British astronomers claim to have found the strongest evidence of life on exoplanet K2-18b, which was first discovered in 2015 and soon emerged as a potential candidate suitable for habitability with water.
The announcement came from astrophysicist Nikku Madhusudhan, a professor of astrophysics and exoplanetary science at the Institute of Astronomy, University of Cambridge, who has been credited with developing the technique of atmospheric retrieval to infer the compositions of exoplanets and coining the term “hycean planet” to describe the class of planets that host a liquid water ocean beneath a hydrogen-rich atmosphere.
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“These are the first hints we are seeing of an alien world that is possibly inhabited,” Nikku Madhusudhan at the University of Cambridge told a press conference on March 15, 2025. The researchers say between 16 and 24 hours of follow-up observation time with the James Webb Space Telescope may help them reach the all-important five-sigma significance. Their findings are reported in The Astrophysical Journal Letters.
The exoplanet K2-18b, 124 light-years away, is 2.6 times the radius and 8.6 times the mass of Earth and orbits its star within the habitable zone, where temperatures could allow liquid water to exist. In the autumn of 2019, two different teams reported detection of water vapor in its hydrogen-rich atmosphere, though the extent of the atmosphere and the conditions of the interior underneath remained unknown.
In 2023, using the instruments on the James Webb Space Telescope, the team looked at K2-18b’s atmosphere in near-infrared light and again found evidence of water vapor, as well as carbon dioxide and methane, besides a tantalizing hint of dimethyl sulfide (DMS), a molecule that, on Earth, is produced only by living organisms, mainly marine phytoplankton.
Since the signs for DMS were extremely weak, Madhusudhan and his colleagues have used a different instrument from JWST, the mid-infrared camera, to observe K2-18b and found in 2015 a much stronger signal for DMS, as well as a possible related molecule called dimethyl disulfide (DMDS), which is also produced on Earth only by life.
“What we are finding is an independent line of evidence in a different wavelength range with a different instrument of possible biological activity on the planet,” Madhusudhan said.
The team claims that the detection of DMS and DMDS is at the three-sigma level of statistical significance, which is equivalent to a 3-in-1000 chance to end up as a fluke. In physics, the standard threshold for accepting something as a true discovery is five sigma, equivalent to a 1-in-3.5 million chance that the data is a chance occurrence.
‘Boy-who-cried-wolf’ situation
However, dismissing the sigma argument, Ryan MacDonald at the University of Michigan says, “We have a boy-who-cried-wolf situation for K2-18b, where multiple previous three-sigma detections have completely vanished when subject to closer scrutiny. Any claim of life beyond Earth needs to be rigorously checked by other scientists, and unfortunately many previous exciting claims for K2-18b haven’t withstood these independent checks.”
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Nicholas Wogan at the NASA Ames Research Center in California concurs with the findings but is not convinced until verified by peer scientists, as the entire data is not yet made public. Even after publication, he says it requires time. “It’s not just like you download the data and you see if there’s DMS – it’s this super complicated process,” says Wogan.
The difficulty in proving that it couldn’t have a non-biological explanation could put K2-18b in the category of a viable biosignature candidate for a long time, says Sara Seager at the Massachusetts Institute of Technology. “It may remain in that category for decades, since the question may never be fully resolved with the limited data exoplanets offer,” she says.