Webb data on K2-18 b sparks debate over potential biosignature gases
This digest was compiled by AI from multiple sources — links to the originals are below.
Astronomers using the James Webb Space Telescope detected tentative signs of dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) in the atmosphere of exoplanet K2-18 b, 124 light-years away. The finding, reported at three-sigma significance, has triggered months of debate over whether the molecules are actually present and whether they could indicate biological activity. Even as the signal remains unconfirmed, independent analyses have questioned its robustness.
The Webb Observations
In 2023, Nikku Madhusudhan and colleagues reported Webb observations using NIRISS and NIRSpec, finding methane at five-sigma significance and carbon dioxide at three sigma in a hydrogen-rich atmosphere. A second paper in April 2025 used the Mid-Infrared Instrument to cover wavelengths from 6 to 12 micrometres, reporting that the data preferred DMS and/or DMDS at approximately three-sigma significance. The authors stressed that more observations were needed, partly because the two gases have overlapping absorption features.
Biosignature Interpretation Challenges
Even if the molecular inference held, the biological interpretation would require further work. DMS is produced abundantly by life in Earth’s oceans, but non-biological sources may exist under different planetary conditions. In 2025, an astronomy team reported DMS in an interstellar molecular cloud, providing observational evidence that the molecule can form without biology. The proposed abundance of at least ten parts per million on K2-18 b would require a production and survival mechanism compatible with the planet's atmosphere.
Independent Analyses
Later work concentrated on whether the spectral signal was robust. Independent teams reanalysed the Webb data, with some finding that the DMS signal could be explained by other atmospheric components or instrumental noise. The debate shifted from whether DMS is a biosignature to whether the spectrum requires either gas at all. The three-sigma significance is below the five-sigma threshold typically required for a definitive detection in particle physics.
What's Next
The Webb team is expected to submit a proposal for additional observing time to confirm the DMS signal. It remains unclear whether the signal will survive deeper analysis or whether alternative explanations will gain consensus.
1 source
Webb data on K2-18 b sparks debate over potential biosignature gases
