TY - JOUR
T1 - A comprehensive reanalysis of K2-18 b's JWST NIRISS+NIRSpec transmission spectrum
AU - Schmidt, Stephen P.
AU - MacDonald, Ryan J.
AU - Tsai, Shang-Min
AU - Radica, Michael
AU - Wang, Le-Chris
AU - Ahrer, Eva-Maria
AU - Bell, Taylor J.
AU - Fisher, Chloe
AU - Thorngren, Daniel P.
AU - Wogan, Nicholas
AU - May, Erin M.
AU - Ferrari, Piero
AU - Bennett, Katherine A.
AU - Rustamkulov, Zafar
AU - López-Morales, Mercedes
AU - Sing, David K.
N1 - Funding: S.P.S. is supported by the National Science Foundation Graduate Research Fellowship Program under grant No. DGE2139757. R.J.M. is supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51513.001, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. S.-M.T. acknowledges support from NASA Exobiology grant No. 80NSSC20K1437 and the University of California, Riverside. M.R. acknowledges support from the Natural Sciences and Engineering Research Council of Canada (NSERC). T.J.B. acknowledges funding support from the NASA Next Generation Space Telescope Flight Investigations program (now JWST) via WBS 411672.07.04.01.02.
PY - 2025/12/1
Y1 - 2025/12/1
N2 - 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.
AB - 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.
KW - Exoplanets
KW - Exoplanet atmospheres
KW - Exoplanet structure
KW - Habitable planets
KW - Mini Neptunes
KW - Exoplanet atmospheric composition
KW - Ocean planets
KW - Exoplanet surfaces
KW - Extrasolar gaseous planets
U2 - 10.3847/1538-3881/ae019a
DO - 10.3847/1538-3881/ae019a
M3 - Article
SN - 0004-6256
VL - 170
JO - Astronomical Journal
JF - Astronomical Journal
IS - 6
M1 - 298
ER -
