A Comprehensive Reanalysis of K2-18 b’s JWST NIRISS+NIRSpec Transmission Spectrum

Journal article

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≤log10CH4≤−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.

Authors: Schmidt, SP; MacDonald, RJ; Tsai, S; Radica, M; Wang, L

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: The Astronomical Journal
• Volume: 170
• Issue: 6
• Article number: 298
• Publication date: 2025-11-05
• Acceptance date: 2025-08-28
• DOI: 10.3847/1538-3881/ae019a
• EISSN: 1538-3881
• ISSN: 0004-6256
• Language: English
• Keywords: Ocean planets; Exoplanet atmospheres; Habitable planets; Extrasolar gaseous planets; Exoplanet atmospheric composition; Exoplanets; Mini Neptunes; Exoplanet structure; Exoplanet surfaces