A Hycean Interpretation of K2-18b Supported by Photochemical Atmospheric Compositional

The nature of the sub-Neptune K2-18b is debated between Hycean and mini-Neptune interpretations.

We test whether self-consistent Hycean atmospheres are compatible with current JWST transmission spectra by combining one-dimensional photochemical modelling, radiative–convective equilibrium calculations, and forward modelling of transmission spectra.

We assume H₂-CH₄-H₂O atmospheres over a liquid ocean, compute altitude-dependent abundances with a 1D photochemical model, and couple them to P-T profiles that avoid runaway greenhouse states.

Using the CH4-dominated 2.8-4.0 μm band, we constrain wavelength-independent offsets between NIRISS SOSS and NIRSpec G395H for multiple reductions, and then scan grids of CO and CO₂ scaling factors, weighted by the CH₄-band offset posteriors, to evaluate oxidised-carbon abundances consistent with the 4-5 μm region. Radiative–convective calculations further map pressures and albedos that yield non-runaway climates.

Over a wide range of temperatures and pressures, liquid oceans can exist, and Hycean models with a 1 bar H₂ envelope, percent-level CH₄ and CO, and CO₂ buffered at ∼10⁻³-10⁻² reproduce the NIRISS and NIRSpec spectra from 0.8 to 5.2 μm without invoking DMS or other additional species.

Our photochemical simulations show that H₂-CH₄-H₂O networks generically drive CO to mixing ratios of order 1-2 %. Mass-balance arguments imply that a ∼1 bar H₂ envelope with percent-level CH₄ requires interior replenishment on gigayear timescales, and the resulting vertical gradients naturally generate flat, CH₄-dominated plateaux in transmission.

While mini-Neptune scenarios remain viable, our results show that Hycean configurations are likewise consistent with the data, and current CO and CO₂ constraints alone are not yet sufficient to rule out Hycean interpretations of K2-18b.

Takuya Fujisawa, Masashi Shimada, Tatsuya Yoshida, Kiyoshi Kuramoto

Comments: 28pages, 15figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2605.17803 [astro-ph.EP] (or arXiv:2605.17803v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2605.17803
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Submission history
From: Takuya Fujisawa
[v1] Mon, 18 May 2026 03:26:08 UTC (6,401 KB)
https://arxiv.org/abs/2605.17803
Astrobiology, exoplanet,