Will the James Webb telescope lead us to alien life? Scientists say we're getting closer than ever.

Imagine a planet twice as wide as Earth, covered in an ocean that smells like sweet cabbage.

Every day, a faint red star warms this ocean world and the uncountable masses of hungry, plankton-like creatures that inhabit it. They rise to the surface by the billions, joining together in a living, floating continent larger than Australia — spewing out a pungent gas as they knit sunlight into food.

The sulfurous gas steams out of the alien bloom, filling the air so fully that a lone telescope floating 700 trillion miles (over a quadrillion kilometers) away can sense it — faintly, for just a few hours every month, when the watery planet glides in front of its small, red star. For those few hours, the alien algae of the pungent planet make themselves known to Earth.

It sounds like science fiction ... but is it?

For the past two years, this question has been the subject of intense debate among alien-hunting scientists, with the James Webb Space Telescope (JWST) at its center. Captured in the powerful telescope's crosshairs is the planet K2-18b, located around 120 light-years from Earth. There's no question that the planet itself is real. But its surface conditions, as well as its likelihood of harboring life, remain contested.

One group of researchers who has studied K2-18b with JWST for the last few years claims to have detected signs of dimethyl sulfide (DMS). This compound, which has a cabbage-like odor, is what many Earthlings think of as "the smell of the sea" and is only known to be produced by living, breathing phytoplankton. The team first reported hints of DMS in K2-18b’s atmosphere in 2023, and has followed up with several papers since.

Outside researchers remain skeptical of this alleged DMS detection, however. They've cautioned that the team's detection relies on questionable data modeling and falls short of the threshold required to signify a new scientific discovery. Only further observations of the planet can truly settle the question.

But what isn't in doubt is that JWST's ultrapowerful infrared vision is giving humans the best-ever shot at finding extraterrestrial life.

Thanks to JWST, "we're learning more just in the last few years than we've learned in the preceding decades about the compositions of atmospheres outside the solar system," Eddie Schwieterman, an assistant professor of astrobiology at the University of California, Riverside who studies exoplanet habitability with JWST, told Live Science.

It's dogma in the search for alien life that where there's an atmosphere, there may also be water on a planet's surface — and where there's flowing water, there may be life. For the first time, JWST is bringing those alien atmospheres into focus.

"We are at a really important time in the search for life, in that we now have the technological capability to do it," said Victoria Meadows, a professor of astronomy at the University of Washington and director of the astrobiology graduate program. "Prior to JWST, we really did not have the capability to do this."

The breath of aliens

In the hunt for habitable planets — those that orbit in the "Goldilocks zone" of their home star, where liquid water can flow on the surface — JWST is in a class of its own.

Unlike Hubble and other optical telescopes, JWST can't directly image the surfaces of faraway planets. Nor can it detect radio waves and other potential "technosignatures" emitted by any advanced alien civilizations that might exist. The signs of life JWST seeks are far more elemental. They're not blurry snapshots of alien trackways or mysterious radio signals, but hints of molecules tumbling invisibly through space, far above a planet's surface.

"The first step in finding life is to find an atmosphere," Sebastian Zieba, a postdoctoral researcher at the Harvard and Smithsonian Center for Astrophysics, told Live Science. "In order to have liquid water on the surface, you need an atmosphere."

Compared with its predecessor — NASA's infrared Spitzer Space Telescope (launched in 2003 and retired in 2020) — JWST is "better in every way," Zieba said. It can look farther across space and detect a broader range of infrared wavelengths than any telescope before it. Infrared emissions are crucial to the hunt for life, because those wavelengths are best at encoding information about the types of molecules that are absorbing or reemitting starlight in a planet's atmosphere.

For JWST to detect hints of an exoplanet's atmosphere, scientists must wait for a transit — the moment when a planet swoops in front of its home star, forcing that star's light to shine through the planet's atmosphere as seen from our perspective on Earth. In the case of K2-18b, for example, that happens once every 33 days.

"The planet passes in front of the star, and it backlights the atmosphere," Meadows said. "It's like a little halo around the planet."

That "halo" contains important clues about an alien world. As starlight streams through the planet's atmosphere, airborne molecules either absorb or reemit different wavelengths of light, changing what JWST sees when observing at those wavelengths. The unique signature of light compiled from these different wavelengths, called a spectrum, can reveal which molecules are in the atmosphere. This information, in turn, allows scientists to infer the planet's size, surface conditions, geography — and chances of supporting life.

For example, Meadows said, if JWST captures the spectrum of a planet that reveals high levels of methane and carbon dioxide absorption in its atmosphere, it could indicate a habitable world akin to Earth in the Archean eon (roughly 4 billion to 2.5 billion years ago), when primitive microbes were breaking down CO2 and spewing vast quantities of methane.

Proving those conditions exist on a planet trillions of miles away is the hard part.

The devil in the data

After making a promising biosignature detection, the challenge then becomes proving that it can't be explained by a geological process, such as volcanism. Then, scientists must demonstrate that their detection meets statistical significance — a rigorous undertaking that requires many repeat observations of the planet and verification from independent researchers using their own data models.

"Webb data is very complex," René Doyon, a professor at the University of Montreal and principal investigator of JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument, told Live Science. "People have been publishing results that are not always consistent. Depending on who reduced the data, you get a different answer."

It's here that early studies of K2-18b have fallen under scrutiny. Despite the tentative detection of DMS reported in two studies by a team of University of Cambridge-led researchers, outside experts have so far been unable to verify the result when looking at the same observations with different data models. Furthermore, the DMS detection only reached the three-sigma level of statistical significance, falling far short of the required five-sigma level. (A three-sigma level is around a 3 in 1000 chance of being a fluke, while a five-sigma value means a result has a probability of 1 in 3.5 million of being a fluke).

Nikku Madhusudhan, a professor of astrophysics at Cambridge and lead author of the two DMS studies, said this is no reason to ignore K2-18b as a candidate for a habitable world "teeming with microbial life."

"We have initial feelers for what we are seeing, but we could be wrong," Madhusudhan told Live Science. "So let's be open to being wrong and get more data. Only then can we confirm what we're seeing."

Schwieterman thinks it was "premature" to announce the detection of DMS on K2-18b, given the questionable statistical significance. However, he agrees that DMS is a promising signature of life that JWST should continue hunting for on other potentially habitable ocean worlds.

"The question we want to ask is, how common are global biospheres in the universe?" Schwieterman said. If there's complex life out there, including intelligent life, then "a big part of that question is, how common are the biospheres from which those more complex forms of life would originate?"

Hitting a "bull's-eye"

Even if life doesn't ultimately materialize on K2-18b, the distant planet is just one of many being targeted by JWST's keen infrared eye.

The telescope's search list includes some of the usual suspects, such as the TRAPPIST-1 system — the single most-studied star system beyond our own. The system contains seven rocky planets, at least three of which may be in the star's habitable "Goldilocks" zone.

So far, JWST has not been able to confirm an atmosphere around any of the three potentially habitable planets TRAPPIST-1b, c, or d. However, a study published Sept. 8 in The Astrophysical Journal Letters has offered preliminary evidence of a possible nitrogen-based atmosphere around TRAPPIST-1e, making it one of the most hopeful contenders for an Earth-like planet beyond our solar system.

Meanwhile, Doyon favors studying a world called LHS 1140 b, located 50 light-years from Earth in the constellation Cetus. Doyon and team's observations with JWST reveal that the exoplanet, once thought to be a rocky "super-Earth" six times as massive as our planet, is a much bigger oddball — or, perhaps, an eyeball.

"It may be a bull's-eye planet," Doyon said, describing a mostly ice-coated planet with a single blue "iris" of liquid water pointed toward its home star.

Using JWST data from two transits of LHS 1140 b, Doyon and colleagues reestimated the mass and radius of the planet and found "it cannot be explained as a rocky planet — it must have something between 10% and 20% of its mass in water," Doyon said. "It's a potential waterworld, and it's right in the habitable zone."

According to Doyon, LHS 1140 b doesn't resemble Earth so much as it resembles our solar system's icy moons Europa and Enceladus, both of which are suspected to harbor subsurface oceans that could support life. But unlike those moons, this planet is so close to its home star that some of its ice may have sublimated into gas, forming an atmosphere. It's even possible that the sun-facing side of the planet (which, like Earth's moon, is tidally locked) could heat up enough for the ice to melt there, revealing a liquid-water ocean beneath a cloudy sky. As such, this warm, blue "iris" could host life.

Doyon thinks this is one the likeliest known exoplanets to harbor an atmosphere.

"If I had to bet a beer on whether it has an atmosphere, it probably has one," he said.

Pushed to the limits

Sadly, Doyon's beer will likely have to wait.

Although Doyon and his colleagues detected "hints" of a nitrogen-rich atmosphere around LHS 1140 b, he said it will take about a dozen more transits to prove whether there are other molecules indicative of an Earth-like atmosphere, such as carbon dioxide. Because LHS 1140 b becomes visible from Earth only four times a year, scientists would have to observe every possible transit for years to come before making any firm conclusions. It's a schedule that "really pushes JWST to its limits," Doyon added.

This underscores one of the telescope's biggest limitations: time.

In 2024, researchers around the world requested a total of more than 78,000 hours of JWST observation time — about nine times more than is available, according to the Space Telescope Science Institute (STScI), which manages JWST proposals each year. Of the more than 2,300 submissions, only 274 proposals were ultimately accepted, with exoplanet habitability research accounting for a small percentage.

That discrepancy is likely to widen with the passage of the Trump administration's proposed budget for 2026, which includes a nearly 50% cut to NASA's science budget, according to Live Science's sister site Space.com. If approved by Congress, the cuts would amount to a roughly 25% to 35% reduction in JWST operations, Neill Reid, multimission project scientist at STScI, said in July at the 246th meeting of the American Astronomical Society in Anchorage, Alaska.

Finding the unforgettable

In the end, JWST may not uncover a smoking gun in the search for extraterrestrial life. But even if it doesn't, it will likely help scientists determine where to search next. Future telescopes will build on JWST's revelations, helping to fill in the missing gaps.

One major gap is oxygen. While the gas makes up about 21% of Earth's atmosphere and is a potent biosignature, "JWST can't do oxygen," Meadows said.

Multiple studies — including one co-authored by Meadows, in which researchers modeled what JWST would see if it studied Earth's atmosphere — have found that the telescope is simply not sensitive enough to detect oxygen. That poses a clear challenge to detecting Earth-like atmospheres.

Forthcoming telescopes could help account for that. For example, the Extremely Large Telescope — a powerful optical/near-infrared telescope being constructed in Chile that could see first light in 2029 — will be more sensitive to oxygen and water in planetary atmospheres than JWST is, Meadows said. It will also be able to peer all the way down to the surfaces of rocky planets — closer to where life and its byproducts are more likely to be, unlike the high upper atmospheres that are JWST's domain.

Further down the line, NASA's recently announced Habitable Worlds Observatory will take a census of planets around sunlike stars close to our solar neighborhood. Parsing visible, infrared and ultraviolet light signatures, the powerful observatory could potentially confirm atmospheres around dozens of Earth-like worlds. Currently, however, there is no planned launch date.

With JWST expected to remain operational at least into the 2030s, its era of discovery is just beginning. Will it find alien life? Maybe, maybe not. But in its first years, it's already leading scientists closer to that first tantalizing glimpse of evidence than any telescope has before.

And once that evidence is found — even if it's on a distant exoplanet that no human or probe will ever lay eyes on — there's no going back. Finding evidence of even one other inhabited planet would imply that there could be countless others out there, raising big questions about the prevalence of life in the universe, and where humans fit into it. The discovery of an alien world would change how we view the cosmos, as well as ourselves.

"Once we find a credible hint of evidence for life on an exoplanet … I don't think we're ever going to forget about that planet," Schwieterman said. "It's going to be both a scientific and cultural touchstone. Kids are going to learn about it in school."

Editor's note: This article was updated on Sep. 10 at 2 p.m. ET to include details from a new study on the exoplanet TRAPPIST-1e.