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

December 2025 • 2025AJ....170..298S

Authors • Schmidt, Stephen P. • MacDonald, Ryan J. • Tsai, Shang-Min • Radica, Michael • Wang, Le-Chris • Ahrer, Eva-Maria • Bell, Taylor J. • Fisher, Chloe • Thorngren, Daniel P. • Wogan, Nicholas • May, Erin M. • Ferrari, Piero • Bennett, Katherine A. • Rustamkulov, Zafar • López-Morales, Mercedes • Sing, David K.

Abstract • 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.

Links

• SIMBAD https://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2025AJ....170..298S
• PDF https://iopscience.iop.org/article/10.3847/1538-3881/ae019a/pdf
• PREPRINT http://arxiv.org/abs/2501.18477
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2025AJ....170..298S
• DATA https://doi.org/10.5281/zenodo.14735688
• ELECTR https://doi.org/10.3847/1538-3881/ae019a