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Arxiv: A Comprehensive Reanalysis of K2-18 b's JWST NIRISS+NIRSpec Transmission Spectrum Published: 1/30/2025 6:51:02 PM Updated: 1/30/2025 6:51:02 PM

Paper abstract: Sub-Neptunes are the most common type of planet in our galaxy. Interiorstructure models suggest that the coldest sub-Neptunes could host liquid wateroceans underneath their hydrogen envelopes - sometimes called 'hycean' planets.JWST transmission spectra of the ~ 250 K sub-Neptune K2-18 b were recentlyused to report detections of CH_4 and CO_2, alongside weaker evidence of(CH_3)_2S (dimethyl sulfide, or DMS). Atmospheric CO_2 was interpreted asevidence for a liquid water ocean, while DMS was highlighted as a potentialbiomarker. However, these notable claims were derived using a single datareduction and retrieval modeling framework, which did not allow for standardrobustness tests. Here we present a comprehensive reanalysis of K2-18 b's JWSTNIRISS SOSS and NIRSpec G395H transmission spectra, including the firstanalysis of the second-order NIRISS SOSS data. We incorporate multiplewell-tested data reduction pipelines and retrieval codes, spanning 60 differentdata treatments and over 250 atmospheric retrievals. We confirm the detectionof CH_4 (~ 4\sigma), with a volume mixing ratio of log CH_4 =-1.15^{+0.40}_{-0.52}, but we find no statistically significant or reliableevidence for CO_2 or DMS. Finally, we quantify the observed atmosphericcomposition using photochemical-climate and interior models, demonstrating thatour revised composition of K2-18 b can be explained by an oxygen-poormini-Neptune without requiring a liquid water surface or life.