Understanding Biosignature
Why Disequilibrium Matters
Oxygen and methane react with each other and should not coexist in an atmosphere for long. On Earth, both are maintained at high concentrations only because life continuously produces them — plants release O₂, microbes release CH₄. Finding both together in an exoplanet atmosphere would be strong (though not conclusive) evidence of biological activity.
JWST's Capability
JWST analyses biosignatures via transmission spectroscopy: when an exoplanet transits its star, some starlight passes through the planet's atmosphere. Different molecules absorb specific wavelengths, leaving a chemical fingerprint. JWST's NIRSpec (0.6–5.3 µm) and MIRI (5–28 µm) cover the wavelength ranges where key biosignature molecules have their strongest absorption features.
Current Candidates
In September 2023, JWST detected carbon dioxide and tentatively identified dimethyl sulfide (DMS) in the atmosphere of K2-18 b, a sub-Neptune in the habitable zone 124 light-years away. On Earth, DMS is produced almost exclusively by marine phytoplankton. However, false positives are possible — volcanic or photochemical processes could also produce some biosignature molecules — so the finding requires further observation and peer review.