A possible sign of life on a distant planet did not survive closer scrutiny
A team reported a gas on the planet K2-18b that on Earth is made almost entirely by living things. Several independent reanalyses of the same data found the signal was not strong enough to claim.
- Institutions
- University of Cambridge, NASA, James Webb Space Telescope
- Reported
- 2025
- Status
- Contested by other researchers or later observations.
No imagery is attached yet. Agency imagery is free to use with a credit line, so anything added here will carry one.
What they found
K2-18b is a planet about eight times the mass of Earth, orbiting a small red star roughly 120 light-years away. In 2023 and again in 2025, a team led from Cambridge reported that observations from the James Webb Space Telescope were consistent with dimethyl sulfide in its atmosphere. On Earth that gas is produced almost entirely by marine life, which is why it is treated as a biosignature, a chemical fingerprint that living things tend to leave. The 2025 paper described the evidence as the strongest yet for such a gas on any exoplanet.
How they found it
When the planet passes in front of its star, a sliver of starlight filters through its atmosphere on the way to us. Different gases absorb different colours, so the light that arrives carries a barcode of what it passed through. Webb reads that barcode. The difficulty is that the signal is tiny, and turning it into a list of gases means fitting models to noisy data, where the answer depends partly on which models you fit.
Why it matters
If a biosignature gas were ever confirmed on a planet around another star, it would be the first evidence that life is not confined to Earth. That is the largest question the field has, which is exactly why the standard of proof has to be high. This episode matters as much for how the claim was tested as for the claim itself.
What this does not mean
This is not a detection of life, and as things stand it is not a confirmed detection of the gas either. Several independent groups reanalysed the same Webb data and found that the evidence for dimethyl sulfide fell below the threshold normally required, with some concluding the data are equally well explained without it. Others have pointed out that the gas can be produced without life at all, so even a confirmed detection would not settle the question. There is also no agreement on what kind of planet K2-18b is: whether it has a liquid water ocean or a deep hydrogen atmosphere with no surface is itself unresolved.