A new paper presents the first mid-infrared spectrum of K2-18b, using the MIRI instrument on JWST. The paper claims a 3σ detection of the biosignature candidate molecules DMS and/or DMDS (the 'and/or' is important). So how does this stack up? Madhusudhan+2025: iopscience.iop.org/article/10.3...

The MIRI transmission spectrum of K2-18b shows multiple bumps and wiggles, which Madhusudhan et al. interpret as compelling evidence for the molecules DMS ([CH3]2 S) and/or DMDS ([CH3]2 S2). On Earth, these molecules are produced mostly by phytoplankton. But there are problems with the analysis.

Most importantly, this is not a 3σ detection. In exoplanet atmosphere research, we use something called 'Bayes factors' (B) to express how confident we are in a spectral signal. • No evidence (B < 3) • Weak evidence (3 < B < 12) • Moderate evidence (12 < B < 150) • Strong evidence (B > 150)