Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b
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| Format: | Preprint |
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2025
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| _version_ | 1866916929668644864 |
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| author | Jaziri, A. Y. Sohier, O. Venot, O. Carrasco, N. |
| author_facet | Jaziri, A. Y. Sohier, O. Venot, O. Carrasco, N. |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2507_14983 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b Jaziri, A. Y. Sohier, O. Venot, O. Carrasco, N. Earth and Planetary Astrophysics 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. |
| title | Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2507.14983 |