On Wednesday, news broke that researchers had found the most compelling evidence yet of a "biosignature"—a chemical present at levels that are only consistent with life—on a distant exoplanet. It didn't take much time for some less-than-reliable news sources to go from there to talk of a planet that "could be 'teeming with life'" and the obvious follow-up, "Scientists reveal what aliens could REALLY look like on exoplanet K2-18b."
Even in the best of circumstances, however, talk of a biosignature is an invitation to scientists to think of alternative chemistries that could explain the results without needing biological activity. And these are not the best of circumstances, as astronomers are pointing to earlier papers that give a range of reasons to be skeptical of the new results; in fact, an astronomer named Chris Glein emailed me to alert me of potential issues the day before the news broke.
To help you understand the controversy, we're going to look at the data that is being presented as evidence of a biosignature and then go through all the reasons that confirming a biosignature is so difficult.
What’s new here
The planet under discussion is called K2-18b, and it's what's termed a sub-Neptune. It has a radius of about 2.5 times that of Earth's and is 8.6 times more massive. Its orbit is also within the inner edge of its star's habitable range, meaning it receives enough radiation from its star that water could be liquid on its surface, depending on other factors like its atmosphere's composition. All of this can be determined based on its orbital properties and the amount of light that it blocks out as it passes between Earth and its host star.
As the new paper acknowledges, these properties are consistent with a number of different planet types, including a small version of Neptune or a rocky planet with a very thick, hydrogen-rich atmosphere. But the researchers behind the new work favor what's called a hycean planet, a merger of hydrogen and ocean. That's because the initial characterization of its atmosphere suggested that, in addition to copious amounts of hydrogen, it had methane and carbon dioxide present but seemingly lacked ammonia and carbon monoxide. The presence of an ocean can account for those chemical properties.
The new paper follows up on a tantalizing hint raised by earlier work: the possible presence of a chemical called dimethyl sulfide. It uses an instrument on the James Webb Space Telescope (JWST) to image K2-18b as it passes in front of its host star. A small portion of the light that reaches Earth does so after having passed through the planet's atmosphere, allowing the chemicals present there to leave a mark on the spectrum of that light.
The research team used two different methods of constructing a spectrum from the JWST data, and the results are in good agreement. They then searched for a combination of molecules that could produce a similar spectrum, starting with a list of 20. They found two: dimethyl sulfide and dimethyl disulfide (we can't tell the difference between the two, given the existing data).
On Earth, the only processes that naturally produce this chemical take place inside cells, and it had previously been suggested to be a biosignature. So, the researchers propose it may be a biosignature here, although they acknowledge that the statistical significance of its signal, currently at three sigma, falls short of declaring a clear discovery. Still, that would qualify it as, in the words of a University of Cambridge press release, the "strongest hints yet of biological activity."
Reasons for doubt
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?
- Are there other ways to produce that signal?
At present, none of those questions can be answered with a definitive yes.
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.
Combined, these raise significant questions about whether K2-18b is in fact a hycean world. And they indicate that, if it's not, it's likely to be far too hot to support life.
Which brings us to whether the signal of a biosignature—the presence of these key wiggles in the JWST spectrum—is real. While the signal is there at three sigma, that's three sigma above a completely featureless spectrum. 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.
Finally, there's a potential problem that the authors of the new paper acknowledge: the spectral information they used comes from Earth. We only have data on dimethyl sulfide's absorption and emission at room temperature and one atmosphere, which is likely to be very different from the conditions it would see in the upper atmosphere of a mini-Neptune. While this wouldn't alter the existence of spectral lines, the differences could broaden them or accentuate their significance, altering how well they match the JWST data.
The last issue is whether, if dimethyl sulfide is really present on K2-18b, it was produced by life as it is here on Earth. The answer appears to be "possibly not": A 2024 paper indicates it's possible to produce the chemical through light-activated reactions. The results suggest that these reactions may max out at lower concentrations of dimethyl sulfide than indicated by the data from K2-18b, but they're certainly a reason for pause and should motivate people to explore whether there are ways to boost the efficiency of the relevant chemical reactions.
Good, or just the best we’ve got?
It should be clear that it's simply not possible to provide a definitive answer to any of the three key questions. So, while this might be, to use Cambridge's phrasing, the "strongest hints yet," that doesn't make them especially strong. And, given the significance of the question they speak to—is there life beyond Earth—maybe the overall strength of the evidence was a far more relevant standard to use here. At least within the body of the press release, the scientists behind the work were appropriately cautious, saying things like "It’s important that we’re deeply sceptical of our own results," and “Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings."
In the end, the potential issues we've laid out above should make it clear that there will be no obvious before-and-after moment of discovery when it comes to finding hints of life over the next few decades. Any hint we get from telescope data will simply be the start of a conversation among astronomers, chemists, atmospheric scientists, and others meant to get definitive answers to all three of the key questions. It will only be after that conversation has settled on "yes" to all of them that we'll be able, in retrospect, to realize a discovery was made.
Astrophysical Journal Letters, 2025. DOI: 10.3847/2041-8213/adc1c8 (About DOIs).
