The Internet is buzzing right now with the headlines that “Scientists find strongest evidence yet of life on an alien planet” (CBC news), “Scientists detect signature of life on a distant planet, study suggests” (CNN), “Astronomers have found the ‘most promising signs yet’ of alien life on a planet beyond our Solar System” (Sky at Night Magazine), or “Tantalising sign of possible life on faraway world” (BBC). But wait, that last headline isn’t from this week — it’s from 2023, and it’s about precisely the same story. That’s because this is not a new discovery — it’s been reported before — and all that happened recently is that the results got published in Astrophysical Journal Letters. Carl Zimmer described the finding this week in the New York Times: 

[A] team of researchers is offering what it contends is the strongest indication yet of extraterrestrial life, not in our solar system but on a massive planet, known as K2-18b, that orbits a star 120 light-years from Earth. A repeated analysis of the exoplanet’s atmosphere suggests an abundance of a molecule that on Earth has only one known source: living organisms such as marine algae.

The molecule is called dimethyl sulfide (DMS) or dimethyl disulfide (DMDS), and on Earth its sole known source is life (specifically, marine phytoplankton algae). But there are a few problems with the claim. Ars Technica has a very nice framing of the problems, and explains:

So why are many astronomers unconvinced? To be compelling, a biosignature from an exoplanet has to clear several hurdles that can be broken down into three key questions:

• Is the planet what we think it is?
• Is the signal real?
• Is life the only way to produce that signal?

At present, none of those questions can be answered with a definitive yes.

We’ll discuss each of these problems briefly.

Just What Kind of Planet Is K2-18b?

The authors of the current study claim that K2-18b could be a Hyacean ocean planet — a very large rocky planet with a hydrogen-rich atmosphere surrounding and sustaining a liquid water ocean that could be filled with life. But many are skeptical of this interpretation, and are suggesting the findings are also consistent with a planet covered in molten magma ocean and a greenhouse-inducing hydrogen atmosphere — a planet highly inhospitable to life. CNN quotes a scientist explaining this possibility:

Astrophysicist Sara Seager, a professor of physics, planetary science, aeronautics and astronautics at the Massachusetts Institute of Technology, said independent teams have completely different interpretations of the planet itself. Seager was not involved in the new research.

“Some propose a Hycean world, others suggest a hot magma ocean — a planet with molten rock beneath a hydrogen-rich atmosphere, which is about as inhospitable as it gets — and still others see it as a mini-Neptune,” Seager said, referring to worlds that are larger than Earth but smaller than Neptune. For reference, K2-18b is 8.6 times as massive and 2.6 times as large as Earth.

Likewise, Science reports:

Christopher Glein, a planetary scientist at the Southwest Research Institute, posted a preprint on arXiv on Sunday suggesting K2-18b may host a vast magma ocean wrapped around a large rocky core — a very different beast from the water-world idea that Madhusudhan’s team advocates. Glein told The New York Times it would take a lot to persuade him there’s life on the planet: “Unless we see E.T. waving at us, it’s not going to be a smoking gun.”

Glein’s preprint paper explains problems with the interpretation that K2-18b is a Hycean exoplanet:

The atmospheric composition determined by transmission spectroscopy in the near-infrared (H₂/He atmosphere with ~1% abundances of CH₄ and CO₂ and no detectable NH₃ or CO) seemed to suggest that K2-18 b is a Hycean world. However, a reanalysis of those data found no statistically significant evidence for the detection of CO₂. This finding may cast doubt on the occurrence of Hycean conditions. Moreover, updated photochemical modeling underscores the difficulty of producing sufficient methane on a Hycean world. It has been emphasized that K2-18 b is too close to its star to support liquid water at its surface due to greenhouse heating, inhibited atmospheric convection, and patchy cloud cover around the substellar point. [Internal citations removed.]

The point that K2-18b is too close to its host star was also a problem raised by Sky at Night Magazine: “The planet is also very close to its star, meaning it is bombarded with a great deal of high-energy radiation, which any organisms on its surface would have to be able to survive.”

Ars Technica pushes this argument further, noting that without clouds on K2-18b (which have not been detected), it would be impossible to sustain an ocean:

The first question is whether we’re actually looking at a hycean world. As the researchers acknowledge in their paper, the presence of an ocean on K2-18b depends very strongly on its weather: “A cloud-/haze-free atmosphere would render the surface too hot to be habitable and/or have water in a supercritical state.” And, as they later acknowledge, the data obtained from the JWST shows no signs of clouds. That doesn’t mean they’re not there, but it certainly doesn’t help the case.

And, in fact, a different research group has already found evidence that the planet isn’t reflecting enough light back into space to keep from boiling away any oceans it tries to form. That manuscript suggests that K2-18b is more likely to be a magma-ocean or gas-dwarf world. And a modeling paper suggests that most potential hycean worlds would suffer from a runaway greenhouse effect unless they receive significantly less illumination than Earth does. Then there’s a draft paper from Glein and his collaborators, which suggests you can get many of the same properties seen in K2-18b from a planet with a deep atmosphere sitting above a magma ocean.

A 2023 article at Big Think by astrophysicist Ethan Siegel argued that K2-18b is “massive, puffy, and more Neptune-like than Earth-like” and thus simply cannot be covered by a liquid-water ocean:

And for large, massive planets that are more like Neptune/Uranus than Earth/Mars/Venus, their stronger gravitational pull makes it easy for them to hold onto the lightest gases of all: hydrogen and helium, whereas for a small, low-mass planet like our own, our gravity is insufficient to prevent solar radiation from boiling those atoms/molecules away.

A recent study has shown that any planet that’s more than about 1.75 times the radius of Earth must be Neptune-like, not Earth-like, and that same study showed that if a hydrogen/helium atmosphere reaches even half-a-percent of the planet’s overall mass, the surface pressure will be tens of thousands of times as great as it is on Earth’s surface, while the temperature will reach into the thousands of degrees. K2-18b, therefore, cannot be an ocean-covered, Earth-analogue world.

With all this skepticism that K2-18b has a liquid-water ocean, I think Zimmer’s article at the New York Times summarized the situation nicely: “Other researchers emphasized that much research remained to be done. One question yet to be resolved is whether K2-18b is in fact a habitable, Hycean world.”

Did They Really Detect DMS/DMDS?

Multiple articles have noted that the detection of DMS/DMDS on K2-18b needs to be independently verified and brought to a higher level of statistical significance before it can be accepted by the scientific community. CNN quotes astrobiologist Eddie Schwieterman of UC Riverside explaining this point:

But Schwieterman said that first, scientists need to confirm that dimethyl sulfide is really present in the atmosphere of K2-18b, which will require validation from multiple independent groups who study the same data and analyze it for the chemical signature of the molecules. Madhusudhan said the data the study team analyzed will be released next week, so other astronomers can do just that.

Next, Schwieterman wants to see additional Webb observations with a higher level of statistical significance to see whether the interpretation of dimethyl sulfide being present holds. Searching for the signatures of these molecules in atmospheres of other similarly sized planets within the habitable zones of their stars would also help, although it’s a process that will take years.

“I do have at (least) one reason to be skeptical, which is that I’d anticipate the presence of ethane (C2H6) to accompany DMS/DMDS if those gases were present,” he said. “This is because UV rays from the star would break apart the DMS/DMDS into components we’d predict would react to form ethane. The absence of ethane makes me think we’ve missed something. Perhaps our models are wrong, or perhaps the DMS/DMDS isn’t there.”

The insightful analysis at Ars Technica further notes that the spectral signature that is being claimed to indicate dimethyl sulfide could easily also indicate other molecules instead: 

For its specific identity as dimethyl sulfide, we only know that it’s the best fit out of the 20 chemicals considered in this paper. There are a whole host of other chemicals that could plausibly be produced on a planet like this that weren’t included in this analysis. The potential presence of a dimethyl sulfide signal at other wavelengths in earlier work may seem to solidify this identification, but a reanalysis of that data found no evidence of a statistically significant signal.

But even if they did find DMS/DMDS, there’s still another crucial question which must be addressed…

Can DMS/DMDS Be Produced Abiotically?

The answer to this question is yes — it is well-established that dimethyl sulfide can be produced from nonbiological sources. In 2024, DMS was detected on what the journal Science described as a “cold, lifeless comet.” That article stated:

Scientists have discovered dimethyl sulfide (DMS), a molecule thought to have only living sources, on a cold, lifeless comet. The finding calls into question the molecule’s usefulness as a biosignature and the significance of an earlier hint of it in the atmosphere of an alien planet.

“This is the first sign of an abiotic source,” says Nora Hänni, a chemist at the University of Bern who presented the discovery last week at the General Assembly of the European Geosciences Union.

That same year a paper in The Astrophysical Journal Letters reported: “Through laboratory photochemical experiments, we show the abiotic production of organosulfur gases, including DMS…” They caution that “H₂S-influenced organic haze chemistry may be an overlooked abiotic source of organosulfur compounds” and conclude:

We have shown that DMS, OCS, CS₂, and simple thiols, species previously considered potentially robust biosignatures in exoplanetary atmospheres, have possible abiotic production pathways via planetary organic haze chemistry. Thus, each organosulfur gas presented here is at risk of being a false-positive biosignature if the abiotic pathways proposed are neglected.

The current study proposing life on K2-18b acknowledges these abiotic mechanisms of producing DMS, but dismisses them, claiming that the don’t produce dimethyl sulfide in high enough amounts to allow them to reach observed concentrations observed on K2-18b before being destroyed. That may be true — but it’s also true for observed biotic production of DSM on Earth. So something else must be going on here. 

Sky at Night Magazine acknowledges the possibility of other processes at work: “Another unknown chemical process could be the source of the molecules detected in K2-18b’s atmosphere.” Indeed, there are good reasons to suspect that something else might be going on: The concentrations of DMS and DMDS on K2-18b are orders of magnitude higher than they are for biotic production here on Earth. Sky at Night explains: 

Yet the concentrations of dimethyl sulfide and dimethyl disulfide in K2-18b’s atmosphere are different from those on Earth. On Earth, dimethyl sulfide and dimethyl disulfide are below one part per billion by volume. On K2-18b, they’re thought to be thousands of times stronger, over ten parts per million.

So if dimethyl sulfide is in fact present on K2-18b in concentrations 1,000+ times greater than on Earth, then something very different is happening there — and we still don’t know what that is. This possibility of non-Earthlike processes has been acknowledged by multiple sources, and further study is needed to rule out abiotic production, as the technical paper says:

Future laboratory experiments and/or theoretical modeling are also needed to fully explore the possible photochemical mechanisms for producing DMS and DMDS in dry, methane-rich, reduced environments, to address potential abiotic sources of these molecules.

Again, the New York Times provides a good summary: “Scientists will also need to run laboratory experiments to make sense of the new study — to recreate the possible conditions on sub-Neptunes, for instance, to see whether dimethyl sulfide behaves there as it does on Earth.” Whatever is happening on K2-18b, it seems unlikely to be similar to what happens on Earth. 

Little Data to Go On

At the end of the day, we must bear in mind that all the data we have from these exoplanets is a small amount of light that is reflected coming from their host star that is reflected off the planet. A news story in the journal Science reminds us just how little information we have to go on: 

Even then, researchers say there should be a very high bar for claiming the presence of life based solely on the gases in a planet’s upper atmosphere. “Everything we know about planets orbiting other stars comes from the tiny amounts of light that glance off their atmospheres,” Oliver Shorttle of Cambridge told BBC. “So it is an incredibly tenuous signal that we are having to read, not only for signs of life, but everything else.”

Researchers would prefer a more thorough knowledge of the planet’s atmosphere and surface to exclude other possibilities. “On Earth [DMS] is produced by microorganisms in the ocean, but even with perfect data we can’t say for sure that this is of a biological origin on an alien world because loads of strange things happen in the Universe,” Catherine Heymans of the University of Edinburgh and Scotland’s Astronomer Royal told BBC. “We don’t know what other geological activity could be happening on this planet that might produce the molecules.”

There are other examples in recent memory where detection of a molecule in an exoplanet led to premature declarations of alien life. A 2023 BBC story recounts:

It is the first time astronomers have detected the possibility of DMS in a planet orbiting a distant star. But they are treating the results with caution, noting that a claim made in 2020 about the presence of another molecule, called phosphine, that could be produced by living organisms in the clouds of Venus was disputed a year later.

Could something similar be happening right now? When the possibility of dimethyl sulfide on K2-18b was first reported in 2023, an article at Big Think said, “I’m betting that you don’t want hype and exaggeration; you want the scientific truth” and concluded we “see no evidence that K2-18b has water; we see no evidence for water there at all. And, most importantly, there is no detection of any biosignature on this world.”  I think perhaps Sara Seager, a planetary scientist at MIT, put it best when she said: “When it comes to K2-18 b, enthusiasm is outpacing evidence.”