K2-18b Hycean interpretation supported

K2-18b is back at the center of an argument that has divided exoplanet researchers since the James Webb Space Telescope began returning detailed spectra of the planet’s atmosphere. A new preprint by Takuya Fujisawa, Masashi Shimada, Tatsuya Yoshida and Kiyoshi Kuramoto, posted to arXiv on May 18 and highlighted by Astrobiology on May 21, says current JWST transmission spectra remain consistent with a Hycean world — a hydrogen-rich planet that could host a liquid-water ocean beneath its atmosphere. The authors did not say the debate is settled. Their paper says K2-18b can be modeled with self-consistent Hycean atmospheres that match JWST observations from 0.8 to 5.2 microns, while also stating that mini-Neptune scenarios remain viable. ### What exactly did the new paper test? The May 18 paper tested whether a Hycean atmosphere could reproduce the transmission spectrum already measured by JWST. Fujisawa and co-authors said they combined one-dimensional photochemical modeling, radiative-convective equilibrium calculations and forward spectral modeling to compare Hycean scenarios with the available data. (arxiv.org) The dataset in question was not a new JWST observing run. (arxiv.org) The study used the existing NIRISS SOSS and NIRSpec G395H transmission spectra for K2-18b and focused on how methane, carbon monoxide and carbon dioxide abundances could fit those observations under a hydrogen-rich atmosphere over a liquid ocean. ### Which JWST measurements are driving the claim? JWST’s NIRISS SOSS and NIRSpec G395H instruments are the core of the argument. (arxiv.org) Fujisawa’s team said they used the methane-dominated 2.8-4.0 micron band to constrain offsets between the two instrument datasets and then scanned carbon-species abundances to match the 4-5 micron region. The paper said Hycean models with a roughly 1 bar H2 envelope, percent-level CH4 and CO, and CO2 at about 10^-3 to 10^-2 could reproduce the NIRISS and NIRSpec spectra without invoking dimethyl sulfide or other added species. (arxiv.org) The authors also said their photochemical simulations naturally produced carbon monoxide at around 1% to 2% mixing ratios. ### Why is K2-18b still disputed? A January 2025 reanalysis by Stephen P. (arxiv.org) Schmidt, Ryan J. MacDonald and colleagues reached a different conclusion from earlier Hycean-leaning interpretations. That team said it confirmed methane at about 4-sigma significance in K2-18b’s atmosphere but found no statistically significant or reliable evidence for carbon dioxide or dimethyl sulfide in the same JWST NIRISS and NIRSpec data. That reanalysis said its revised composition could be explained by an oxygen-poor mini-Neptune without requiring a liquid-water surface or life. Fujisawa’s new paper addresses that same dispute from the other direction, arguing that present CO and CO2 constraints are not yet strong enough to rule out Hycean configurations. ### What does the new study actually claim — and not claim? The strongest claim in the new paper is narrower than “K2-18b is habitable.” Fujisawa and co-authors said Hycean atmospheres are “likewise consistent with the data,” not that they have been proven correct. (arxiv.org) The paper also says mass-balance arguments imply that maintaining a roughly 1 bar hydrogen envelope with percent-level methane would require interior replenishment over gigayear timescales. (arxiv.org) That makes the study partly a chemistry-and-climate consistency test, not just a spectral curve fit. ### What should readers watch next? The next phase is likely to center on the same spectra, not a simple headline reversal. Competing groups are now arguing over how robust the inferred CO2, CO and other species are in JWST observations of K2-18b, and those abundance constraints are central to whether the planet is framed as a Hycean world or a gas-rich mini-Neptune. (arxiv.org) Astrobiology’s May 21 post points readers to the new paper, while the underlying preprint remains available on arXiv with Fujisawa, Shimada, Yoshida and Kuramoto listed as authors. (arxiv.org) Any stronger shift in the case for K2-18b will depend on future peer review, follow-on analyses and additional JWST-based atmospheric interpretation work. (astrobiology.com)