Is there really alien life on this exoplanet? We asked 10 experts.

In 2020, scientists claimed to have found a sign of life on Venus: hints of a stinky gas called phosphine that’s made by microbes on Earth. The claim was swiftly challenged and, years later, is still mired in controversy. Now, another stinky gas has sparked its own alien life debate—this time, for an exoplanet.

Researchers announced on April 16 that they’d used data from the James Webb Space Telescope (JWST) to detect a gas called dimethyl sulfide (DMS) in the atmosphere of an exoplanet called K2-18b, which orbits in its alien star’s habitable zone. On Earth, DMS is mostly made by microscopic phytoplankton. On other planets, it could be what’s called a biosignature—a sign of life.

Cambridge University, which hosts several of the researchers involved in the detection, was quick to promote the finding as the “strongest hints yet of biological activity outside the solar system.” Some media outlets trumpeted the DMS as a likely sign of life. But scientists who weren’t involved in the discovery aren’t as euphoric.

“I’m pretty skeptical of this claim, and I wish the press coverage better reflected the skepticism of the astronomical and astrobiological community,” wrote astrobiologist Joshua Krissansen-Totton of the University of Washington in an email.

For Clara Sousa-Silva, an astrochemist at Bard College who was involved in the 2020 Venus biosignature debacle, the situation is disappointingly familiar. “We did not learn enough from the ‘phosphine on Venus’ drama,” she says.

National Geographic got in touch with ten independent experts to find out what to make of this biosignature claim. (Not all are quoted below, but their views are represented.) The takeaway: It’s an exciting discovery, perhaps even an important step on the way to discovering alien life one day. Emphasis on “one day.” We did not just find aliens.

Here’s what you need to know about DMS on K2-18b.

What are the researchers claiming?

If you follow exoplanet news, you might be feeling a bit of déjà vu. In 2023, the same research team led by Cambridge astrophysicist Nikku Madhusudhan published JWST observations hinting at DMS on K2-18b.

Based on the same JWST data, the researchers also concluded that K2-18b was a type of habitable planet called a “Hycean” world. Madhusudhan and his colleagues coined the term in 2021 to describe a group of hypothetical planets bigger than Earth, smaller than Neptune, mostly made of water, and wreathed in thick veils of hydrogen and helium. Under the right conditions, they could have temperate surface oceans hospitable for life.

The DMS detection from 2023 fell short of the typical statistical standards for discoveries in astronomy. This new study, published in the Astrophysical Journal Letters,  is a follow-up using an instrument on JWST that’s sensitive to light at different wavelengths than the original study.

While the last DMS detection was weaker than tentative, this one appears to be much stronger. Madhusudhan and his colleagues claim that their detection of DMS (and/or a similar molecule called dimethyl disulfide or DMDS) reaches a “three sigma” level of significance. That’s statistical jargon meaning there’s less than a 0.3 percent chance that the DMS detection was made by chance—still lower than the gold standard five sigma cutoff for statistical significance, but far more convincing than before.

“Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have,” Madhusudhan said in Cambridge’s press release.

Is the dimethyl sulfide even there?

Other scientists aren’t as sanguine. Some are skeptical that the DMS (or DMDS) is even there at all.

“It’s really interesting, a great showcase of the capabilities of JWST,” astronomer Laura Kreidberg of the Max Planck Institute for Astronomy said in a voice memo. “But, yeah, I wouldn’t bet my house on it.”

Scientists can use JWST to identify gases in exoplanet atmospheres via chemical fingerprints in starlight that has filtered through the planets’ atmospheres. Those chemical fingerprints show up as wiggles in graphs of the starlight’s intensity versus its wavelength. The new study tried to match these spectral wiggles to 20 molecules. That’s “more molecules than astronomers often do, it’s just astronomers don’t often claim aliens,” says Sousa-Silva. Most of those molecules aren’t structurally similar to DMS and DMDS, either, she notes, so it wasn’t exactly a targeted screen of possible false-positives. 

Another researcher, astronomer Ryan MacDonald at the University of Michigan went further, criticizing the three sigma claim as “statistical hacking” on Bluesky.

Kreidberg is more forgiving. “I think that the discovery team did a great job, really careful work with the data. But as someone in this business myself, I can say that it is just really hard.” 

Is K2-18b even habitable?

Though K2-18b orbits in its star’s habitable zone, it is not a second Earth. At 2.6 times the radius and 8.6 times the mass of our planet, it is a mysterious and alien world—one that might not even be habitable.

When an independent team re-analyzed the Cambridge team’s 2023 observations of K2-18b, they found no evidence of DMS, nor of carbon dioxide—a blow to the Hycean world scenario, which predicted plenty of carbon dioxide. Based on their analysis, the independent team thinks K2-18b is an inhospitable gassy ball with no surface whatsoever. Another team has suggested an even less hospitable alternative: The planet could have an ocean not of water, but of magma.

Madhusudhan points out that the analysis reporting no carbon dioxide on K2-18b has not yet been peer-reviewed. “There are open questions, but they don’t preclude habitability,” he says. “The evidence for CO₂ is certainly there.”

Even if K2-18b is a Hycean world, that doesn’t mean it is habitable. Without a reflective deck of clouds, the planet’s ocean would broil beneath its hydrogen blanket. That’s the likely fate of any ocean that might exist on K2-18b, at least according to a study posted last week.

“The simplest explanation of this planet is a very thick gas-giant atmosphere with no habitable surface,” says exoplanet scientist Nick Wogan of NASA Ames. “There are so many challenges with making a habitable (or inhabited) K2-18b work.”

If the dimethyl sulfide is there, was it made by life?

Still, let’s say scientists confirm the DMS signal, and K2-18b turns out to be a habitable Hycean world. You still might want to hold off on popping the “we found aliens” champagne.

Until scientists can rule out abiotic explanations—ones that don’t involve living things—for DMS and/or DMDS, these gases won’t be true biosignatures for K2-18b. And as Harrison Smith, an astrobiologist at the Earth-Life Science Institute in Japan, and his colleague Cole Mathis at Arizona State University argued in a 2023 essay, ruling out false positives for exoplanets is very hard.

“At least Venus is a planet we know. We know what it looks like, and what the environment is like,” says Sousa-Silva.  Without knowing what K2-18b’s geochemistry and atmosphere are really like, scientists can’t confidently exclude the possibility that alien chemistry, not alien life, is the source of the DMS.

And we already know that nature can produce DMS without life. Last year, chemist Nora Hänni at the University of Bern and her colleagues found DMS on comet 67P—not exactly a habitable world. Other researchers have found it in interstellar space. And last year, chemist Eleanor Browne of the University of Colorado, Boulder and her colleagues showed that DMS can be produced in light-fueled chemical reactions in lab experiments with synthetic atmospheres.

“There’s no reason to understand [DMS] as a unique consequence of life,” says Mathis. “I just, for the life of me, cannot figure out exactly what the argument is about: why they think this could even potentially be indicative of life, given that we’ve seen abiotic sources.”

The study authors acknowledge some of these challenges. Madhusudhan says that neither comets nor interstellar material are feasible sources of the high concentrations of DMS and DMDS his team detected. But finding DMS in unexpected, dead environments shows that we still have a lot to learn about how it forms.

Embracing uncertainty

Other uncertainties haunt the detection. We don’t know how life began on Earth, so we can’t know if conditions on K2-18b — even if they’d be hospitable for earthlings — could have got life started in the first place. And even if life did evolve there, who’s to say that it’d produce DMS — and if it does produce DMS, why haven’t scientists spotted other biosignature gases?

Still, despite its many, many caveats, most researchers we spoke with agree that there’s reason to celebrate this new study of K2-18b. “It’s really an achievement. Thirty years ago, we didn’t even know that there are exoplanets,” says Hänni.

Peter Vickers, a philosopher of science at Durham University who’s studied life detection claims, was initially skeptical. “But then the more I looked at it, the more I thought that it actually is quite significant and shouldn’t be underplayed either,” he says.

For Madhusudhan’s part, he doesn’t think caution and excitement are mutually exclusive. Even a sliver of evidence for alien life is a “transformational achievement” he says, but there’s a big step from there to a true life detection claim. “We need to recognize both: the achievement and the caution.”

If we do ever find life beyond our solar system, it won’t happen all at once. We’ll slide slowly into certainty, pushed along by findings like this one—hints that there’s something more to discover if only we’d look closer. And this result is, without a doubt, an invitation to look closer at K2-18b. If we find life there, on one of the first potentially habitable planets we’ve inspected closely, says Vickers, we’ll have to assume life is common everywhere; if life is rare, the odds of just stumbling upon the right planet are astronomically low.

“We’re still only at the question asking stage, but it’s amazing that we can ask this question,” says astrobiologist Michael Wong of Carnegie Science. “What a lucky time to be alive.”