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- K2-18b is an exoplanet 124 light-years away and is larger and more massive than Earth. Scientists said it might have a deep global ocean under a hydrogen atmosphere. Could it support life?
- New observations from the Webb Space Telescope show there is a molecule called dimethyl sulfide in its atmosphere, a possible signature of life. Webb had previously tentatively detected it before, but the new observations show a stronger signal.
- The results don’t prove there is life on K2-18b, but they are tantalizing. Other scientists are quite skeptical, however.
Life on exoplanet K2-18b?
In 2023, scientists announced they had tentatively identified the gas dimethyl sulfide – a possible biosignature of life – in the atmosphere of K2-18b, an exoplanet 124 light-years away. On April 17, 2025, scientists at the University of Cambridge in the U.K. said they found the gas again with the Webb Space Telescope, but this time with a stronger signal. They said the dimethyl sulfide appears to be thousands of times more abundant on K2-18b than on Earth. However, more data are needed to fully confirm its presence and whether it is connected to life … or not. And many scientists are still skeptical.
The New York Times first broke the news on April 16, 2025.
The researchers published their peer-reviewed results in The Astrophysical Journal Letters on April 17, 2025.
A hint of dimethyl sulfide
K2-18b is a super-Earth or sub-Neptune world, orbiting in the habitable zone – where liquid water could exist – of its star. Its exact classification is also still a matter of debate among scientists, which has a lot of bearing on the reported discovery. It’s about 8.6 times as massive and 2.6 times larger than Earth, and orbits a red dwarf star about 124 light-years away.
When scientists announced the possible detection of dimethyl sulfide (DMS) in September 2023, using the Webb Space Telescope, the news spurred much debate. It was a potentially exciting discovery, to be sure. The gas is a potential biosignature, a chemical, molecular or other trace of biological life. But the detection was weak and far from conclusive. Astronomers would need to observe the planet again with Webb to try to determine if the gas really was there or not.
Nikku Madhusudhan, a University of Cambridge astrophysicist, was involved in the previous research and is the lead author of the paper about the latest findings. He said:
We didn’t know for sure whether the signal we saw last time was due to dimethyl sulfide, but just the hint of it was exciting enough for us to have another look with Webb using a different instrument.
Webb had also previously found methane and carbon dioxide in the atmosphere of K2-18b. Those signals were much stronger, however, and considered to be confirmed. The dimethyl sulfide signal was a lot weaker, or of “low statistical significance,” in more scientific terms.
New observations with Webb
For the older initial observations, Webb used its Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec) instruments. But for the new observations, Webb used its Mid-Infrared Instrument (MIRI) instead. Madhusudhan said:
This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations. The signal came through strong and clear.
Co-author Måns Holmberg at the Space Telescope Science Institute in Baltimore, Maryland, added:
It was an incredible realization seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests.
Madhusudhan also discussed the new findings in a livestream that you can replay and watch here:
https://www.youtube.com/watch?v=v=yc0757j2R8s
Video via Cambridge University Astronomy.
More dimethyl sulfide than on Earth?
One big surprise from the results is the apparent amount of dimethyl sulfide in the planet’s atmosphere. If the results are accurate – and they are still to be confirmed – then K2-18b has thousands of times more of the gas in its atmosphere than Earth does. On Earth, it’s less than one part per billion. But on K2-18b, it is an estimated 10 parts per million.
The new observations revealed the tentative existence of a similar gas, dimethyl disulfide. Both molecules are from the same chemical family and could be potential biosignatures.
On Earth, marine organisms such as plankton produce almost all the dimethyl sulfide. But it can also form without life and has been detected in comets and gas clouds in space. So its presence, by itself, isn’t a slam dunk for life. Not yet, anyway.
Is K2-18b a Hycean world?
Some studies suggest that K2-18b is a Hycean world, a rocky planet covered by a global ocean, but with a hydrogen atmosphere. It could be similar to Earth in some ways, but also utterly alien. As Madhusudhan noted:
Earlier theoretical work had predicted that high levels of sulfur-based gases like dimethyl sulfide and dimethyl disulfide are possible on Hycean worlds. And now we’ve observed it, in line with what was predicted.
But even that is still up for debate among scientists. Other studies say that it might be more of a sub-Neptune, with a deep, dense atmosphere and no solid surface or ocean at all. Whichever scenario is correct has, of course, direct implications for the possibility of life on K2-18b.
Skepticism abounds
Madhusudhan made a strong statement in the Cambridge press release, saying:
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.
But at the same time he acknowledges that even the new results are preliminary and are open to debate, saying:
It’s important that we’re deeply skeptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them. That’s how science has to work.
Co-author Savvas Constantinou at Cambridge’s Institute of Astronomy added:
Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings.
But other scientists are also skeptical, not only about the results but even how the results were obtained. Sarah Hörst, a planetary scientist, said on Bluesky:
Sarah’s rules of looking for life with only chemical signatures: 1) The smaller the molecule(s) the more likely they can also be produced by abiotic processes. 2) One molecule, if it can be produced by any abiotic process, will never be enough to definitively claim detection of life.
You can read some more detailed threads from Chris Lintott and Ryan MacDonald below:
An astonishing headline reporting on new observations from a team led to Nikku Madhusudhan claims they’ve found ‘hints of life’ on a planet orbiting a dwarf star some 124 light years away. What’s going on? (1/n) www.bbc.co.uk/news/article… ? ?
— Chris Lintott (@chrislintott.bsky.social) 2025-04-17T03:24:30.587Z
??, ? ???????????? ??? ??? ???? ???????? ?? ??-???’? ??????????.K2-18b is back in the news, now with a bold claim that biosignature molecules (DMS and/or DMDS) have been ‘detected at 3?’.Most exoplanet astronomers are extremely sceptical about these claims, let’s see why (1/n).??? #exoplanet
— Dr Ryan MacDonald (@distantworlds.space) 2025-04-17T16:08:01.577Z
So overall, the new results are tantalizing, but still not proof of life on K2-18b. And the reaction from quite a few other scientists shows how such possible evidence must be very carefully vetted.
Bottom line: New observations by the Webb Space Telescope have reignited the debate about possible life on K2-18b, a potentially habitable exoplanet 124 light-years away.
Source: New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI
Via University of Cambridge
Read more: Did Webb find signs of life on exoplanet K2-18 b?
Read more: Is K2-18b really a habitable super-Earth?
#Life #K218b #Exciting #results #met #skepticism
