading to its elongated shape. This likely happened early in Pluto's history.

The icy rock that hit Pluto was probably around 454 miles (730 km) in diameter, the study authors said. Because of Pluto's icy core, the impact did not melt and liquefy portions of the planet as might happen in an impact in warmer climes, allowing the impacting body to sink into the planet's core.

Instead, the impactor likely flattened on Pluto's surface. Even now, it may sit just under the smooth nitrogen ice that covers Sputnik Planitia.

"Somewhere beneath Sputnik is the remnant core of another massive body that Pluto never quite digested," Asphaug said.

The simulations further suggest that Pluto does not hold a subsurface ocean under its icy outer layers, the researchers reported. Because Pluto's heart has a lower mass than the rest of the dwarf planet's surface, it should have gradually migrated toward the pole as Pluto rotated over millennia. But the formation is near the planet's equator — an odd position researchers had previously theorized might be due to the dynamics of a huge body of subsurface liquid water.

The new research suggests that a liquid ocean isn't necessary to explain the placement of Pluto's heart, Jutzi said in the statement.

"In our simulations, all of Pluto's primordial mantle is excavated by the impact, and as the impactor's core material splats onto Pluto's core, it creates a local mass excess that can explain the migration toward the equator without a subsurface ocean, or at most a very thin one," Jutzi said.

A pair of dwarf planets lurking in the outer reaches of the solar system could still be geologically active, increasing the chances that they could support alien life, a new study suggests. The findings could also change what we know about dwarf planets in general.

There are five confirmed dwarf planets in the solar system: Ceres, Haumea, Eris, Makemake and the ex-planet Pluto. All of these planetary pretenders, apart from Ceres, are located in or around the Kuiper Belt, a disk of comets and other small objects beyond the orbit of Neptune.

In 2015, NASA's New Horizons probe passed Pluto and spotted signs of recent geological activity in the dwarf planet's thinning atmosphere. Follow-up observations have shown that Pluto is covered in giant, icy volcanoes, which may still be active. Scientists believe that Pluto's surprising geological activity is caused by interactions with its moon Charon. As a result, they thought it was unlikely for other dwarf planets without moons to be similarly active. 

But in the new study, which was uploaded Feb. 10 to the preprint server arXiv, researchers took a closer look at chemical spectra from two other dwarf planets, Eris and Makemake, views of which were recently captured by the James Webb Space Telescope (JWST). After analyzing the data, the team realized that some of the gases found on both dwarf planets' icy surfaces could be explained only by some form of current or recent geological activity, such as hydrothermal vents of cryovolcanic (ice volcano) activity. (These findings have not yet been peer-reviewed.)

The results also hint that some of the other dwarf planets could be geologically active, too.

Related: What's the best evidence we've found for alien life?

The JWST data included spectra of all of the frozen gases in the ice covering Eris and Makemake. But the researchers were really only interested in methane, a greenhouse gas that is also found on Earth and is made up of one carbon atom bonded to four hydrogen atoms.

In planetary science, there are two main types of methane: abiotic methane, which is left behind from chemical reactions that occurred during planetary formation; and thermogenic methane, which is created by hydrothermal or geothermal processes. The main difference between the two types is the ratio of hydrogen and deuterium — an isotope, or alternative version, of hydrogen with an extra neutron in its nucleus — found among their molecules.

Abiotic methane has a high ratio of deuterium-to-hydrogen (D/H), while thermogenic methane has a lower D/H ratio. The spectra revealed that the methane in the atmospheres of both Eris and Makemake had a low D/H ratio, meaning that it is mostly thermogenic. This suggests that the gas is either being produced by ongoing geological activity or relatively recent activity, within the past few million years.

These findings came as a surprise to the researchers — especially regarding Makemake, which is around 60% the size of Pluto. Eris is around the same size as the ex-planet, which makes it less surprising that it may also be geologically active. However, Makemake should be far too small for these types of processes to occur.

The findings also raise the chances of life developing on these planets, the researchers said. For example, hydrothermal vent activity is a leading candidate for sparking self-replicating life-forms on Earth and is a key reason scientists look to water worlds such as Enceladus and Mimas — both moons of Saturn — as potential places where alien life may occur.

A future probe should be sent to Eris and Makemake to "explore these worlds up close" and further assess their geological potential, the researchers wrote.

Every year, we learn more about the intricacies of the solar system, and 2023 has been no different. This year, scientists shed more light on our cosmic neighborhood with some intriguing discoveries. From shrinking planets and new moons to an icy supervolcano and mysterious spots, here are the 10 wildest things we learned about the solar system in 2023. 

Mercury is shrinking 

Starting with the planet closest to the sun, Mercury, an October study revealed that the solar system's tiniest planet might be getting even smaller.

Experts have long known that Mercury has shrunk significantly since it first formed. Giant cracks, known as scarps, on the planet's surface were created as the planet contracted over long periods of cooling. 

Scientists originally thought these scarps were billions of years old, but the new research showed that some were just 300 million years old, which suggests the planet has been shrinking for most of its life — and it could still be happening today. 

This year, scientists also discovered that salty glaciers in some of Mercury's craters could provide the necessary conditions for some extreme life-forms to thrive.

Oxygen found on Venus 

Moving farther from the sun, researchers announced in November that oxygen had been detected in Venus' atmosphere for the second time, confirming that the element is present on the hellish planet. 

The thin layer of molecular oxygen (unbonded oxygen atoms) was spotted by NASA's Stratospheric Observatory for Infrared Astronomy, a telescope attached to a large plane that flew above most forms of interference in Earth's atmosphere between 2014 and 2022.

However, this oxygen is not a sign of life on Venus. Instead, researchers suspect the gas is created as solar radiation breaks down carbon dioxide and carbon monoxide molecules in the atmosphere. This oxygen would also be useless to future human colonists because it is not breathable and is found in low concentrations.

This year, scientists also discovered that Venus has thousands more volcanoes than we previously realized and that a majority of them could still be active.

The moon is older than we thought 

Closer to home, a study published in October revealed that the moon may be at least 40 million years older than we previously thought.

Researchers examined tiny impact crystals, known as zircon crystals, within lunar rock samples brought back by NASA's Apollo 17 mission. The new analysis revealed that these crystals,which were left by a colossal collision between Earth and a Mars-size planet that birthed the moon, were older than similar crystals found in other lunar samples.

"These crystals are the oldest known solids that formed after the giant impact," the researchers wrote, and they act as "an anchor for lunar chronology."

This year, astronomers also detected a mysterious heat-emitting blob on the moon's far side and spotted a separate chunk of the moon orbiting Earth.

Mars is spinning faster and faster

One of the biggest stories to come from Mars this year is that the Red Planet's spin has been getting quicker every year, and scientists don't know why.   

Data from NASA's InSight mission shows that Mars' rotation is accelerating at a rate of 4 milliarcseconds per year, which means its days are growing slightly shorter. (A milliarcsecond is one one-thousandth of an arcsecond, which is a unit of angularity.)

Two leading theories explain the speed-up. The first is that ice accumulation at the planet's poles is causing a slight change in how its mass is distributed, and the second is a phenomenon called postglacial rebound, where landmasses rise up after millennia buried under ice.

This year, scientists also solved the mystery of the planet's largest-ever "marsquake" and calculated the minimum number of people needed to start a Martian colony.

Jupiter has a dozen new moons 

This year, scientists confirmed the existence of 12 new moons around Jupiter, bringing the planet's satellite count to 92. At the time, this set a new solar system record.

The newly discovered Jovian moons are small, ranging from 0.6 to 2 miles (1 to 3.2 kilometers) wide, and most of them have wide orbits — nine of the 12 moons take more than 550 days to orbit Jupiter.

The moons were discovered in 2021 and 2022 but were just confirmed this year by the International Astronomical Union's Minor Planet Center.

This year, NASA's Juno spacecraft also discovered organic compounds on the surface of Jupiter's largest moon, Ganymede. The molecules likely came from the moon's subsurface ocean, hinting that it could sustain life.

But Saturn has even more moons

If you thought Jupiter gained lots of moons this year, just wait until you get a load of Saturn's new satellites. This year, scientists officially added 62 moons to the ringed planet's roster, bringing its total to 145 and setting a new record that is unlikely to be broken by Jupiter again. 

Some of the new moons were just 1.6 miles (2.5 km) wide. But all of the newly detected satellites are "irregular moons," meaning they follow distant, elliptical orbits around their host planet and often move in retrograde, meaning in the opposite direction of Saturn's rotation. Some are also clumped together, suggesting they were once a single body that broke apart.

This year, scientists also spotted a key organic compound shooting out of Saturn's icy ocean moon Enceladus, and researchers also worked out that the astronomer who discovered Saturn's largest moon, Titan, was probably nearsighted.  

Polar vortex spotted on Uranus 

Moving into the outer planets, new images of Uranus showed a gigantic "polar vortex" swirling around the ice giant's north pole, suggesting the planet's atmosphere is not as inert as we previously believed.

A polar vortex is a swirling ring of hot or cold air that circles a planet's polar region. These vortices have been spotted on Venus, Earth, Mars, Jupiter, Saturn and Neptune. And in 1986, NASA's Voyager 2 probe spotted a polar vortex around Uranus' south pole on its journey out of the solar system.

The swirling rings are thought to be caused by high-altitude atmospheric jet streams, although the mechanisms differ from planet to planet.

Earlier this year, scientists also spotted a polar vortex on the sun for the first time.

Neptune keeps growing mysterious spots 

This year, astronomers captured the first, clear image of one of Neptune's mysterious dark spots from Earth and photographed a never-before-seen bright spot at the same time.

Temporary dark spots have been documented on the solar system's most distant planet since 1989, when Voyager 2 spotted one as it passed by. Scientists think the dark spots are probably gigantic storms, like Jupiter's "Great Red Spot," but they are unsure why the spots appear so dark. 

The recent bright spot has never been seen before and lasted only a few weeks, so its origins are even more mysterious. But the different spots are likely linked, researchers said.

This year, astronomers also noticed that all of Neptune's clouds had suddenly vanished. The disappearing act is likely linked to a peak in solar activity as the sun reaches the explosive peak in its roughly 11-year solar cycle, known as the solar maximum. 

Pluto has an icy supervolcano 

Demoted planet Pluto has been hiding a gigantic cryovolcano the size of Yellowstone's supervolcano, an October study revealed.

Cryovolcanoes, also known as ice volcanoes, are simply volcanoes that erupt with cryomagma — ice, water and various gases — instead of molten rock. They've been found in several places across the solar system, including the dwarf planet Ceres and at the heart of explosive comets, such as the "devil comet."

Pluto's newly discovered volcano, named Kiladze Caldera, was originally pegged as a crater in images captured by NASA's New Horizons mission. But after taking a second look at the data, scientists detected traces of cryomagama around the depression, which is evidence that it has erupted multiple times.

Scientists don't know if it's still active, but if it is, it could produce enough cryomagma to cover Los Angeles.

Planet Nine might not exist (but it also might) 

Planet Nine is a hypothetical planet that is rumored to exist somewhere in the outer reaches of the solar system.

The Planet Nine hypothesis arose when astronomers detected gravitational anomalies in the Kuiper Belt — a group of comets and asteroids beyond the orbit of Neptune. These anomalies suggest that an invisible planet up to 10 times the size of Earth is lurking beyond the Kuiper Belt.

But in a preprint paper uploaded in September, scientists proposed that these gravitational anomalies could be explained by an alternative theory of gravity, known as modified Newtonian dynamics (MOND). 

MOND suggests that entities toward the edge of the Milky Way, such as the solar system, experience a greater gravitational tug from our galaxy's supermassive black hole heart than traditional gravity allows. Therefore, the Kuiper Belt object may be tugged away by our galaxy and not an elusive planet. However, MOND is not a universally accepted theory, so for now, our best guess is that Planet Nine does exist. 

Demoted planet Pluto may have an ice volcano on its surface the size of Yellowstone, scientists recently discovered.

The newly discovered volcano, Kiladze Caldera, was originally pegged as a crater via images captured by NASA's New Horizons mission. After taking a second look at data from New Horizons, scientists now think Kiladze Caldera has erupted multiple times over its history, spewing around "a thousand kilometers of cryo-lava" — or enough to almost cover the city of Los Angeles — in each of its largest eruptions, the researchers wrote in a study submitted to the journal Icarus and published on the preprint database arXiv.

Cryovolcanoes, also known as ice volcanoes, are simply volcanoes that erupt with ice, water, and various gasses instead of molten rock. They've been identified in a few places in our solar system, including the dwarf planet Ceres located in the main asteroid belt. They're even the culprit behind Enceladus's famous plumes, seen by the spacecraft Cassini.

Plus, "there are at least two other cryovolcanic structures on Pluto," lead author and NASA planetary scientist Dale Cruikshank told Newsweek. These other icy volcanoes are known as Wright Mons and Piccard Mons.

Researchers had plenty of evidence to suspect Kiladze was more than a common impact crater. Not only did it look a bit odd for a crater, it was also surrounded by water ice — which is usually hidden below methane snows and gunk from Pluto's natural equivalent of smog — and nestled between faults and other tectonic features.

The clincher came when the team found evidence of ammonia mixed in with the water ice around Kiladze. Ammonia lowers the freezing point of water, possibly allowing water to flow as liquid cryo-lava. Interestingly, ammonia is also an important molecule for the chemistry that makes amino acids, and eventually life.

The fact that Kiladze is surrounded by water ice also suggests that it's quite young, or at least erupted fairly recently (in geologic timescales, anyways). Over time, the water ice is covered up by other materials, so an uncovered area must be younger. "We estimate that the age of Kiladze and surroundings (since the last eruption) is only a few million years," said Cruikshank.

The big outstanding question is: where did the water cryo-lava come from? Pluto "once had an internal ocean that was global in scale," explained Cruikshank, harkening back to a distant time when Pluto was still warm from the collisions that led to its birth. Leftover heat in the dwarf planet's core could be keeping that ocean liquid now, combined with freeze-resistant chemicals like ammonia, and occasionally bursting out through cryovolcanoes like Kiladze. Alternatively, the subsurface ocean may have frozen over long ago, but small pockets of water remain feeding structures like Kiladze.

"This is a mystery for the next generation of planetary scientists to solve," Cruikshank told the space news site Universe Today in an email.

Astronomers will have to wait a while to celebrate the first complete orbit of Pluto since its discovery.

Pluto was discovered on Feb. 18, 1930, using the Lowell Observatory in Flagstaff, Arizona. American astronomer Clyde Tombaugh found a moving object clearly beyond the orbit of Neptune. That object was later called Pluto, the ruler of the Greek underworld in that culture's mythology.

There's a long-running debate about whether Pluto is a planet or a dwarf planet. Concerning its orbit, however, astronomers don't disagree that the world has yet to complete a single orbit since Tombaugh first spotted Pluto in imagery.

It takes Pluto 248.09 Earth years to complete one orbit around the sun. Plug that information into a timeanddate.com calculator along with its discovery date, and you'd find that Pluto will complete its first full orbit since its discovery on Monday, March 23, 2178.

But there is far more to this intriguing world's orbit than the amount of time lapsed since its discovery, as astronomers told Live Science.

Related: Why do the planets in the solar system orbit on the same plane?

The larger worlds of our solar system tend to orbit near the ecliptic, which is the plane of the solar system. Pluto, however, is quite inclined compared with Earth and many others at 17 degrees.

The world Eris exceeds that at 44 degrees, noted the Lowell Observatory's Will Grundy, co-investigator on NASA's New Horizons mission that encountered the Pluto system in 2015 and Arrokoth in January 2019. The craft is now moving into the Kuiper Belt.

"Inclinations of smaller planets tend to be higher, as a result of dynamical interactions between planets early in solar system history: Mercury's is seven degrees, Eris' is seven degrees, Makemake's is 29 degrees, and Haumea's is [28.2] degrees," Grundy said.

The same is true of small world eccentricity, which describes how far an orbit is from a true circle. While Earth's orbit is almost circular, Pluto's is elongated with an eccentricity of 0.25. By comparison, Mercury's is 0.205, Eris' is 0.44, Makemake's is 0.16 and Haumea's is 0.20. 

The deviations present among these various worlds, he said, "are valuable clues to dramatic events that happened early in solar system history, calling for a complicated history of planetary migrations and close encounters and maybe even some planets being kicked out of the solar system entirely to become free-floating rogue planets." 

Such perturbations are still dynamic enough that complicated calculations are required to navigate a spacecraft through these orbits, he said.

New Horizons principal investigator Alan Stern, of the Southwest Research Institute, noted there are four things to consider about Pluto's orbit. The first two are its inclination and eccentricity.

The third is Pluto's resonance with Neptune. This large gas giant is locked in a gravitational dance with Pluto that keeps both worlds in a consistent pattern of orbits with each other (which we also see at places like Saturn, which have a lot of moons in resonant orbits).

The fourth is what happens because of that resonance. Pluto comes closer to the sun than Neptune does for about 20 years of every orbit. "It is like clockwork," Stern said, explaining that when this happens, Neptune is always on the opposite side of the sun. "The two can never collide, because they're in this resonance."

Grundy added a fifth element to the list of important features of Pluto's orbit: Pluto and its largest moon, Charon, are relatively close in size; Charon is about half the mass of Pluto. He said that rather than thinking of Pluto as a world to itself, "think of that common center of mass" that Pluto and Charon share as they orbit the sun. 

Simply put, Grundy said, "Pluto and Charon are really a double planet," and that should be considered when mapping the orbit of the system.

Pluto is a resident of the region that astronomers call the Kuiper Belt, which is filled with icy worlds collectively known as Kuiper Belt Objects (KBOs). Stern noted that these objects are very different from one another, and there are at least four known types, so the best way to think about KBOs are as worlds occupying "the same zip code" in the outer reaches of the solar system.

As Pluto proceeds in its orbit, at times it is closer or farther from the sun and will react to the stronger or weaker sunlight. For example, Stern said, when the heating is stronger, Pluto will have a more massive atmosphere. "The increased sunlight is increased energy, and that energy sublimates more ice into gas, making for a thicker, more massive atmosphere," he explained.

On the surface, changes also occur. "Ices are moving around from place to place, just like on the Earth," Stern said. "With the seasons [on Earth], it snows in the winter, and if you took pictures of the Earth from another planet, you would see more bright zones than you would in the summertime on a given hemisphere. Then in the summer, you [see] the ice retreats. It's the same thing on Pluto and on Mars and on other objects as well."

New Horizons has enough power available to run for the next 20 years, and it is focusing on heliophysics (studying the sun) and astrophysics as it continues its mission, Stern said. Of course, the data from its Pluto encounter in 2015 continues to be available and analyzed for future science investigations, too.

As for Lowell, the observatory continues to celebrate its role in Pluto research with programs, exhibits of artifacts used in the discovery and tours of the telescope. The observatory also hosts an annual "I Heart Pluto Festival" to honor its historic connections to that distant world.

Originally published on Live Science.

An area of Pluto that researchers think was formed from the eruption of ice volcanoes is unique on the dwarf planet and in the solar system, a new study suggests.

NASA's New Horizons mission, which launched in 2006, took detailed photos of the surface of Pluto, a dwarf planet and the largest object in the Kuiper Belt. Now, a new analysis examines images of an area containing two main mounds that scientists have proposed are ice volcanoes. In the study, the researchers conclude that the surface around these mounds was likely formed by fairly recent activity of the ice volcanoes, or cryovolcanoes.

The finding raises the possibility that these volcanoes may still be active and that liquid water, or something like it, flows or recently flowed under the surface of Pluto. Recent activity also means that there is likely more heat in Pluto's interior than scientists previously thought. Given other recent research, the scientists say their work could even raise the possibility of life existing under Pluto's surface.

Related: 7 solar system worlds where the weather is crazy

The researchers analyzed photographs of a region dominated by two large mounds, called Wright Mons and Piccard Mons, which scientists think are cryovolcanoes. Wright Mons is a mount 2.5 to 3 miles (4 to 5 kilometers) high and about 90 miles (150 km) wide, while Piccard Mons is about 4 miles (7 km) high and 150 miles (250 km) wide. 

The suspected ice volcanoes also have extremely deep depressions at their peaks — the one on Wright Mons is about as deep as the mount is high. Many parts of the area also have an unusual, lumpy or "hummocky" appearance, made up of undulating, rounded mounds. The researchers think smaller mounds, formed from ice volcanoes, could have accumulated over time to form these two main mounds.

"There was no other areas on Pluto that look like this region," Kelsi Singer, a planetary scientist at the Southwest Research Institute in Boulder, Colorado and the lead author of the study, told Space.com. "And it's totally unique in the solar system."

Unlike other areas of Pluto, this area has few to no impact craters, indicating that the surface was formed relatively recently in geological time. Based on the lack of craters, the area is likely no older than one or two billion years old, with some areas likely being less than 200 million years old, Singer said.

In some ways, cryovolcanoes are analogous to volcanoes on Earth, since much of Pluto's surface is made of ice, and temperatures on Pluto are far below water's freezing point. That means that liquid water, or something like it which is at least partially fluid or mobile, would be like magma on Earth, coming up to the surface after an eruption and freezing, or hardening, into a solid.

"It's probably not coming up completely liquid — it's probably more like a slushy thing where you have some liquid and some ice, or it could even be more like a flowing solid," Singer said, which could be "more like ketchup or silly putty." It could even be more solid ice that can still flow.

"We all know that ice can flow because we have glaciers that flow on Earth," she said.

Though scientists don't totally understand how cryovolcanic activity on Pluto might work, it is likely powered by radiogenic heat created by the decay of radioactive elements in the dwarf planet's interior. A similar phenomenon is also one of the sources of heat in the Earth's interior, although Pluto does not have plate tectonics, the complex system of shifting continental crust that underlies geologic activity on Earth. Scientists call geologic activity like that on Pluto "general tectonics," which can still create features like faults in rock but does not have tectonic plates.

Pluto's cryovolcanoes display some similarities to shield volcanoes on Earth, which are low-profile volcanoes which form from the steady accumulation of lava flows into rounded structures. (Think of the Hawaiian island volcanoes, rather than an eruption like Mount Saint Helens or Vesuvius.) But shield volcanoes usually form from very liquid lava, unlike what scientists think happened on Pluto. 

Some volcanoes on Earth and other planets also have a depression in their middle called a caldera, formed when a newly erupted volcano collapses into the void left by all the material it spewed out. But the depression on Wright Mons is so deep that the volcano would have had to lose about half of its volume to be similar in shape to Mauna Loa, a shield volcano in Hawaii that is one of the largest volcanoes on Earth and has a comparatively small caldera, though the two structures are similar in volume, Singer said.

There's still a lot researchers don't know about these features, how they were formed, and how cryovolcanism works on Pluto. The idea that liquid water could exist beneath the surface of Pluto raises the chances of life existing on Pluto from practically non-existent to slightly more plausible, given other research suggesting that Pluto was hot when it first formed and could still have a liquid ocean under its icy surface.

"I think that it is a little more promising, and that there might be some heat and liquid, potentially liquid water closer to the surface," Singer said. "But there's still some big challenges for poor microbes that want to live on Pluto."

The research is described in a paper published Tuesday (March 29) in the journal Nature Communications.

Follow us on Twitter @Spacedotcom and on Facebook. 

Pluto's atmosphere is going through a strange transformation, scientists are finding. 

The icy dwarf planet, which lies over 3 billion miles (4.8 billion kilometers) away from Earth in the Kuiper Belt, caught astronomers' attention as it passed in front of a star back in 2018. 

With the star backlighting Pluto, the team of researchers was able to make observations of the dwarf planet and its atmosphere. With this unique view, they came to a surprising conclusion, which they describe in a new study. 

The astronomers found evidence that Pluto's atmosphere is starting to disappear. 

Related: Why Isn't Pluto a Planet Anymore?

Using telescopes at multiple sites in both the U.S. and Mexico, the team observed Pluto and its thin atmosphere, which is primarily made of nitrogen, like that of Earth. Pluto's atmosphere is supported by the vapor pressure of ices on the dwarf planet's surface. So, if ice warms up on Pluto, it can dramatically alter the density of its atmosphere, according to a statement from Southwest Research Institute (SwRI), the home institution of multiple members of the research team.

For about 25 years, Pluto has been moving farther and farther away from the sun, so its surface temperature has been going down. And with these recent observations, the researchers found evidence showing that Pluto's atmosphere is actually refreezing back onto its surface as the dwarf planet gets colder and colder. Pluto is so far from the sun that, as time goes on, it will get distinctly farther away (and colder) before getting closer to the sun in other regions of its immense orbit.

Thanks to a phenomenon known as thermal inertia, Pluto'ssurface pressure and atmospheric density continued to rise until 2018. Essentially, Pluto had residual heat from when it was closer to the sun. However, the inertia starting to wear off and, as Pluto gets colder, more and more of its atmosphere will freeze back onto its surface and "disappear."

"An analogy to this is the way the sun heats up sand on a beach," SwRI staff scientist Leslie Young, who studies the interaction between icy solar system bodies and their surfaces and atmospheres, said in the same statement. 

"Sunlight is most intense at high noon, but the sand then continues soaking up the heat over the course of the afternoon, so it is hottest in the late afternoon. The continued persistence of Pluto’s atmosphere suggests that nitrogen ice reservoirs on Pluto’s surface were kept warm by stored heat under the surface. The new data suggests they are starting to cool," Young said.

So how did they figure this out just by watching Pluto move in front of a star?

The researchers watched the star fade as Pluto moved in front of it and then come back into view once the dwarf planet had passed. Using the rate that the star came in and out of view, a transition that lasted about 2 minutes, they were able to determine the density of the dwarf planet's atmosphere, according to the statement.

This method relies upon what is known as an "occultation," an event that happens when one cosmic object is hidden by another passing in front of it. Studying occultations is an old and well-worn technique in the world of astronomy, and researchers have used it to study Pluto's atmosphere since 1988, Eliot Young, a SwRI senior program manager, said in the statement. 

"The New Horizons mission obtained an excellent density profile from its 2015 flyby, consistent with Pluto’s bulk atmosphere doubling every decade, but our 2018 observations do not show that trend continuing from 2015," Young added, referring to the pioneering NASA mission that gave the world its first up-close look at Pluto..

In observing Pluto as it passed in front of the star, the team noticed a "central flash" in the middle of the path of the dwarf planet's shadow. The flash, caused by Pluto's atmosphere refracting light into the center of the shadow, changed the light curve that usually happens during occultation from a "u-shape" into a "w-shape."

"The central flash seen in 2018 was by far the strongest that anyone has ever seen in a Pluto occultation," Young said. "The central flash gives us very accurate knowledge of Pluto’s shadow path on the Earth."

Young discussed the results of this study Oct. 4 at the 53rd American Astronomical Society Division for Planetary Sciences Annual Meeting.

Email Chelsea Gohd at cgohd@space.com or follow her on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.

We may live on a planet covered in water but Earth is not the only ocean world in our solar system. From dwarf planet Ceres, to Jupiter’s heavily cratered moon Callisto, check out 10 extraordinary places on our galactic doorstep. 

A rare version of the radioactive element plutonium embedded in Earth's crust below the deep sea is providing new clues as to how heavy metals form in the stars.

The new research finds that the isotope, called plutonium-244, may arrive on Earth in tandem with iron-60, a lighter metal known to form in supernovas, explosions that occur during the death throes of many types of stars. This finding suggests that supernovas may create both heavy metals — although it's possible that other events, such as the mergers of neutron stars, are responsible for at least some of the plutonium-244.

Understanding how heavy elements formed is one of the top three most burning questions in physics, said Anton Wallner, a nuclear physicist at the Australian National University and the Helmholtz Center Dresden-Rossendorf, a research center in Germany. Half of elements heavier than iron are built in the hearts of stars through a fairly well-understood process of fusion. The other half, though, requires a high density of free neutrons to form. This means they must form in a more explosive environment than a typical star core — supernovas, perhaps, or massive events such as a neutron-star merger or a collision of a black hole and a neutron star.

Related: Elementary, my dear: 8 elements you’ve never heard of

Along with collaborators in Japan, Australia and Europe, Wallner was interested in finding out if he could discover fingerprints of these celestial events on Earth. There are some radioactive versions of heavy metals that don't occur naturally on the planet. In particular, the researchers were on the hunt for plutonium-244, a variation of plutonium with a half-life of 80.6 million years. This means it takes 80.6 million years for radioactive decay to eat away at half of the initial plutonium produced. Any plutonium-244 originally present during Earth's formation has long since decayed, so any atoms the researchers could find would have to be extraterrestrial in origin.

"Can we find plutonium-244 on Earth?" Wallner said. "Then we know it's coming from space."

Rare metals 

To hunt for these rare atoms, the researchers turned to samples of Earth's crust from nearly 5,000 feet (1,500 meters) below the Pacific Ocean. These rocks form so slowly that a millimeter of crust records 400,000 years of history, Wallner told Live Science. The sample covered the past 10 million years.

The researchers then probed the samples for iron-60 — the extraterrestrial version of iron that forms in supernovas — and for plutonium-244. They found both. 

It was no surprise to find iron-60, Wallner said, as previous research had already shown fluctuations in iron-60 levels in deep-sea sediments and crust over time. The findings confirmed what researchers had previously suspected: There were two increases in iron-60 — one that occurred between 4.2 million and 55 million years ago, and one that happened sometime before 7 million years ago. These influxes of the metal may have been the result of two fairly nearby supernovas, Wallner said.

"The supernova that happened and produced the iron-60 must have been spectacular at the time," he said. "It must have been similar [in brightness] to the full moon, so you would see it even in daytime."

In the past, the researchers did not have sensitive enough methods to accurately count the extremely rare atoms of plutonium-244 scattered in Earth's crust. But in the new study, using cutting-edge technology and methods, they did. The timing of this extraterrestrial plutonium's arrival on Earth is a bit harder to pin down, as the researchers had to search layers of crust corresponding to between 3 million and 5 million years of history. However, the influx of plutonium-244 did correlate with the influx of iron-60.

"The ratio of plutonium-244 to iron-60 seems to be constant," Wallner said. This suggests that both might come from a common origin.

Forged in stars 

Although the coordinated arrival of plutonium-244 and iron-60 suggests that both could have come from supernovas, a lot of questions remain. Computer models that attempt to mimic the formation of elements within supernovas really struggle to generate heavy-element formation, Wallner said. The ratio of iron-60 to plutonium-244 found in the new study suggests that the plutonium-244 would be a lot less prevalent than iron-60 after the stellar explosion, perhaps just a small percentage of the total elements formed.

It's also possible, Wallner noted, that the plutonium-244 atoms discovered in the deep-sea crust didn't come from a supernova at all. The plutonium-244 could have been formed in an earlier event and may have been floating aimlessly in deep space when a blast of iron-60 whooshed through, pushing the heavier plutonium-244 along with it. In that situation, both elements would have arrived on Earth at the same time, but the plutonium-244 would be a lot older.

To explore that possibility, the researchers want to look at different classes of atoms with different half-lives. The half-lives act like a clock so that scientists can determine a range of estimates for the ages of the elements. If the plutonium-244 were found in concert with an element of a much shorter half-life, for example, it would suggest that both were younger and fresher. It would also suggest that the amount of plutonium-244 produced in a supernova was lower and that more of it may have come from other events, like a neutron-star merger.

The research team is already studying a piece of crust 10 times larger than the one in this research. Having a larger piece of crust will allow researchers to expand their search for plutonium-244 atoms and get a more precise timeline of when those atoms arrived on Earth.

"What is fascinating is that you find some six or 10 atoms which you can identify in the end as not from Earth but from space, and then you get some hints about where it had been produced and when it had been produced," Wallner said.

The research was published today (May 13) in the journal Science.

Originally published on Live Science.

Back in 2017, a point of strange light flashed through the solar system. Three years later, a team of scientists claims to have figured out what it was.

That light was the first interstellar visitor ever detected in solar space. Scientists put forth several eye-popping theories from that brief glance: Maybe it was a piece of discarded alien technology. Maybe it was a cosmic dust bunny, made of incredibly fragile frozen hydrogen. Now, another team of researchers claims to have cracked the problem of 'Oumuamua: It was a nitrogen iceberg, they argue, snapped off from a frigid alien Pluto.

Scientists reach for exotic theories to explain 'Oumuamua because it didn't look like any comet anyone had ever seen before. Not only did it hail from interstellar space, but it also flashed like a lighthouse beacon. No telescope snapped a decent enough image of it to reveal its shape, but that regular pattern of shifting light suggests that the object was spinning and waeither wide and flat like a frisbee or very long and thin like a cigar.

Related: The 12 strangest objects in the universe

Even stranger: It accelerated as it passed the sun, as if it were a weak rocket engine pumping out some sort of propellant. Comets do this too, but their propellant — water vapor — is visible to telescopes. Whatever was pushing 'Oumuamua along wasn't.

Both previous popular theories for 'Oumuamua's origin were designed to explain both features of the distant, flashing dot.

The alien technology theory, advanced by Harvard astrophysicist Avi Loeb, suggests that the object was a light sail — a wide, ultrathin and ultralight disk manufactured by some alien civilization that accelerated due to the pressure of solar radiation. The theory fits the data, but other scientists argued that jumping to an alien explanation isn't necessary. 'Oumuamua is weird, yes, but mainly because astronomers just didn't get a very good look at it, researchers wrote in 2019 in the journal Nature.

The cosmic dust bunny theory proposes that the object was more or less a cigar-shaped, puffy cloud of solid hydrogen and interstellar dust. Evaporating hydrogen would accelerate such an object passing near the sun just as water vapor coming off of a conventional comet. The problem with this theory is it's not clear whether there's any place in the universe cold enough to form a hydrogen iceberg, or whether such an iceberg would survive long enough to reach the solar system.

Hydrogen freezes just a few degrees above absolute zero — something that's never been seen happening anywhere in space — and even the distant light of faraway stars and the faint glow of the cosmic microwave background would cook a block of free-floating hydrogen iceberg to death if it drifted through the universe for too long.

Now, a new set of research discounts both theories in favor of a new one: the nitrogen iceberg hypothesis.

In a pair of papers published March 16 in the Journal of Geophysical Research: Planets, two Arizona State University researchers argue that a nitrogen iceberg fits the data at least as well as a hydrogen iceberg: Such an iceberg could have become a wide disk as its surface slowly evaporated during its travels through the galaxy, just like a bar of soap flattens over the course of many showers. And, like hydrogen, evaporating nitrogen wouldn't be visible to telescopes.

Even better: Astronomers have actually seen nitrogen ice form in space. Pluto's surface is studded with shiny chunks of it, which were seen when NASA's New Horizons probe flew by the dwarf planet in 2015.

Over time, the researchers wrote, collisions with other distant solar-system objects have probably knocked about trillions of nitrogen ice fragments at least 165 feet (50 meters) wide off of Pluto's surface. About 80% of those fragments were likely ejected into interstellar space. If other solar systems have their own Plutos, the researchers argued, there are likely plenty of interstellar chunks of the nitrogen out there. And calculations suggest that a disk of solid nitrogen just under a football field in diameter and about 25 feet (7.5 meters) thick would tumble through the solar system just like 'Oumuamua did.

"We've probably resolved the mystery of what 'Oumuamua is," Steven Desch, an Arizona State University astrophysicist and co-author of the new papers, said in a statement.

Garrett Levine, a Yale researcher who was not involved in the current research, but  has studied the conditions necessary to produce hydrogen icebergs, said the new work is very compelling but not yet definitive. 

Between the hydrogen and nitrogen explanations, he said, neither one is a clear winner. And astronomers can't solve the mystery of 'Oumuamua based solely on the brief observation of the interstellar object as it exited the solar system, which was conducted at the absolute limits of telescope technology, Levine said.

Of course, hydrogen icebergs exist "at the very edge of feasibility."

Related: 6 reasons astrobiologists are holding out hope for life on Mars

But nitrogen icebergs present their own problems. While hydrogen is the most common element in the universe, nitrogen is a lot rarer and the process described for making nitrogen icebergs would still only produce a fairly small and diffuse population of lonely nitrogen objects wandering the universe. ,

The Vera C. Rubin Observatory in Chile, which comes online in 2022, might resolve the question, Levine said. Current telescopes can't quickly identify and study 'Oumuamua-like objects. But the future telescope is designed to scan vast swaths of sky for faint, moving lights and closely observe them. It should spot interstellar visitors much more frequently and collect far more data on them. 

Levine’s back-of-the-envelope calculations estimate that if hydrogen icebergs were being produced in an ultracold interstellar dust cloud, a ballpark of about 10 ‘Oumuamua like objects would enter our solar system during Vera Rubin's first decade-long observing run that might be visible to the new telescope. But if 'Oumuamua was an interstellar nitrogen iceberg, Levinehe said, the new telescope would be lucky to even spot one.  

If there are lots of 'Oumuamuas out there, Levine said, that's likely a sign that the hydrogen hypothesis was correct. If no more 'Oumuamuas turn up, however, that would suggest the nitrogen iceberg may have been a better explanation, Levine said.

Originally published on Live Science.

The haze shrouding Pluto might be made up of ice crystals possessing cyanide hearts, a new study finds.

Hazes, which are made of tiny motes of dust, smoke, ice and other substances, are not unique to Earth — scientists have detected hazes enveloping Mars, Venus, Saturn and Jupiter. 

Nor are hazes limited to planets. Saturn's largest moon Titan, the only moon known to have clouds, is cloaked in a thick orange haze similar in makeup to smog on Earth. NASA's Voyager 2 spacecraft also detected haze in the atmosphere of Neptune's largest moon Triton.

Related: Pluto and Titan are surprisingly hazy, but scientists are fuzzy on exactly why 

NASA's New Horizons mission, which flew by Pluto in 2015, unexpectedly revealed Pluto also possesses haze hugging the dwarf planet's frozen surface. Since Pluto, Titan and Triton are all icy worlds with atmospheres rich in nitrogen, methane and carbon monoxide, researchers had thought they might all possess similar kinds of hazes.

Previous research found Titan's haze was the result of solar ultraviolet rays triggering chemical reactions in the icy moon's upper atmosphere. These created simple organic compounds that went on to help form ever larger and more complex organic molecules.

However, scientists now find the hazes on Pluto and Triton might have significantly different origins than Titan's.

"Although Titan and Pluto seem very similar, their haze properties can be drastically different," study lead author, Panayotis Lavvas, a planetary scientist at the University of Reims Champagne-Ardenne in Reims, France, told Space.com.

When the researchers analyzed computer models of haze formation, they discovered that if the same chemical reactions at work on Titan happened on Pluto, they could only create about half the haze particles that prior work detected on the dwarf planet. (Since Pluto's atmosphere is roughly 175 degrees Fahrenheit, or 80 degrees Celsius, colder than Titan's, its atmospheric chemistry runs slower.)

In the new study, Lavvas and his colleagues suggest that as on Titan, sunlight triggers chemical reactions in Pluto's upper atmosphere, forming simple compounds such as hydrogen cyanide, a molecule made up of one hydrogen, one carbon and one nitrogen atom. Their models suggested hydrogen cyanide might freeze to produce tiny ice particles.

As these microscopic ice particles settle downward due to gravity, other gases condense around these seeds to coat them. As such, haze particles on Pluto may be largely made of a variety of ices, instead of more complex organic compounds as on Titan. The sizes, amounts and atmospheric distributions of these ice grains in the computer models closely match those of the haze particles detected on Pluto, the scientists noted.

In addition, these ice particles on Pluto should interact less with incoming solar energy than haze particles on Titan, the researchers said. As such, they should play a smaller role in controlling heat in Pluto's atmosphere than previously thought, Lavvas said.

When it comes to Triton, since it possesses an even colder atmosphere than Pluto's, the scientists noted its hazes should prove even icier.

"This comparison of Titan, Pluto and Triton illuminates the important differences in the mechanisms of haze formation in planetary atmospheres," Lavvas said.

The scientists detailed their findings online Dec. 21, 2020, in the journal Nature Astronomy.

Follow Charles Q. Choi on Twitter @cqchoi. Follow us on Twitter @Spacedotcom and on Facebook. 

Pluto's icy heart is beating.

The dwarf planet's famous heart-shaped feature, which NASA's New Horizons spacecraft discovered during its epic July 2015 flyby, drives atmospheric circulation patterns on Pluto, a new study suggests.

Most of the action comes courtesy of the heart's left lobe, a 600-mile-wide (1,000 kilometers) nitrogen-ice plain called Sputnik Planitia. This exotic ice vaporizes during the day and condenses into ice again at night, causing nitrogen winds to blow, the researchers determined. (Pluto's atmosphere is dominated by nitrogen, like Earth's, though the dwarf planet's air is about 100,000 times thinner than the stuff we breathe.)  

Related: Destination Pluto: NASA's New Horizons mission in pictures

These winds carry heat, particles of haze and grains of ice westward, staining the ices there with dark streaks.

"This highlights the fact that Pluto's atmosphere and winds — even if the density of the atmosphere is very low — can impact the surface," study lead author Tanguy Bertrand, an astrophysicist and planetary scientist at NASA's Ames Research Center in California, said in a statement. 

And that westward direction is interesting in itself, considering that Pluto spins eastward on its axis. The dwarf planet's atmosphere therefore exhibits an odd "retrorotation," study team members said.

Bertrand and his colleagues studied data gathered by New Horizons during the probe's 2015 close encounter. The researchers also performed computer simulations to model Pluto's nitrogen cycle and weather, especially the dwarf planet's winds.

This work revealed the likely presence of westerly winds — a high-altitude variety that races along at least 2.5 miles (4 kilometers) above the surface and a fast-moving type closer to the ground that follows Sputnik Planitia's western edge.

That edge is bounded by high cliffs, which appear to trap the near-surface winds inside the Sputnik Planitia basin for a spell before they can escape to the west, the new study suggested.

"It's very much the kind of thing that's due to the topography or specifics of the setting," planetary scientist Candice Hansen-Koharcheck, of the Planetary Science Institute in Tucson, Arizona, said in the same statement. 

"I'm impressed that Pluto's models have advanced to the point that you can talk about regional weather," added Hansen-Koharcheck, who was not involved in the new study.

New Horizons' Pluto flyby revealed that the dwarf planet is far more complex and diverse than anyone had thought, featuring towering water-ice mountains and weird "bladed" terrain in addition to the photogenic heart (whose official name, Tombaugh Regio, honors the discoverer of Pluto, Clyde Tombaugh).

The new study, which was published online Tuesday (Feb. 4) in the Journal of Geophysical Research: Planets, reinforces and extends that basic message.

"Sputnik Planitia may be as important for Pluto's climate as the ocean is for Earth's climate," Bertrand said. "If you remove Sputnik Planitia — if you remove the heart of Pluto — you won't have the same circulation."

• Pluto flyby anniversary: The most amazing photos from NASA's New Horizons
• New Horizons probe's July 14 Pluto flyby: Complete coverage
• Ultima Thule in pictures: Flyby views of 2014 MU69 by New Horizons

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

Even though Pluto was officially downgraded from planetary status over a decade ago, fans of the solar system's underdog are still rooting for the small cosmic body. Case in point: NASA Administrator Jim Bridenstine, who stepped into the long-standing debate over Pluto's planethood during press remarks on Friday (Aug. 23).

"Just so you know, in my view, Pluto is a planet, and you can write that the NASA administrator declared Pluto a planet once again, Bridenstine said during a FIRST robotics event in Colorado this week. 

Video of the exchange was posted on Twitter by meteorologist Cory Reppenhagen of 9News. 

That verdict goes against the official decision made by the International Astronomical Union (IAU) back in 2006, which resulted from a vote among astronomers. 

But much to Pluto fans' delight, Bridenstine reiterated his dedication. Pluto is a planet, he said. "I'm sticking by that. It's the way I learned it, and I'm committed to it."

Pluto was discovered by American astronomer Clyde Tombaugh in 1930. Some researchers began calling Pluto's planethood into question in the late 1990s, after it became clear that Pluto was far from alone in the Kuiper Belt, the ring of icy bodies beyond Neptune's orbit. 

Following years of debate, and the 2005 discovery of Eris, a distant object even larger than Pluto, the IAU stripped Pluto of its planetary status.

Instead, the IAU determined, Pluto and similar bodies should be classified as dwarf planets. Planethood requires that an object orbit the sun, have a nearly round shape, and "clear its orbital neighborhood," according to the IAU, and the astronomers who voted in the 2006 decision weren't convinced Pluto met that last criterion.

But the decision was highly controversial, and remains so today. Many scientists and laypeople alike advocate for Pluto's planetary status. One of the most prominent is Alan Stern, principal investigator of NASA's New Horizons mission, which flew by Pluto in 2015, revealing a stunningly complex and diverse world with large mountains and vast nitrogen-ice plains.

Stern has long decried the IAU's decision as unscientific, claiming that it was made primarily to keep the number of "official" planets at a manageable number. 

• Should Pluto Be a Planet Again? Informal Vote Offers Support After Experts Debate 
• Pluto Flyby Anniversary: The Most Amazing Photos from NASA's New Horizons 
• Dwarf Planets of Our Solar System (Infographic) 

Follow Passant Rabie on Twitter @passantrabie. Follow us on Twitter @Spacedotcom and on Facebook. 

Editor's Note: This story was updated at 11:20 a.m. E.D.T. on June 24

In most maps of the solar system, you can expect to see the eight canonical planets (plus whatever Pluto is at the moment) trailing the fiery orange sun like polite little ducklings in a row. In biologist Eleanor Lutz's new map of the solar system, which shows the precise orbital paths of more than 18,000 near celestial objects, you'll be lucky if you can even find Mars.

Lutz is a doctoral candidate at the University of Washington who spends her evenings turning public data sets into hyperdetailed works of art. In her new project, called the Atlas of Space, she's borrowed more than a decade of data compiled by the likes of NASA, the U.S. Geological Survey and other science organizations to create some of the most accurate maps of the solar system that will fit on your bedroom wall. [10 Interesting Places in the Solar System We'd Like to Visit]

The map shown here, which Lutz posted to her website on June 10, was created from orbital data taken from a dozen different public databases. Going above and beyond most textbook space maps, this guide to the cosmos shows the asteroid belt between Mars and Jupiter and the Kuiper Belt beyond Neptune in gorgeous, chaotic detail.

"This map shows each asteroid at its exact position on New Years' Eve 1999," Lutz wrote on her site. "This includes everything we know of that's over 6.2 miles (10 kilometers) in diameter — about 10,000 asteroids — as well as 8,000 randomized objects of unknown size."

This is our solar system in macro. Over the coming weeks, Lutz also plans to share some more intimate views of Earth's nearest cosmic neighbors, including topographic maps of Mercury and Venus. While these lovely maps may not take you to another world, they'll probably blow your mind a little bit.

Want a taste? Take a look at the Kuiper Belt (the outermost ring of green asteroids) on the bottom of the map.

"You might notice that Pluto is shown inside Neptune's orbit," Lutz wrote. "It turns out that about 10% of the time, Pluto is actually closer to the sun than Neptune."

Editor's Note: A caption in the second image of this story was updated. Mercury is not the hottest planet, though it is quite hot.

• The 12 Strangest Objects in the Universe
• 15 Amazing Images of Stars
• 9 Strange Excuses for Why We Haven't Met Aliens Yet

Originally published on Live Science.

Ultima Thule is lumpy, and scientists aren't sure why.

Just after midnight on Jan. 1, NASA's New Horizons spacecraft zoomed past Ultima Thule, a small, frigid object that lies about 1 billion miles (1.6 billion kilometers) beyond the orbit of Pluto. 

The photos New Horizons captured during that epic encounter revealed a world unlike any other ever seen up close. The 21-mile-wide (33 km) Ultima Thule is bilobed and therefore resembles a reddish snowman — if that snowman had been flattened like a pancake.

Related: New Horizons' Ultima Thule Flyby in Pictures

"These are not spherical lobes at all," New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said April 23 during a presentation at the Outer Planets Assessment Group meeting at NASA headquarters in Washington, D.C. 

"That caught us by surprise," Stern added. "I think it caught everybody by surprise."

New Horizons imagery also revealed a number of abutting mound-like features on the larger of the two lobes, which mission team members call Ultima. (The smaller lobe, naturally, is Thule.)

"They seem to be raised, but exactly what causes them we're not sure," Stern said. "It's still early days."

An early hypothesis held that the mounds resulted from convection of low-temperature ice, which was driven by the heat generated by the radioactive decay of aluminum-26. But further work suggests that this is an unlikely scenario, Stern said. The team now thinks the mounds may be the retained outlines of the small planetesimals that came together to form the Ultima lobe long ago. 

"But there could be other processes as well," Stern said. "So, this is an active topic of debate."

Ultima Thule coalesced from a cloud of rocky, icy material far from the sun. These smaller chunks first formed two larger objects, which then apparently orbited a common center of mass as a binary pair, Stern said. These two bodies then slowly merged to form Ultima Thule (which is officially known as 2014 MU69).

The mission team has been able to put some "speed limits" on that merger, thanks to computer simulations. For example, if the two lobes had come together at about 22 mph (35 km/h), they likely wouldn't have merged at all; the collision would have been a glancing one, and Ultima and Thule would have gone their separate ways, Stern said.

A collision at 11 mph (18 km/h) would lead to a merger, but not one generating an object with two relatively intact lobes like Ultima and Thule; there would be considerable distortion, Stern said. So, the New Horizons team thinks the collision occurred at even slower speeds — perhaps around 5.5 mph (8.9 km/h). 

Indeed, the result of simulations with a 5.5-mph merging speed "is strikingly like what we actually observe," Stern said.

Mission scientists have seen no evidence of any type of atmosphere on Ultima Thule, nor have they spotted any signs of satellites or ring systems, he added during Tuesday's talk. But the team isn't done looking on these fronts; it will take another 16 months or so for New Horizons to finish beaming all of its encounter data home to Earth.

The Ultima Thule flyby was the second such encounter for New Horizons, which famously cruised past Pluto in July 2015, providing the first good looks at that complex and surprisingly active world. And we may get further close-ups from New Horizons in the future. The spacecraft is in good health and has enough fuel to zoom past a third object, if NASA approves another mission extension, Stern has said. 

• New Horizons' Historic Flyby of Ultima Thule: Full Coverage
• Destination Pluto: NASA's New Horizons Mission in Pictures
• New Horizons May Make Yet Another Flyby After Ultima Thule

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter@michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

Pluto may have been downgraded to a dwarf planet, but its mysteries still loom large. When NASA's New Horizons reconnaissance probe flew past Pluto and its moon Charon in 2015, the resulting footage revealed a novel world of icy peaks, glacial planes and frozen volcanoes not seen anywhere else in the solar system.

Now, researchers are looking at that footage again for clues about one of the solar system's most enigmatic regions: the vast ring of icy debris known as the Kuiper Belt.

In a new study published today (Feb. 28) in the journal Science, a team of planetary scientists led by researchers at the Southwest Research Institute in Boulder, Colorado, pored over New Horizons maps of Pluto and Charon to count the scars left by billions of years of collisions with rogue Kuiper Belt objects (KBOs). These icy bodies orbit the Kuiper Belt at the edge of the solar system (and Pluto itself is the largest of them). [When Space Attacks: The 6 Craziest Meteor Impacts]

By studying impact craters, the researchers discovered that Pluto and Charon had been battered by far more large objects than small ones over the last 4 billion years. This suggests that the Kuiper Belt is primarily populated by big, ancient objects that date to close to the formation of the solar system.

"Craters give you a window into the past," lead study author Kelsi Singer, a senior research scientist at the Southwest Research Institute and member of the New Horizons team, told Live Science. "We can use the number of craters to say how old a surface is, which helps us learn more about the Kuiper Belt as a whole."

Scars of Pluto

Generally, parts of a planet's surface speckled with lots of craters are thought to be relatively old, whereas regions without any craters are considered to be new developments, Singer said. On Pluto, for example, there is a bright sheet of nitrogen ice known as The Heart, named for its shape. Because there are no impact craters on this region, it is believed to be relatively young compared to the rest of Pluto's surface.

In contrast, past evidence suggested that some crater-rich regions of Pluto are about 4 billion years old, Singer said. By closely studying the sizes of the craters in those regions, the researchers can get a snapshot of the types of objects moving through the Kuiper Belt billions of years ago, not long after the solar system formed.

In the new study, the team examined nearly 3,000 impact craters from New Horizons' 2015 observations. Something stood out: While the craters came in a wide range of sizes, very few craters came from small objects measuring between 1 and 2 kilometers (0.6 and 1.2 miles) in diameter.

"This was surprising to us, because we based a lot of our expectations about the Kuiper Belt on what we knew about the asteroid belt [between Mars and Jupiter]," Singer said. "It turns out there are a lot fewer small objects in the Kuiper Belt than we thought. That tells us something about the area's collisional history."

How? Well, small celestial objects are created by collisions between larger objects, Singer said. A low number of small objects in the Kuiper Belt likely means that fewer collisions have occurred there over time — and that means that many of the objects orbiting in that region are more likely to be "primordial" relics of the early solar system, Singer said.

These findings fit with recent observations of the KBO called Ultima Thule, a 21-mile-long (34 km), snowman-shaped object orbiting about 1 billion miles (1.6 billion km) beyond Pluto's orbit.

"When New Horizons got to Ultima Thule in January, it looked like a fairly primordial body," Singer said. "There's maybe one big impact crater on it, and it doesn't look like it was ever broken apart and reformed."

If the Kuiper Belt is indeed full of ancient objects like these, studying the region's mysteries could shed light on the solar system's earliest days, Singer said. For its part, New Horizons will continue plunging into the frontier of icy debris at the edge of our solar system. What the probe will find next is anyone's guess.

• Crash! 10 Biggest Impact Craters on Earth
• Space Rocks! Photos of Meteorites for Sale
• Spaced Out! 101 Astronomy Images That Will Blow Your Mind

Originally published on Live Science.

The best-ever photos of Ultima Thule have made it down to Earth, and they heighten the intrigue about the frigid and faraway world.

On New Year's Day, NASA's New Horizons spacecraft zoomed past the 21-mile-long (34 kilometers) Ultima Thule, setting a record for the most distant planetary encounter in history. (Ultima lies about 1 billion miles, or 1.6 billion km, beyond Pluto, which New Horizons flew by in July 2015.)

New Horizons has been beaming flyby data and imagery home to its handlers ever since, and will continue to do so for approximately another 18 months. The early returns have been spectacular, revealing Ultima Thule to be a weird, snowman-pancake hybrid with mysterious features, such as a bright "collar' where the body's two lobes meet. [New Horizons at Ultima Thule: Full Coverage]

And now we have our best look yet. New flyby photos released today (Feb. 22) show Ultima in even more stunning detail, capturing bright ring-like features and dark pits, whose origins remain mysterious.

"Whether these features [the pits] are craters produced by impactors, sublimation pits, collapse pits or something entirely different is being debated in our science team," New Horizons Deputy Project Scientist John Spencer, of the Southwest Research Institute (SwRI) in Boulder, Colorado, said in a statement.

The newly released images boast a resolution of about 110 feet (33 meters) per pixel. They're the highest-resolution pictures New Horizons has ever taken, and may ever take during its operational life, mission team members said.

The photos were captured just 6.5 minutes before the craft's closest approach on Jan. 1. At the time, New Horizons was 4,109 miles (6,628 km) from Ultima Thule and 4.1 billion miles (6.6 billion km) from Earth, mission team members said. (Closest approach brought the probe within a mere 2,200 miles, or 3,500 km, of Ultima.)

There was no guarantee we'd ever get to see Ultima Thule — whose official name is 2014 MU69 — in this dazzling light. Capturing the newly released images was a "stretch goal," that demanded a precise square-up of Ultima in the narrow field of view of New Horizons' telescopic camera.

"Getting these images required us to know precisely where both tiny Ultima and New Horizons were — moment by moment — as they passed one another at over 32,000 mph [51,500 km/h] in the dim light of the Kuiper Belt, a billion miles beyond Pluto. This was a much tougher observation than anything we had attempted in our 2015 Pluto flyby," New Horizons Principal Investigator Alan Stern, also of SwRI, said in the same statement.

"These 'stretch goal' observations were risky, because there was a real chance we'd only get part or even none of Ultima in the camera's narrow field of view," Stern added. "But the science, operations and navigation teams nailed it, and the result is a field day for our science team! Some of the details we now see on Ultima Thule's surface are unlike any object ever explored before."

New Horizons launched in January 2006 to lift the veil on Pluto, which had remained largely mysterious since its 1930 discovery. The Ultima Thule flyby was the centerpiece of an extended mission that runs through 2021.

New Horizons is in good health and has enough fuel to fly by yet another distant object, if NASA grants another mission extension, Stern and other mission team members have said.

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate) is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.

The final photos that NASA's New Horizons spacecraft snapped of Ultima Thule during the probe's epic Jan. 1 flyby reveal the distant object to be far flatter than scientists had thought, mission team members announced today (Feb. 8).

"We had an impression of Ultima Thule based on the limited number of images returned in the days around the flyby, but seeing more data has significantly changed our view," New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said in a statement. [New Horizons at Ultima Thule: Full Coverage]

"It would be closer to reality to say Ultima Thule's shape is flatter, like a pancake," Stern added. "But more importantly, the new images are creating scientific puzzles about how such an object could even be formed. We've never seen something like this orbiting the sun."

The $720 million New Horizons mission launched in January 2006 to perform the first-ever flyby of Pluto. The probe aced this encounter in July 2015, revealing the dwarf planet to be a spectacularly diverse world of surprisingly varied and rugged landscapes.

New Horizons then set out an extended mission, which centered on a close flyby of Ultima Thule, which is officially known as 2014 MU69. This object is about 21 miles (34 kilometers) long and lies 1 billion miles (1.6 billion km) beyond Pluto's orbit. (Ultima is currently 4.1 billion miles, or 6.6 billion km, from Earth.)

Initial imagery taken during New Horizons' approach suggested that Ultima Thule is shaped like a bowling pin. But that impression changed shortly before closest approach, which occurred just after midnight on New Year's Day and brought the probe within 2,200 miles (3,540 km) of the mysterious body. Photos snapped around that time indicated that Ultima Thule is composed of two lobes, both of which appeared to be roughly spherical.

A snowman, with a distinctly reddish hue.

But the newly released images have forced another rethink. New Horizons took the long-exposure photos about 10 minutes after closest approach; the central frame in the sequence was snapped from a distance of 5,494 miles (8,862 km), mission team members said.

The new views were captured from a different angle than the snowman-suggesting photos, and they show Ultima Thule's outline against a number of background stars. By noting which of these stars went dark as Ultima blocked them out, mission scientists were able to map out the object's (surprisingly flat) shape.

"This really is an incredible image sequence, taken by a spacecraft exploring a small world 4 billion miles away from Earth," Stern said. "Nothing quite like this has ever been captured in imagery."

While the newly released images are the final ones New Horizons snapped of Ultima, they're far from the last pieces of data we'll see from the probe. It'll take a total of about 20 months for New Horizons to send home all of its flyby imagery and measurements, mission team members have said.

And Ultima Thule may not be the spacecraft's final flyby target. New Horizons is in good health and has enough fuel to zoom by another distant body, if NASA grants another mission extension, Stern has said. (The current extended mission runs through 2021.)

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us @Spacedotcom or Facebook. Originally published on Space.com.

The weirdly clustered orbits of some far-flung bodies in our solar system can be explained without invoking a big, undiscovered "Planet Nine," a new study suggests. 

The shepherding gravitational pull could come from many fellow trans-Neptunian objects (TNOs) rather than a single massive world, according to the research.

"If you remove Planet Nine from the model, and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs," study lead author Antranik Sefilian, a doctoral student in the Department of Applied Mathematics and Theoretical Physics at Cambridge University in England, said in a statement. [The Evidence for 'Planet Nine' in Our Solar System (Gallery)]

The hunt for Planet Nine — or, as some prefer to call it, Planet X or Giant Planet Five — began in earnest in 2014. That year, astronomers Chad Trujillo and Scott Sheppard proposed the existence of a large, unseen "perturber" beyond Neptune, whose gravitational influence could explain oddities in the orbits of distant objects like the dwarf planets Sedna and 2012 VP113.

In January 2016, Konstantin Batygin and Mike Brown contributed more evidence, announcing that other TNOs also appeared to bear this gravitational imprint. Batygin and Brown estimated that the perturber is perhaps 10 times more massive than Earth and lies about 600 astronomical units (AU) from the sun on average. (One AU is the Earth-sun distance — about 93 million miles, or 150 million kilometers.)

The case has been building ever since, as astronomers have found more and more "clustered" TNOs; the tally is up to about 30 at the moment. 

But Planet Nine's existence isn't a slam dunk: Some astronomers think the orbit-shaping tug is more likely coming from many small bodies. The new study, which Sefilian conducted with Jihad Touma of the American University of Beirut, explores this latter scenario.

The duo's modeling work suggests that the strength-in-numbers explanation does indeed work — if the mass of the Kuiper Belt, the ring of bodies beyond Neptune, is a few to 10 times that of Earth. This is a pretty big "if," given that most estimates peg the Kuiper Belt's mass at less than 10 percent that of Earth (and one recent study put the figure at 0.02 Earth masses).

But other solar systems are known to harbor massive disks of material in their outer reaches, Sefilian and Touma noted. And our failure to spot one around our own sun doesn't mean it doesn't exist, they stressed.

"The problem is, when you're observing the disk from inside the system, it's almost impossible to see the whole thing at once. While we don't have direct observational evidence for the disk, neither do we have it for Planet Nine, which is why we're investigating other possibilities," Sefilian said.

"It's also possible that both things could be true — there could be a massive disk and a ninth planet," he added. "With the discovery of each new TNO, we gather more evidence that might help explain their behavior."

The new study has been accepted for publication in the Astronomical Journal. You can read it for free at the online preprint site arXiv.org.

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate) is out now. Follow him on Twitter @michaeldwall. Follow us @Spacedotcom or Facebook. Originally published on Space.com. 

The most distant celestial object ever explored may well have moons, and astronomers are trying hard to find them.

In the wee hours of Jan. 1, NASA's New Horizons spacecraft zoomed past the small, frigid object Ultima Thule, which lies more than 4 billion miles (6.4 billion kilometers) from Earth. The probe has beamed home just a tiny fraction of its flyby data so far, but mission team members are already starting to get the goods on the far-flung rock.

For example, scientists now know that the 21-mile-long (33 km) Ultima Thule is composed of two roughly spherical lobes, which apparently began their lives as independent, free-flying objects. The duo quickly spiraled closer and closer together, joining up in the solar system's earliest days to form a reddish "snowman." [New Horizons at Ultima Thule: Full Coverage]

Modeling work suggests that the two constituent bodies, dubbed "Ultima" and "Thule," likely completed one rotation every 3 or 4 hours around the time when they hooked up, mission team members said. But New Horizons' observations show that the present-day Ultima Thule takes about 15 hours to make a full spin.

"So, how did they slow down? Well, the best way to understand that is if there were another moon, or two or three, orbiting this system," Mark Showalter, a New Horizons co-investigator from the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, said during a news conference Thursday (Jan. 3).

"Essentially, what those moons would do is put the brakes on the two bodies in the middle — slow them down" by carrying away the duo's angular momentum, he added.

So the hunt for Ultima Thule satellites — which began in earnest a while back, when the mission team was investigating potential hazards that could complicate the epic New Year's Day flyby — is hardly a lark.

The mission team has ruled out the existence of any sizable moons at least 500 miles (800 km) away from Ultima Thule, or within 100 miles (160 km) of the object, Showalter said. But that middle zone is a big question mark and will remain so until late January, when New Horizons beams home observations covering the region.

And, crucially, that in-between zone is the most likely place for satellites to exist in the system, Showalter said.

He and his colleagues are really hoping they turn up at least one moon, for such a find would help them tease out key details about Ultima Thule that they'll be hard-pressed to determine any other way.

"Any moon at all, on any orbit at all, will tell us the mass and the density to pretty decent usable precision," Showalter said. "And so we’re very, very excited about that prospect."

Even if the search ultimately comes up empty, that doesn't mean Ultima Thule — which is officially known as 2014 MU69 — never hosted moons, he added. As "braking" satellites carry away angular momentum from their systems' central bodies, these moons move farther and farther out into space. So, it's possible that Ultima Thule once had such satellites, but these moons moved so far away that they were eventually lost.

The $700 million New Horizons mission launched in January 2006, tasked with returning the first up-close images of Pluto. The mission aced this goal when it cruised past the dwarf planet in July 2015, revealing Pluto to be a world of stunning beauty and geological diversity.

The Ultima Thule flyby is the centerpiece of New Horizons' extended mission, which runs through 2021. The spacecraft has enough fuel and power, and is in good enough health, to potentially fly past a third object, if NASA grants another mission extension, team members have said.

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate) is out now. Follow him on Twitter @michaeldwall. Follow us @Spacedotcom or Facebook. Originally published on Space.com. 

p.p1 {margin: 0.0px 0.0px 10.0px 0.0px; font: 10.0px Arial; color: #000000; -webkit-text-stroke: #000000} p.p2 {margin: 0.0px 0.0px 10.0px 0.0px; font: 10.0px Arial; color: #00006d; -webkit-text-stroke: #00006d} span.s1 {font-kerning: none} span.s2 {text-decoration: underline ; font-kerning: none; color: #00006d; -webkit-text-stroke: 0px #00006d} span.s3 {text-decoration: underline ; font-kerning: none}

New Horizons' flyby of the distant object Ultima Thule now has its own soundtrack. 

Astrophysicist Brian May, lead guitarist for the band Queen, released a new single called "New Horizons" just after midnight EST (0500 GMT) on New Year's Day to highlight the flyby, which peaked about 30 minutes later when the NASA spacecraft zoomed within 2,200 miles (3,500 kilometers) of Ultima Thule. 

Ultima lies about 4 billion miles (6.4 billion km) from Earth and is now the farthest-flung celestial body ever to be visited by a spacecraft. [New Horizons at Ultima Thule: Full Coverage]

"This project made music in my head, and that's what you're hearing," May told reporters on Monday (Dec. 31). 

"This mission is about human curiosity," he added. "It's about the need of humankind to go out and explore."

A brand-new single

After its famous flyby of Pluto in July 2015, New Horizons began an extended mission to visit another object in the Kuiper Belt, a band of icy rocks beyond Neptune's orbit. That second target is officially known as 2014 MU69, and has been nicknamed Ultima Thule by the mission team. 

In visiting one of the most primitive and pristine objects in the solar system's dark outer reaches, New Horizons is probing the building blocks of planets, and the solar system's earliest history. 

The 22-mile-wide (35 km) Ultima Thule is the first small Kuiper Belt object ever visited. This makes the encounter incredibly intriguing, because "there is nothing more exciting in a world of exploration than going to a place about which you know nothing," May said. "The sky's the limit for what we could find out."

According to May, New Horizons principal investigator Alan Stern, a planetary scientist at the Southwest Research Institute in Colorado, called him up to request he write a song for the mission. May said he initially had some reservations.

"I thought this was going to be hard, because I can't think of anything that rhymes with Ultima Thule," he said.

May said that, as he considered the mission's goals, he began to hear in his head "the music of an object plummeting through space faster than anything before." He realized that, rather than trying to focus on the specifics of the mission itself, he should instead work to incorporate the spirit of exploration embodied by the spacecraft. [Destination Pluto: NASA's New Horizons Mission in Pictures]

May sat down with English lyricist Don Black, who penned iconic songs such as "Born Free," which May described as "a very forward-looking song." The next morning, May woke up with what he described as two simple verses in his inbox from Black — verses that inspired him and kicked him into action.

Tucked into the new single are quotes from the late cosmologist Stephen Hawking. The song both begins and concludes with those quotes, while a third is tucked in the middle.

May played his new single just after midnight on New Year's Day, when it became available on iTunes and various streaming platforms. A YouTube video that May said he and his colleagues made on a "shoestring budget" will also be available, and will also tell the story of the mission team.

"This became a song which is an anthem to human endeavor," May said. "The human race explores because it needs to know."

Art and science

Astronomy isn't a new field for May, who is a full member of the New Horizons team. Before he began his career with the classic-rock band Queen, May spent four years working in a postgraduate physics program. He said he left science because he didn't think he was good enough, and so turned his attention more to music. But his success with Queen didn't quite rid him of his childhood desire to become an astronomer.

"I always had that thing in me, that unfinished circle," he said.

May returned to London's Imperial College andreceived his Ph.D. in 2007. Since then, he has worked with stereoscopic images, helping to make three-dimensional scientific photographs. He was introduced as a New Horizons team member during the Pluto flyby in 2015.

One of the biggest boundaries May said he struggles with is the forced dichotomy between art and science. "I was brought up in school to believe that if you were a scientist, you could not be an artist," he said. "I've kind of fought [against] that all my life."

May has found many astronomers and astronauts to be musicians and artists. "It feels as though [human]kind is coming back around and putting the two halves of its existence back together," he said.

May said he feels as though his spirit is anchored in Victorian times, when people made no distinction between science and art. He compared the scientific process explored as part of the New Horizons team to the musical creation process, with everyone bringing something to the table.

"The greatest things happen when you're interacting with people," May said.

Previously, May had composed only one science-related song — "39," the story of a group of space explorers who return home to find that, during their one year of travel, a century had passed on Earth. However, after creating "New Horizons," he said that he feels ready to consider creating other science-related music.

"I think my head has moved," May said. "I think I'm in a place where I am ready to make space music now."

As for the new song, did he ever find something to rhyme with Ultima Thule?

"Do you think I would make such a foolie of myself?" May laughed.

Follow Nola Taylor Redd on Twitter @NolaTRedd. Follow uson Twitter@Spacedotcom and on Facebook. Original article on Space.com. 

Sing along if you know the words: When the moon hits your eye, like a big pizza pie, that's a planet!

That's what ancient Greek stargazers thought, anyway. And according to a team of astronomers writing online Aug. 29 in the journal Icarus, maybe it's time we started thinking of Earth's trusty satellite — plus demoted dwarf planets like Pluto and Ceres — as a full-fledged planets once again.

That might sound odd, but it's well within the consensus established by centuries of scientific literature, the new study argues. The authors of that study scoured more than 200 years of scientific research to try to answer two simple questions: What makes a planet, according to the scientific community; and does that definition fall in line with the criteria that the International Astronomical Union (IAU) used in 2006 when they famously stripped Pluto of its planethood? [Top 10 Amazing Moon Facts]

The IAU's judgment on Pluto (and therefore all celestial bodies seeking planethood) does not speak for the scientific community, the authors determined.

"There are 120 examples I found of scientists in the recent published literature violating the IAU definition, calling something a planet even though the IAU definition says it's not a planet," lead study author Philip Metzger, a planetary scientist at the University of Central Florida, told NBCNews.com. "The reason planetary scientists do this is because the IAU definition is not useful for science."

What makes a planet?

After 76 years of textbook planet status, Pluto was demoted to a dwarf planet in 2006 when the IAU voted that the icy orb failed to meet a set of criteria established by decades of scientific literature.

To be a planet, they said, Pluto must: 1) Orbit the sun; 2) Be massive enough to it pull itself into a sphere shape under its own gravity; 3) "Clear its neighborhood" of debris and other celestial bodies, proving it has gravitational dominance in its corner of the solar system.

Pluto fails that third criterion. Because it orbits in the Kuiper Belt — a massive ring of asteroids and planetoids that stretches beyond the orbit of Neptune — Pluto is surrounded by thousands of other celestial bodies and hunks of debris, each exerting its own gravity. Pluto, then, is not the gravitationally dominant object in its neighborhood — and therefore, the IAU said, not a full-fledged planet.

According to Metzger and his colleagues, however, that third bit of the criterion is unnecessarily narrow — and not reflected in the consensus of past research. After looking at hundreds of published astronomy papers going back to the 1800s, the researchers found that, while most writers agreed that planets should be spherical and orbit the sun, the "clearing the neighborhood" rule appeared only in a single 1801 paper.

The researchers concluded that this Pluto-excluding rule is "arbitrary and not based on historical precedent," and should be therefore ignored when defining any present or future planet-size objects in our universe.

"When Galileo described the moons of Jupiter, he described them as planets," study co-author Kirby Runyon, a planetary geologist at the Johns Hopkins University Applied Physics Laboratory, told NBCNews.com. "So, the actual historical precedent is to consider round worlds that orbit other planets as planets, too. And we consider dwarf planets to be full-fledged planets."

By this revised standard Pluto is a planet. So is Earth's moon, and the moons of Jupiter and Saturn. That's a controversial view that has already garnered some astro-criticism — and the study authors are fine with that. If anyone disagrees with their assessment, the researchers wrote, it should be up to the consensus of scientific literature to change it, not a vote by a single authority like the IAU.

The study will be detailed in the February 2019 print issue of Icarus.

Originally published on Live Science.

There's a "hydrogen wall" at the edge of our solar system, and NASA scientists think their New Horizons spacecraft can see it.

That hydrogen wall is the outer boundary of our home system, the place where our sun's bubble of solar wind ends and where a mass of interstellar matter too small to bust through that wind builds up, pressing inward. Our host star's powerful jets of matter and energy flow outward for a long stretch after leaving the sun — far beyond the orbit of Pluto. But at a certain point, they peter out, and their ability to push back the bits of dust and other matter — the thin, mysterious stuff floating within our galaxy's walls — wanes. A visible boundary forms. On one side are the last vestiges of solar wind. And on the other side, in the direction of the Sun's movement through the galaxy, there's a buildup of interstellar matter, including hydrogen.

And now NASA researchers are pretty sure that New Horizons, the probe that famously skimmed past Pluto in 2015, can see that boundary.

What New Horizons definitely sees, the researchers reported in a paper published Aug. 7 in the journal Geophysical Research Letters, is some extra ultraviolet light — the kind the researchers would expect such a wall of galactic hydrogen to produce. That replicates an ultraviolet signal the two Voyager spacecraft — NASA's farthest-traveling probes, which launched in the late 1970s — spotted all the way back in 1992. [Images: Dust Grains from Interstellar Space]

However, the researchers cautioned, that signal isn't a sure sign that New Horizons has seen the hydrogen wall, or that Voyager did. All three probes could have actually detected the ultraviolet light from some other source, emanating from much deeper in the galaxy, the researchers wrote.

But Alice, the instrument on board New Horizons responsible for this finding, is much more sensitive than anything the Voyagers had on board before beginning their own journey out of the solar system, the researchers wrote. And they said they expect Alice to function 15 to 20 more years.

New Horizons will continue to scan the sky for ultraviolet light twice a year, the researchers wrote, and report what it sees back to Earth.

Editor's note: This story originally included a quote mistakenly attributed to a press release by the researchers. The quoted phrase was in fact written by reporter Lisa Grossman of Science News in her coverage of this paper, and his been removed.

Originally published on Live Science.

Don't call it Planet Nine. 

That nickname for the big world that may lurk unseen in the far outer solar system doesn't show the proper amount of respect to the discoverer of the original ninth planet, Pluto, a group of researchers argues in a new message to their colleagues.  

The International Astronomical Union (IAU) famously reclassified Pluto as a "dwarf planet" in 2006. That decision remains highly controversial today, as made clear by the new note, which appeared in the July 29 issue of the Planetary Exploration Newsletter.

The message, titled "On the Insensitive Use of the Term 'Planet 9' for Objects Beyond Pluto," reads:

"We the undersigned wish to remind our colleagues that the IAU planet definition adopted in 2006 has been controversial and is far from universally accepted. Given this, and given the incredible accomplishment of the discovery of Pluto, the harbinger of the solar system's third zone — the Kuiper Belt — by planetary astronomer Clyde W. Tombaugh in 1930, we the undersigned believe the use of the term 'Planet 9' for objects beyond Pluto is insensitive to Professor Tombaugh's legacy.

"We further believe the use of this term should be discontinued in favor of culturally and taxonomically neutral terms for such planets, such as Planet X, Planet Next or Giant Planet Five."

Nearly three dozen researchers signed onto this message. They are:

• Paul Abell
• Michael Allison
• Nadine Barlow
• James Bauer
• Gordon Bjoraker
• Paul Byrne
• Eric Christiansen
• Rajani Dhingra
• Timothy Dowling
• David Dunham
• Tony L. Farnham
• Harold Geller
• Alvero Gonzalez
• David Grinspoon
• Will Grundy
• George Hindman
• Kampalayya M. Hiremath
• Brian Holler
• Stephanie Jarmak
• Martin Knapmeyer
• Rosaly Lopes
• Amy Lovell
• Ralph McNutt
• Phil Metzger
• Sripada Murty
• Michael Paul
• Kirby Runyon
• Ray Russell
• John Stansberry
• Alan Stern
• Mike Summers
• Henry Throop
• Hal Weaver
• Larry Wasserman
• Sloane Wiktorowicz

Leonard David is author of "Mars: Our Future on the Red Planet," published by National Geographic. The book is a companion to the National Geographic Channel series "Mars." A longtime writer for Space.com, David has been reporting on the space industry for more than five decades. Follow us @Spacedotcom, Facebook or Google+. This version of the story published on Space.com.

While hunting for the proposed Planet Nine, a massive planet that some believe could lie beyond Pluto, a team of scientists, led by Scott Sheppard from the Carnegie Institution for Science, found the 12 moons orbiting Jupiter. With this discovery, Jupiter now has a staggering 79 known orbiting moons — more than any other planet in the solar system.

Of the 12 newly discovered moons, 11 are "normal," according to a statement from the Carnegie Institution for Science. The 12th moon, however, is described as "a real oddball," because of its unique orbit and because it is also probably Jupiter's smallest known moon, at less than 1 kilometer (0.6 miles) in diameter, Sheppard said in the statement. [Photos: The Galilean Moons of Jupiter]

In the spring of 2017, these researchers were searching for Planet Nine in the region past Pluto, and "Jupiter just happened to be in the sky near the search fields where we were looking," Sheppard said. This gave the team a unique opportunity to search for new moons around Jupiter in addition to objects located past Pluto, according to the statement.

These two groups of prograde and retrograde moons consist of "irregular" satellites, or moons whose orbits have irregular, or noncircular, shapes.

In addition to these two groups, Jupiter has "regular" satellites, or moons with nearly circular orbits. These regular satellites consist of an inner group of four moons that orbit very closely to the planet and a main group of four Galilean moons that are Jupiter's largest moons.

The newly discovered "oddball" moon has a prograde orbit, but it orbits farther from Jupiter than the other moons in the larger prograde group and it takes about one and a half Earth years to complete an orbit. The satellite's oddness comes from its tiny size and the fact that, although it's out in the realm of the retrograde moons, it's orbiting in the opposite direction to them. Researchers have proposed naming the "oddball" Valetudo, after the Roman goddess of health and hygiene.

Valetudo is more than just the odd moon out; it's also a serious collision hazard.

Because it's orbiting in the opposite direction of the nine "new" retrograde moons, and across their paths, there is a high risk that it will hit one of them, according to the statement.

"This is an unstable situation," Sheppard said. "Head-on collisions would quickly break apart and grind the objects down to dust."Some of Jupiter's moons and moon groupings, including the "oddball," could have formed from collisions like this, according to the statement.

While researchers aren't certain if this is exactly what happened, understanding how and when Jupiter's moons formed could help scientists to better understand the early solar system as a whole, the statement said.

For example, a large amount of gas and dust would push very small moons (moons between 1 and 3 kilometers (.6 and 1.9 miles) in diameter) toward their planet. This means that such gas and dust couldn't have been present when earlier, larger moons collided and created these small moons. So, by that logic, moons of that small size must have formed after the era of planet formation, a time when a disk of gas and dust swirled around the sun and formed planets, according to the statement.

Email Chelsea Gohd at cgohd@space.com or follow her @chelsea_gohd. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

The long-simmering argument about Pluto's planethood has just flared up again.

For more than 75 years after its 1930 discovery, Pluto was regarded as our solar system's ninth planet — a distant and frigid oddball, to be sure, but a member of Earth's immediate family nonetheless. Then, in 2006, the International Astronomical Union (IAU) reclassified Pluto as a "dwarf planet," a newly created category that the organization explicitly stressed made Pluto distinct from the eight "true" planets.

A true planet, according to the IAU's newly devised definition, must meet three criteria: It must circle the sun and no other object (so, moons are out); it must be big enough to be rounded into a sphere or spheroid by its own gravity, but not so large that its innards host the fusion reactions that power stars; and it must have "cleared its neighborhood" of other orbiting bodies. [Photos of Pluto and Its Moons]

Pluto failed at this last hurdle, because its neighborhood — the ring of icy bodies beyond Neptune known as the Kuiper Belt — is far from cleared. 

Many scientists and Plutophilic members of the public objected strongly to the IAU's decision, on various grounds. For starters, some folks pointed out, the new planet definition rules out anything not orbiting the sun — meaning that the hundreds of billions of exoplanets in our Milky Way galaxy aren't planets at all, at least according to the IAU. 

And the "clear your neighborhood" requirement seemed ridiculous to many researchers, including Alan Stern, the principal investigator of NASA's New Horizons mission, which famously flew by Pluto in July 2015. Stern has been a vocal proponent of Pluto's planethood and has argued that the IAU's decision stemmed at least partly from a very nonscientific desire to keep the solar system's planetary stable down to a "manageable" number.

Which brings us to the most recent flare-up. Stern and planetary scientist David Grinspoon have just published a book about the Pluto flyby, called "Chasing New Horizons: Inside the Epic First Mission to Pluto" (Picador, 2018). On Monday (May 7), The Washington Post published a "Perspectives" piece the two scientists wrote titled, "Yes, Pluto Is a Planet."

In the piece, Grinspoon and Stern took aim at the IAU's "hastily drawn" and "flawed" planet definition, reserving special ire for the "clearing your neighborhood" requirement.

"This criterion is imprecise and leaves many borderline cases, but what's worse is that they chose a definition that discounts the actual physical properties of a potential planet, electing instead to define 'planet' in terms of the other objects that are — or are not — orbiting nearby," the scientists wrote. "This leads to many bizarre and absurd conclusions. For example, it would mean that Earth was not a planet for its first 500 million years of history, because it orbited among a swarm of debris until that time, and also that if you took Earth today and moved it somewhere else, say out to the asteroid belt, it would cease being a planet." [Destination Pluto: NASA's New Horizons Mission in Pictures]

The duo pushed instead for a much simpler "geophysical planet definition," which was presented last spring at a planetary science conference in Texas. And this definition is indeed simple; boiled down, it holds that planets are "round objects in space that are smaller than stars." 

Under this definition, Pluto and other dwarf planets, such as Ceres and Eris, are considered planets, as are large moons like Jupiter's Europa, Ganymede, Io and Callisto and Saturn's huge satellite Titan (as well as Earth's own moon). Indeed, the solar system's planet count would easily top 100 if everyone agreed to use the geophysical definition.

But getting such widespread agreement about this, and about Pluto's "official" classification, will be a hard row to hoe. For example, astrophysicist and author Ethan Siegel argued in a piece for Forbes on Tuesday (May 8) that a cosmic object's environmental context is important to understanding the object's nature.

"The simple fact is that Pluto was misclassified when it was first discovered; it was never on the same footing as the other eight worlds. The 2006 move by the IAU was an incomplete attempt to repair that mistake," Siegel wrote.

The geophysical definition, he added, "is a step in the opposite direction: It's a step towards making a larger, more confusing mistake that will render a definition meaningless to the majority of people who use it."

They roamed Earth millions of years ago, have spurred blockbuster movies and fascinated the studied paleontologist and the curious toddler. Yes, dinosaurs.

Dinosaur Detective: Find Out What You Really Know

And then there's the pithy take by California Institute of Technology astronomer Mike Brown, whose discovery of outer-solar-system objects helped spark the rethink of Pluto's place in the solar system.

"So, hey, Pluto is still not a planet. Actually, never was. We just misunderstood it for 50 years. Now, we know better. Nostalgia for Pluto is really not a very good planet argument, but that's basically all there is. Now, let's get on with reality," Brown wrote via Twitter, where his handle is @plutokiller. 

Brown also wrote a book, titled "How I Killed Pluto and Why It Had It Coming" (Spiegel & Grau, 2010), so his feelings on the topic are pretty well-known.

Will the geophysical planet definition catch on? Will the IAU welcome Pluto back into the "true planet" fold, along with Ceres, Europa, Titan, Earth's moon and many other objects? Who knows? But it seems clear that people will be fighting about this stuff for a long time to come.  

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

As Earthlings, we have the privilege of ooohing and aaahing at total solar eclipses, those dazzling celestial events in which the moon blocks the sun's light from hitting our planet. But is Earth the only world in our solar system that experiences this spectacular phenomenon?

The answer is no. Total solar eclipses can happen on other planets too, as long as they have moons that are big enough to cover the sun's disk from the planet's perspective and orbit the planet on the same plane as the sun, astronomers told Live Science.

A total solar eclipse occurs when a planet, its moon and the sun are aligned along the same plane, and a substantially sized moon passes between the planet and its sun, totally blocking the sun's light from reaching the planet.

"To get a solar eclipse, the first thing you need is a moon," said Christa Van Laerhoven, a postdoctoral fellow of astronomy at the University of British Columbia in Canada. "This immediately rules out solar eclipses on Mercury or Venus" — two planets without moons, she said. [What Would Earth Be Like with Two Suns?]

Mars has two moons — Phobos and Deimos — but both are too small to create total solar eclipses that would be visible from the Red Planet. Rather, these moons can make partial eclipses for any potential life-form (or Mars rover) watching from the ground, Van Laerhoven said.

"The view from those little moons is more interesting: They see Mars eclipsing the sun very frequently, and during some seasons, it happens every day," astronomer Matija Cuk wrote on the Cornell University blog "Ask an Astronomer."

The gas giants — Jupiter, Saturn, Uranus and Neptune — can all have total solar eclipses, as they have large moons and the sun appears small to them, Cuk said. But because these planets are made of gas, it would be impossible to stand on them and see such solar eclipses, he said.

However, if you had a special spaceship that could hover near the swirling gas giants, you could very well glimpse a solar eclipse. Jupiter has up to 67 moons, including Ganymede, the largest moon in the solar system. Because Jupiter's moons orbit on the same plane as the sun, the planet can have solar eclipses, Cuk and Van Laerhoven said.

In fact, if you could land on one of Jupiter's moons, you could see its other moons eclipse the sun, the astronomers said.

But what about dwarf planets, such as Pluto? "Charon [Pluto's largest moon] is large enough and close enough to Pluto to produce total solar eclipses for Pluto," Van Laerhoven said. But because the same side of Pluto and Charon always face each other, "only one side of both Pluto and Charon will ever experience eclipses," Cuk wrote.

On Earth, the moon is almost perfectly suited to making eclipses. The moon is the right size — that is, it appears to be the same size or larger than the apparent size of the sun, as seen from Earth. "This means that when the moon passes in front of the sun, the photosphere [the sun's luminous outer shell] gets covered, but the corona [the sun's upper atmosphere] remains visible," Van Laerhoven said. [Why Total Solar Eclipses Are Total Coincidences]

She noted that Earth's moon is slowly moving farther away from our planet, so in the distant future, the moon's apparent size will be too small to entirely cover the sun, at least from Earth's perspective. This means that one day, the moon won't be able to cause total solar eclipses but only annular eclipses, in which a "ring" of the sun's disk is still visible, Van Laerhoven said. Experts speculate that Earth will experience its last total solar eclipse about 600 million years from now.

For now, however, the moon is in prime location to cause a total solar eclipse.

"The reason we don't get solar eclipses every month is because the moon's orbital plane is slightly misaligned from Earth's orbital plane around the sun," Van Laerhoven said. "If they are misaligned, that decreases your chances of having a solar eclipse. You only get it when things happen to be lined up."

REMEMBER: Looking directly at the sun, even when it is partially covered by the moon, can cause serious eye damage or blindness. NEVER look at a partial solar eclipse without proper eye protection. Our sister site Space.com has a complete guide for how to view an eclipse safely.

Original article on Live Science.

On the two-year anniversary of the New Horizons probe's flyby of Pluto, mission scientists unveiled two detailed global maps of the dwarf planet and its largest moon, Charon. The combined data can now give the public insight about the mountains, volcanoes and canyons of these distant celestial neighbors.

"The complexity of the Pluto system — from its geology to its satellite system to its atmosphere — has been beyond our wildest imagination," Alan Stern, New Horizons principal investigator from the Southwest Research Institute in Boulder, Colorado, said in a statement. The data New Horizons collected during its encounter with Pluto on July 14, 2015 continues to reveal more secrets about the dwarf planet, according to the release.

New Horizons' scientists used a technique that accounted for how something appears when photographed from different places. The phenomena, also known as parallax, allowed scientists to determine how elevated or sunken the features imaged by their Long-Range Reconnaissance Imager (LORRI) on New Horizons were. The probe's Multispectral Visible Imaging Camera measured elevations that appear in the photographs as assorted colors.

Some of the features shown on the map are the icy plains of Pluto's Sputnik Planitia and the ridges of Tartarus Dorsa. The new maps show that the former ninth planet of the solar system is home to cryovolcanoes, or ice volcanoes, as well as mountain peaks, craters and canyons. Pluto's largest moon, Charon, is also dotted with craters and overlaid with expansive plains.

"Everywhere we turn are new mysteries," Stern said. "These new maps from the landmark exploration of Pluto by NASA's New Horizons mission in 2015 will help unravel these mysteries and are for everyone to enjoy."

Follow Doris Elin Salazar on Twitter @salazar_elin. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

In 2015, scientists learned that there’s a giant red "whale" on Pluto. This dark-colored region could be the mark of a giant impact — the same one that produced Pluto’s huge moon Charon, according to a group of researchers in Japan.

The surface of Pluto — the biggest object inside the Kuiper Belt, the ring of ice bodies beyond Neptune's orbit — remained mysterious for decades. Astronomers knew the dwarf planet as little more than a blurry orb until NASA's New Horizons probe revealed its surprisingly complex features in high definition during a flyby in July 2015. [Destination Pluto: NASA's New Horizons Mission in Pictures]

Thanks to that mission, we now know that Pluto has towering ice mountains, blue skies, a 620-mile-wide (1,000 kilometers) heart-shaped nitrogen glacier, jagged faults and, potentially, an underground ocean.

One of the most prominent features on Pluto is the informally named Cthulhu Regio, also called "the whale," which stretches across 1,900 miles (3,000 km). Cthulhu Regio is pockmarked with craters, which suggests it is billions of years old — much older than the craterless, young "heart" it borders. Scientists have said the dark region's reddish coloring might come from tholins, which are complex hydrocarbons.

To further investigate how the whale got its coloring, Yasuhito Sekine, an associate professor at the University of Tokyo, conducted heating experiments on organic molecules, like formaldehyde, that would have been present on the newly forming Pluto soon after the formation of the solar system. Sekine found that he could produce the same dark, reddish color after heating solutions above 122 degrees Fahrenheit (50 degrees Celsius) for several months. 

Meanwhile, Hidenori Genda, an associate professor at the Tokyo Institute of Technology, conducted computer simulations of a giant impact on Pluto. Genda found that the same impact that would have created Pluto's moon Charon, which is about half the size of Pluto, could have created a huge pool of hot water near Pluto's equator. And as this giant hot-water pool cooled, reddish complex organic materials would have formed, according to the findings, which were presented at the annual meeting of the European Geosciences Union in Vienna last month and were also published in the journal Nature Astronomy earlier this year. "For the terrestrial planets in our solar system, giant impacts are common," Genda told Space.com. "Our results suggest that giant impacts are common in [the] outer system beyond the Neptune orbit."

The color of an impact

The color changes across Pluto "make an interesting pattern, and we don't have good ideas to explain all of these features, so we are all still in the initial stages [of] exploring different hypotheses to explain these variations," said Kelsi Singer, a co-investigator on the New Horizons extended mission at the Southwest Research Institute in Colorado. 

But Singer, who wasn't involved in the new study, isn't entirely convinced by the newly described impact scenario. She told Space.com in an email that it is unlikely that the "whale" has remained mostly the same for the past 4 billion years or so, because the region has a lot of variation within it.

"There are some heavily cratered areas, and some smoother, almost crater-free areas that have younger ages," Singer said. "You could perhaps argue that if the dark layer was very thick (more than a few km) you could keep it around for 4 billion years or longer and still have other craters form in it, tectonics fracturing it, and keep it dark." 

But Singer said that in most areas of the whale, the dark material does not seem to be thick; thinner patches of the dark material are sitting on top of a brighter surface. Singer thinks a simpler explanation for the whale's coloring could be that the dark material formed from methane being processed by radiation on Pluto's surface or in the atmosphere.

To fully confirm the chemical makeup and impact history of Pluto, scientists might need to send additional space telescopes to observe the dwarf planet, or perhaps, one day, a probe to sample the icy surface.

"If we can get some information on chemical compositions of complex organic matters in the whale region, it will help us to confirm or deny the impact origin of this region," Genda said. "UV spectra will give us that information, but unfortunately, New Horizons did not have a UV spectra instrument. Ultimately, sample return from the whale region can reveal the origin of this region." 

Follow Megan Gannon @meganigannon, or Space.com @Spacedotcom. We're also on Facebook and Google+. Original article on Space.com.

What's a planet? What's a dwarf planet? Should we make a distinction? Should we really care about these definitions in the first place?

As we learn more about the outer solar system, the boundaries begin to blur.

A tiny celestial body called 2014 UZ224 and informally known as DeeDee (for “distant dwarf”) is a distant world about 92 astronomical units, or Earth-sun distances, from our sun. Recent observations from the Atacama Large Millimeter/submillimeter Array (ALMA) revealed that DeeDee is roughly 395 miles (635 kilometers) across, which would give it enough mass to be spherical.

Why does it matter if DeeDee is round? In 2006, a controversial vote by the International Astronomical Union defined three parameters for a planet. Simply speaking, the IAU says a planet must be in orbit around the sun, have enough mass to be round, and have cleared the neighborhood around its orbit — meaning it needs to be gravitationally dominant and hold any nearby bodies within its orbit.

It's the last part of the definition that most aggravates those who argue that Pluto – redefined as a "dwarf" planet under the IAU – is more planet than not. The argument is that the rocky planets of Earth, Mars, Venus, and Mercury have also not cleared their neighborhoods, as many asteroids co-orbit along with them.

The planetary geologist Kirby Runyon, a Ph.D. student at the Johns Hopkins University Applied Physics Laboratory, wrote a paper this year proposing a geophysical-based definition of a planet that would dispense with the orbital criterion and basically include any round celestial body that isn’t a star. The idea hatched by Runyon and his co-authors — which include Alan Stern, the principal investigator of the New Horizons mission to Pluto — would increase the number of purported planets in our solar system to over 110, including Earth’s moon and DeeDee.

RELATED: Behind the Push to Get Pluto Its Planetary Groove Back

"What it's really showing is the diversity of planets in our solar system,” said Runyon of the DeeDee news, “and giving us a better understanding of planets in the rest of the galaxy.”

"DeeDee is almost certainly made out of ices — water ices, methane, and carbon dioxide — which is similar to what Pluto is made of," he added. "These are very soft materials, compared with rocky silicate. It's more easily pulled into a sphere than rock or metal."

Adding more fodder to the debate over the definition, when the New Horizons spacecraft flew by Pluto in 2015, it unveiled a world of surprising complexity, ranging from mountainous areas to vast nitrogen-ice lakes.

"We call Pluto a 'dwarf' planet, but it's just an adjective for 'planet,’” Runyon said. “It's still a planet, and that's where we take umbrage with the IAU.”

"Astronomers aren't experts in planetary science, and they basically passed a bunch of B.S. off on the public back in 2006 with a planet classification so flawed that it rules the Earth out as a planet, too," Stern remarked in 2016. "A week later, hundreds of planetary scientists, more people than at the IAU vote, signed a petition that rejects the new definition. If you go to planetary science meetings and hear technical talks on Pluto, you will hear experts calling it a planet every day."

RELATED: Remind Me Again, Why Isn't Pluto a Planet?

As a geomorphologist, Runyon is most particularly interested in the shape of landforms on worlds around the solar system. His dissertation deals with wind-blown sand geology on Mars, as mapped by the Mars Reconnaissance Orbiter's HiRISE (High Resolution Imaging Science Experiment) camera.

But he'll still keep his eye on the outer solar system. New Horizons is set to fly by a tiny world called 2014 MU69 on Jan. 1, 2019. While MU69 is too small to be even a dwarf planet, Runyon will be looking for features on MU69 such as ripples, which were also seen during the Rosetta mission during observations of Comet 67P/Churyumov–Gerasimenko. He'll also look at the shape of craters, among other things.

"MU69 has never warmed up since the formation of the solar system," he said. "It will be really interesting to see how different or complex it might be. There's a laundry list of things we'll be looking for, and hopefully we'll be surprised."

Originally published on Seeker.

To Pluto and beyond!

Nearly two years after its historic encounter with the dwarf planet Pluto, NASA's New Horizons spacecraft is getting ready for its next big adventure in the icy outskirts of the solar system.

Now, the spacecraft is on its way to a small, ancient object located about 1 billion miles (1.6 billion kilometers) beyond Pluto in the Kuiper Belt. This distant region surrounds the solar system and is filled with trillions of icy rocks that have yet to be explored. The new target was discovered by the Hubble Space Telescope in June 2014, and it was dubbed 2014 MU69.

Pluto, which officially lost its planetary status shortly after New Horizons launched in 2006, is also a Kuiper Belt object (KBO), and the largest of its kind. New Horizons became the first spacecraft to visit the Pluto system when the probe flew by the dwarf planet and its moons on July 14, 2015. [Destination Pluto: NASA's New Horizons Mission in Pictures]

A world of discoveries

It took the spacecraft about 16 months to beam back all of its data from the Pluto flyby, and planetary scientists have had a ball with that data. 

"The New Horizons flyby of the Pluto system was completely successful, and now we've got all the data on the ground and we're putting a bow around it," Alan Stern, the New Horizons principal investigator at Southwest Research Institute, said in a Facebook Live event on Thursday (Jan. 19). 

Thanks to New Horizons, scientists now have a global map of Pluto and the most detailed images yet of the dwarf planet's bizarre, mountainous landscape and icy volcanoes. Tall mountain ranges seen on Pluto also suggest recent geological activity on the dwarf planet's surface.

New Horizons additionally beamed back a gorgeous photo of a huge, heart-shaped basin (unofficially called "Tombaugh Regio") that quickly became Pluto's most famous feature, taking the internet by storm and gracing the front page of hundreds of newspapers worldwide. The New Horizons science team has said Pluto's "heart" seems to indicate the presence of a subsurface ocean. 

The Pluto flyby also provided an opportunity to study Pluto's moons, particularly Charon. Researchers discovered that Charon and Pluto are both tidally locked, meaning the same side of the moon always faces the dwarf planet and vice-versa. As a result, Pluto's heart is always facing Charon. A giant red spot discovered on Charon's surface revealed that the moon is taking some of its atmosphere from Pluto.

"One thing that we discovered is that small planets can be just as complex as big planets, and that really blew away our expectations," Stern said, adding that all the new findings from Pluto "wet our appetite for future exploration of the Kuiper Belt."

Exploring beyond Pluto

While the team continues to analyze the plethora of data — something that could go on for decades — it's also busy planning for the next big stage of the mission, the flyby of 2014 MU69. That will occur in January 2019.

Pluto is the largest object known to exist in the Kuiper Belt, but MU69 is much smaller and more representative of the trillions of other KBOs, Kelsi Singer of the New Horizons science team told Space.com. Pluto is comparable to the size of North America at 1,475 miles (2,370 km) in diameter, while MU69 is less than 30 miles (about 45 km) across.

But MU69 isn't just any old KBO. Singer said that the object "has a special kind of orbit that makes it possibly a type of object that is primordial and left over from early solar system formation. So we think that we'll be able to look at what the building blocks of the solar system were like by going to this special object that has a special orbit."

Plutonium power

Part of the rationale for choosing MU69 as the next target was that it had a good location given the amount of fuel left on the New Horizons spacecraft. 

"MU69 turned out to be really interesting, but we also had limited options," Singer said. Using the Hubble Space Telescope, "we were searching the area of space where we had enough fuel left in the spacecraft to get to any objects that were there," she said. Three good potential targets were located, but the other two "were just on the edge of where [the spacecraft] had enough fuel to get to." 

New Horizons runs on a radioactive plutonium power supply that could keep the spacecraft going through the mid-2030s, Glen Fountain, the New Horizons encounter project manager at Johns Hopkins University's Applied Physics Laboratory, said during the Facebook Live event.

But after the 2019 flyby of MU69, the spacecraft probably won't have much fuel left for special maneuvers, Singer said. "We won't be able to switch directions, but we'll still keep going out. It's possible that we'll be able to observe some other objects, but we haven't identified any of them yet. So we're going to keep an eye out to see what we can find."

Get ready for MU69!

For now, the team will remain focused on planning the MU69 flyby and sifting through data from Pluto. The researchers need to plan the spacecraft's every move far ahead of time; because of a 6-hour delay in communications with the distant spacecraft, they won't be able to tell the probe what to do in real time. Instead, the team must program New Horizons at least several months in advance to do every observation and data transmission. 

The spacecraft will take photos of MU69 along the way, starting out with pictures of a single-pixel speck from afar, Singer said. During the flyby, New Horizons will be able to get even closer to MU69 than it did with Pluto, because the small object has much less gravity. This means that the photos of MU69 will have a higher resolution than the photos of Pluto. Singer said that's something she and the team look forward to seeing. 

In April, New Horizons will be halfway to MU69 from Pluto, with 21 months of spaceflight left to go. 

Email Hanneke Weitering at hweitering@space.com or follow her @hannekescience. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

A new face has been added to the solar system's family portrait: Scientists have discovered a new dwarf planet looping around the sun in the region beyond Pluto.

The dwarf planet, called 2014 UZ224, measures about 330 miles (530 kilometers) across and is located about 8.5 billion miles (13.7 billion km) from the sun, NPR reported today (Oct. 11). For comparison, Pluto's largest moon, Charon, is about 750 miles (1,200 km) in diameter, and reaches a maximum distance of about 4.5 billion miles (7.3 billion km) from the sun.

A year on 2014 UZ224 (the time it takes the dwarf planet to orbit the sun) is about 1,100 Earth years. One Pluto year, for c is about 248 Earth years. The new object was also confirmed by the Minor Planet Center. [Meet the Solar System's Dwarf Planets]

David Gerdes, a professor of astronomy at the University of Michigan, told NPR that the new dwarf planet was discovered using an instrument called the Dark Energy Camera (DECam). The universe is not only expanding but accelerating in that expansion, and "dark energy" is the name scientists have given the mechanism powering that expansion. The DECam was built to observe the movement of galaxies and supernovas (exploding stars) as they move away from the Earth. The goal is to provide more clues that will help reveal what dark energy actually is or where it comes from.

A project called the Dark Energy Survey is using observations from the DECam to create a map of the universe that provides information relevant to the study of dark energy. The DES maps have already been used to study dark matter (which makes up about eighty percent of all the mass in the universe but whose exact nature is still a mystery) and to find previously unidentified objects.

Part of the DES includes taking images of a few small patches of the sky "roughly" once per week, according to themission website, and that's what made this new discovery possible. While stars and galaxies appear in the same place in the sky, an object that is relatively close to Earth and orbiting the sun might appear to move over the course of a week or a few weeks.

A few years ago, Gerdes asked some visiting undergraduates to look for unidentified solar system objects in the galaxy map, according to NPR. The challenge was slightly difficult because the repeated observations would take place at irregular intervals, Gerdes said, but the students developed computer software to work with the irregularities and spot moving objects.

It took two years to confirm the detection of 2014 UZ224, NPR reports, and while its exact orbital path is still unclear, the scientists behind the discovery say they think that 2014 UZ224 is the third most-distant object in the solar system.

The smallest object in the solar system that has earned the title of "dwarf planet" (prior to this new discovery) is Ceres, which lies in the asteroid belt between Jupiter and Mars. Ceres is 590 miles (950 km) across. The object 2014 UZ224 might be too small to be considered a dwarf planet, Gerdes told NPR, but that will have to be decided by the International Astronomical Union (which made the controversial decision to demote Pluto to a dwarf planet). There are four other recognized dwarf planets in the solar system, but scientists think there could be dozens — or even more than 100 — objects that size that have yet to be discovered, according to NASA. 

Planet nine?

The region beyond Neptune's orbit is known as the Kuiper Belt, a disk that is believed to contain thousands of icy, rocky bodies. Beyond that is a region known as the Oort Cloud — a sphere of icy, rocky bodies that surrounds the rest of the solar system. Most comets originate in the Kuiper Belt or the Oort Cloud, but their wide orbits bring them close to the sun.

While the outer regions of the solar system are thought to be made up mostly of objects smaller than Pluto, there may be another planet nearly the size of Neptune lurking in this outer territory. Recent research has shown that the movement of known bodies in the outer solar system may point to the existence of this ninth planet (which scientists have dubbed Planet Nine); that research has prompted efforts to spot the new planetwith telescopes.

Follow Calla Cofield @callacofield.Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

A new simulation of how Pluto got its "heart" suggests that the dwarf planet most likely has a deep ocean beneath its surface.

Scientists have long suspected that Pluto has liquid water hidden underground. When NASA's New Horizons mission first set sail to the outskirts of the solar system, scientists were already planning to investigate whether the dwarf planet harbors water.

When New Horizons flew past Pluto in July 2015 and beamed its observations back to Earth, scientists found evidence suggesting that Pluto had water at some point. However, they weren't sure whether Pluto's had an existing ocean, or if it had frozen solid over time.

In a new study, scientists have determined that Pluto's subsurface ocean probably does exist, and that liquid water beneath the dwarf planet's icy shell is at least 60 miles (100 kilometers) deep and about as salty as the Dead Sea on Earth. For perspective, the deepest part of Earth's ocean is about 7 miles (11 km) deep, and Earth is about 150 times the size of Pluto.

However, many details about this possible ocean remain elusive.

"Thermal models of Pluto's interior and tectonic evidence found on the surface suggest that an ocean may exist, but it's not easy to infer its size or anything else about it," Brandon Johnson, lead author of the study and an assistant professor at Brown University's Department of Earth, Environmental and Planetary Sciences, said in a statement. "We've been able to put some constraints on its thickness and get some clues about [its] composition."

Johnson and his team discovered this information about Pluto's ocean by simulating the huge asteroid impact that left the flat, icy patch called Sputnik Planum on Pluto's surface, the western half of Pluto's famous heart-shaped feature. But first, they had to look at Charon, Pluto's largest moon. Charon and Pluto are both tidally locked, so they always face each other with the same sides. Sputnik Planum is found on the side of Pluto that constantly faces Charon, and that positioning implies that the area is what scientists call a "positive mass anomaly," meaning it contains more mass than the rest of Pluto's surface, on average.

At first, this doesn't make any sense. The side of Pluto that faces Charon has a gigantic crater, which is "basically a hole in the ground," Johnson said. "You're taking a bunch of material and blasting it out, so you expect it to have negative mass anomaly. But that's not what we see with Sputnik Planum. That got people thinking about how you could get this positive mass anomaly."

That's where the asteroid simulation comes in. Brown and his team ran several simulations of a 200-km (125 miles) asteroid — the appropriate size for Sputnik Planum's 900-km (560 miles) basin — striking Pluto's icy surface with varying depths of liquid water underneath. During this major impact, the asteroid created a huge dent on Pluto. As the asteroid's remains bounced off of the surface, Pluto's interior bounced back, too.

During this rebound, Pluto's interior was pulled toward the surface at the impact site. If the moving material is dense enough, the weight could make up for the missing mass at the impact site and even out Pluto's weight distribution. So the side of the dwarf planet that appears as though it should be lighter actually weighs about the same as the other side. This process is called "isostatic compensation," and it appears to be the reason why Pluto's dented side is so heavy, the researchers said in the new study; after the asteroid took away Pluto's icy crust, denser water from Pluto's interior evened out the missing mass.

But what really tipped the scale for Pluto was the subsequent sea of nitrogen ice that filled Pluto's heart after the impact: The amount of nitrogen ice in Sputnik Planum doesn't weigh enough to explain the tidal locking alone, Johnson said.

"This scenario requires a liquid ocean," Johnson said. "What this tells us is that, if Sputnik Planum is indeed a positive mass anomaly — and it appears as though it is — this ocean layer of at least 100 kilometers has to be there. It's pretty amazing to me that you have this body so far out in the solar system that still may have liquid water."

The new work was detailed Sept. 19 in the journal Geophysical Research Letters.

Email Hanneke Weitering at hweitering@space.com or follow her @hannekescience. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

Pluto may be sharing its atmosphere with its largest moon, Charon, creating the visually striking red spot at the satellite's north pole.

New research suggests that conditions on the two worlds over the past few billion years would allow Pluto's traveling atmosphere to freeze out on the frigid moon Charon, while radiation would quickly transform the methane and nitrogen ices to a sticky residue known as tholins.

"Methane is volatile enough that it can only stick to the surface during the long, cold polar winters," Will Grundy, lead author of the new study, told Space.com by email. Grundy, a planetary scientist at the Lowell Observatory in Arizona, is part of NASA's New Horizons mission that made a historic flyby of Pluto in July 2015. After modeling conditions on Charon over the past few billion years, he and his colleagues found that, after methane from Pluto froze, radiation stripped away the hydrogen. (Methane is made up of hydrogen and carbon.) The carbon left behind joined with other molecules to make heavier fragments of materials able to stick around even after temperatures warmed [Amazing Pluto Flyby Photos by NASA's New Horizons]

"As more and more fragments join up, they build progressively bigger and more carbon-rich, complicated molecules until they are so complex that we don't even try to give them chemical names, just describe them with generic terms like 'organic molecules' or 'tholins,'" Grundy said. "These are what produce the reddish color."

The research was published online Sept. 14 in the journal Nature.

A captured atmosphere

When New Horizons made its historic flyby of Pluto, it also studied the moons around the dwarf planet. The largest of these moons, Charon, is nearly the size of Pluto itself, leading many scientists to classify the pair as a double-planet system. New Horizons revealed that the massive moon sports a red spot at its northern pole.

Based on early measurements taken by the New Horizons spacecraft, Grundy and many othersimmediately speculated that the red spot was made of tholins created by the transfer of Pluto's atmosphere to Charon. They theorized that some of the material could freeze and that, later, radiation could convert it to tholins.

Pluto's small size means that it has trouble holding on to much of an atmosphere; many scientists questioned whether much of an atmosphere would remain by the time New Horizons made its visit. Observations revealed that Pluto manages to hold on to much of the gas around it, but some still escapes the dwarf planet's grasp.

"Pluto's atmosphere escapes radially outward in all directions, but Charon's gravity is strong enough to intercept a few percent of that flow," Grundy said. [Fly Through Giant Canyon on Pluto's Moon Charon | Video]

To measure the probability that Pluto's captured atmosphere became Charon's red spot, Grundy and colleagues modeled how temperatures changed over the moon's history. Charon's winters are cold, with polar temperatures only a few degrees higher than absolute zero (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius) at coldest. Because the winters last so long — well over 100 Earth-years — the imported atmosphere has enough time to freeze.

The warmer temperatures near Charon’s equator allows less of the material to fall to the surface, which is why the red spot doesn't extend across the face of the moon. Charon itself is too small to hold on to its own gaseous atmosphere very long, so as the 100-year winters end, the atmosphere evaporates, leaving behind the larger hydrocarbons.

"For the most part, Charon's surface is too warm for methane to stick, so the methane molecules that make it to Charon's surface just bounce around there until they either escape back to space again or find a place cold enough to stick," Grundy said. "The winter pole does get cold enough for methane to stick; so methane will accumulate there, but only until the sun rises again in the spring and warms it back up."

Winter on Charon

During Charon's long winters, radiation from the sun and galactic cosmic rays convertedthe atmosphere to heavier material that does not evaporate rapidly when temperatures rise. When the winter pole tilted back toward the sun, ushering in spring, the remaining methane and nitrogen evaporated, leaving behind material that gradually changed to the reddish organics spotted at the surface. Grundy and his team estimate that about 30 centimeters (12 inches) of tholins would be produced at the poles over Charon's billion-year lifetime. The process should continue in the winter poles today, with material slowly building up its red spot.

Although Charon's southern pole was shrouded in darkness during the flyby, the researchers analyzed the light reflecting off of the dayside of Pluto to identify a slow dimming unrelated to the illumination, suggesting that the south pole may also boast its own spot.

"We don't have any color data to say that it's red, but in the black-and-white images lit by Pluto shine, it does show a darkening that is similar in darkness and size to what's seen around the northern pole," Grundy said. "The two poles take turns being the winter pole, just like on Earth."

Follow Nola Taylor Redd on Twitter @NolaTRedd or Google+. Follow us at @Spacedotcom, Facebook or Google+. Originally published on Space.com.