An international team of astronomers has reported what may be the strongest evidence yet of biological activity beyond our solar system, detecting chemical fingerprints in the atmosphere of the exoplanet K2-18b that, on Earth, are produced almost exclusively by living organisms. The findings, announced by researchers at the University of Cambridge and published in The Astrophysical Journal Letters, were made using the James Webb Space Telescope and have set off a wave of cautious excitement, and pointed scepticism, across the global scientific community.
K2-18b is a so-called “sub-Neptune” world about 124 light-years from Earth in the constellation Leo. Roughly 8.6 times the mass of our planet, it orbits a cool red dwarf star within the habitable zone, the region where liquid water could plausibly exist. Lead researcher Nikku Madhusudhan and his team say Webb’s instruments picked up atmospheric signatures consistent with dimethyl sulphide (DMS) and dimethyl disulphide (DMDS), molecules produced on Earth primarily by marine phytoplankton. You can read more about the original announcement on the University of Cambridge research news page.
What the Telescope Actually Saw
Webb does not photograph exoplanets directly. Instead, it uses a technique called transmission spectroscopy: when a planet passes in front of its host star, a sliver of starlight filters through the planet’s atmosphere, and different molecules absorb specific wavelengths, leaving telltale dips in the spectrum. The Cambridge-led team reported these dips at a statistical confidence level of roughly three sigma, meaning there is about a 0.3 per cent chance the signal is a random fluke. That is well short of the five-sigma standard typically required for a confirmed discovery in physics, a point the authors themselves emphasise.
“This is the strongest evidence yet that there is possibly life out there,” Madhusudhan told reporters, while stressing that the result must be independently verified. NASA’s own briefing on the James Webb Space Telescope mission underlines that atmospheric characterisation of small, temperate worlds is one of the observatory’s flagship goals, and that K2-18b is now among its most-studied targets.
Why Researchers Are Hesitant
Several independent groups have already pushed back. Critics note that DMS has now been tentatively detected on a comet and in interstellar gas clouds, environments where no biology could plausibly exist, suggesting non-living chemical pathways may produce the molecule under conditions very different from Earth’s oceans. Others question whether K2-18b is even a “Hycean” world, the warm, ocean-covered, hydrogen-atmosphere class of planet that Madhusudhan’s team has championed. Some models suggest the planet could instead host a magma ocean beneath a thick gas envelope, an environment hostile to life as we know it.
Reporting from outlets including the BBC’s science coverage has highlighted this divide, with planetary scientists urging the public to treat the headlines as a promising lead rather than a confirmed discovery. Sara Seager of MIT, who was not involved in the study, has previously cautioned that biosignature claims have a long history of being walked back as data improve.
The Bigger Picture for Astrobiology
Even with the caveats, the announcement marks a milestone in observational astrobiology. Just a decade ago, characterising the atmosphere of a temperate sub-Neptune was beyond the reach of any instrument. Webb has now done it twice for K2-18b, first detecting carbon dioxide and methane in 2023, and now flagging potential biosignatures. The trajectory suggests that within a few years, astronomers may be able to compile detailed atmospheric inventories for dozens of small, potentially habitable worlds.
The next steps are clear: more Webb observing time has already been requested, and rival teams will reanalyse the public data with their own atmospheric models. Future missions, including the European Space Agency’s Ariel telescope and NASA’s planned Habitable Worlds Observatory, are designed precisely to settle questions like this one. Whether K2-18b ultimately turns out to be inhabited, sterile, or something stranger than either, the episode is reshaping how seriously, and how carefully, scientists discuss the search for life beyond Earth.
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