JWST Detects Potential Biosignature on K2-18b
Summary
Astronomers led by nikku-madhusudhan at the University of Cambridge detected dimethyl sulfide (DMS) and potentially dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b using the jwst’s Mid-Infrared Instrument (MIRI). On Earth, both molecules are exclusively produced by microbial life, making this the strongest potential biosignature ever detected on an exoplanet. Published in The Astrophysical Journal Letters on April 17, 2025.
Key Claims
- DMS and DMDS detected at three-sigma significance (0.3% chance of random occurrence), below the five-sigma threshold required for definitive discovery.
- K2-18b is a potential “Hycean world” — a habitable planet covered in liquid water with a hydrogen-rich atmosphere — located 124 light-years from Earth.
- The planet has 8.6 times Earth’s mass and 2.6 times Earth’s diameter, orbiting within its star’s habitable zone.
- DMS/DMDS concentrations are estimated at thousands of times stronger than Earth levels, raising questions about plausibility of biological production at such scales.
- The absence of ethane (C2H6) suggests atmospheric models may be incomplete.
- Independent analysis cast doubt on the findings, with “overwhelming consensus” that enthusiasm exceeded the strength of evidence.
Detection Method
The team used jwst instruments across multiple observation campaigns:
- NIRISS and NIRSpec: Previously detected methane and CO2 in K2-18b’s atmosphere.
- MIRI: Used for the new DMS/DMDS detection via transmission-spectroscopy at mid-infrared wavelengths.
Scientific Community Response
| Researcher | Affiliation | View |
|---|---|---|
| Eddie Schwieterman | UC Riverside | Skeptical but intrigued; sustaining DMS requires ~20x Earth production — “a high bar, but plausible” |
| David Clements | Imperial College London | ”A step in the right direction” |
| Sara Seager | MIT | ”Enthusiasm is outpacing evidence”; finding “will remain in the candidate category indefinitely” |
Next Steps
- An additional 16-24 hours of Webb observation time could enable definitive claims.
- Data to be released for independent validation.
- Confirmation requires five-sigma significance and validation from multiple independent groups.
Relevant Quotes
“This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations. The signal came through strong and clear.” — nikku-madhusudhan
“It’s important that we’re deeply skeptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them.” — nikku-madhusudhan
“A bigger question in my mind is whether we as a species are prepared to find life as we don’t know it.” — nikku-madhusudhan
Follow-up / Related Findings
- 2026 radio technosignature null — A VLA + MeerKAT 33-day search of K2-18 found no radio technosignatures; the paper itself describes the DMS/DMDS biosignature interpretation as “heavily disputed.” See src-k2-18b-technosignature-null-2026.
- 2026 abiotic biosignature modeling — Photochemical work refining abiotic O₂/O₃ ceilings is methodologically relevant to any future biosignature claim on M-dwarf rocky planets. See src-oxygen-false-positive-biosignatures-2026-03 and false-positive-biosignatures.