Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b

Abstract

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 (Kzz). This study explores the parameter space of metallicity, C/O ratio, and Kzz 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.