K2-18b is a planet about 124 light-years away, bigger than Earth, orbiting in the zone where liquid water could exist. A team led by the University of Cambridge, using the James Webb Space Telescope, reported a possible sign of a gas called dimethyl sulfide (DMS) in its atmosphere. Because DMS on Earth comes mainly from ocean life, the team called it a possible biosignature.
The reasons for caution are important. The 2025 result was at three-sigma confidence, meaning about a 0.3% chance of being a fluke. That sounds strong, but physicists usually require five-sigma, a much tougher standard, before announcing a discovery, and three-sigma hints have disappeared before with more data.
Independent teams reanalysed the same data. Several found the DMS signal was not statistically significant, even while confirming other gases like methane, so experts did not all see the same thing in the same numbers. On top of that, a gas made by life on Earth could be produced by non-living chemistry elsewhere. The honest takeaway is that K2-18b offers intriguing hints that deserve more observation, not evidence of life.