2025AJ....170..298S

Sub-Neptunes are the most common type of planet in our galaxy. Interior structure models suggest that the coldest sub-Neptunes could host liquid water oceans underneath their hydrogen envelopes-sometimes called “hycean” planets. JWST transmission spectra of the ∼250 K sub-Neptune K2-18 b were recently used to report detections of CH₄ and CO₂, alongside weaker evidence of (CH₃)₂S (dimethyl sulfide, or DMS). Atmospheric CO₂ was interpreted as evidence for a liquid water ocean, while DMS was highlighted as a potential biomarker. However, these notable claims were derived using a single data reduction and retrieval modeling framework, which did not allow for standard robustness tests. Here, we present a comprehensive reanalysis of K2-18 b's JWST NIRISS SOSS and NIRSpec G395H transmission spectra, including the first analysis of the second-order NIRISS SOSS data. We incorporate multiple well-tested data reduction pipelines and retrieval codes, spanning 60 different data treatments and over 250 atmospheric retrievals. We confirm the detection of CH₄ (≈4σ), with a volume mixing ratio range $-2.14\leqslant {\mathrm{log}}_{10}{{\rm{CH}}}_{4}\leqslant -0.53$, but we find no statistically significant or reliable evidence for CO₂ or DMS. Finally, we assess the retrieved atmospheric composition using photochemical-climate and interior models, demonstrating that our revised composition of K2-18 b can be explained by an oxygen-poor mini-Neptune without requiring a liquid water surface or life.