K2-18b is a "Hycean" candidate — a sub-Neptune that may hide a liquid-water ocean under a hydrogen-rich atmosphere, orbiting a cool red dwarf in the habitable zone. In 2023 and again in 2025, JWST data appeared to show dimethyl sulfide, a gas that on Earth comes almost exclusively from marine life. It's been argued over ever since.
Artist's concept of exoplanet K2-18b. Credit: NASA, CSA, ESA, J. Olmsted (STScI)
A Cambridge-led team reported tentative DMS/DMDS signals, then a stronger claim in April 2025 — described at the time as the strongest evidence yet for possible biological activity beyond the solar system.
An independent NASA analysis, combining new and old JWST data across three teams, found no conclusive evidence of DMS — while confirming the planet's water-rich, methane-and-CO2 atmosphere.
Some re-analyses (Welbanks and colleagues, and a JHU/APL team) found no statistically significant signal at all. A 2026 study led by Pica-Ciamarra and Madhusudhan, searching 661 candidate molecules, still ranked DMS the best-fitting explanation — modestly, and short of a firm detection.
Even the most optimistic reading sits around 3-sigma confidence — well short of the 5-sigma bar astronomy treats as a confirmed discovery. And DMS itself isn't an airtight biosignature: the amounts implied on K2-18b would be roughly 20 times higher than anything Earth's oceans produce, which is exactly the kind of detail that makes the "or is it just unfamiliar chemistry" camp hard to dismiss. This page will get updated as new JWST observing time resolves more of it — as of mid-2026, it hasn't.
NASA — How Webb Supports the Search for Life Beyond Earth
University of Cambridge — the original 2025 detection announcement