Unraveling The Non-equilibrium Chemistry Of The Temperate Sub-Neptune K2-18 b

The search for habitable, Earth-like exoplanets faces major observational challenges due to their small size and faint signals.

M-dwarf stars offer a promising avenue to detect and study smaller planets, especially sub-Neptunes-among the most common exoplanet types. K2-18 b, a temperate sub-Neptune in an M-dwarf habitable zone, has been observed with HST and JWST, revealing an H₂-rich atmosphere with CH₄ and possible CO₂.

Conflicting interpretations highlight the importance of non-equilibrium chemistry, which is critical for constraining atmospheric parameters like metallicity, C/O ratio, and vertical mixing (K_zz). This study explores the parameter space of metallicity, C/O ratio, and K_zz for K2-18 b using the non-equilibrium chemistry model FRECKLL and JWST data. We generated spectra from a 3D grid of models and compared them to observations to refine atmospheric constraints.

A fixed pressure-temperature profile was used to capture first-order chemical trends, acknowledging some uncertainties. Our best-fit model favors high metallicity (266^{+291}{-104} at 2 sigma) and high C/O ratio (C/O > 2.1 at 2 sigma). CH4 is robustly detected (log₁₀[CH₄] = -0.3^{+0.1}{-1.7} at 1 mbar), while CO₂ remains uncertain due to spectral noise. Kzz has no clear impact on the fit and remains unconstrained.

Non-equilibrium models outperform flat spectra at > 4 sigma confidence, confirming atmospheric features. Minor species, such as H₂O and NH₃, may be present but are likely masked by dominant absorbers. Our results highlight the limits of constant-abundance retrievals.

The atmosphere has a high C/O ratio suggesting possible aerosol formation. Better constraints require higher-precision data. Future JWST NIRSpec G395H and ELT/ANDES observations will be critical for probing habitability and refining models.

A. Y. Jaziri, O. Sohier, O. Venot, N. Carrasco

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2507.14983 [astro-ph.EP] (or arXiv:2507.14983v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.14983
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Submission history
From: Adam Yassin Jaziri
[v1] Sun, 20 Jul 2025 14:39:00 UTC (748 KB)
https://arxiv.org/abs/2507.14983
Astrobiology,