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

¹ LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France
² Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Université Paris Cité and Univ Paris Est Creteil, CNRS, LISA, 75013 Paris, France
⁴ ENS Paris-Saclay, Université Paris-Saclay, Gif-sur-Yvette 91190, France

^★ Corresponding author.

Received: 12 May 2025
Accepted: 17 July 2025

Abstract

Context. The search for habitable Earth-like exoplanets can present significant observational challenges, due to their small size and faint signals. M-dwarf stars provide an opportunity to detect and characterize smaller planets, particularly sub-Neptunes, which are among the most common exoplanetary types. K2-18 b, a temperate sub-Neptune in the habitable zone of an M-dwarf star, has been studied using HST and JWST, revealing an H₂-rich atmosphere with detected CH₄ and possible CO₂. However, previous debates on atmospheric composition emphasize the importance of non-equilibrium chemistry models. These models are crucial for interpreting exoplanetary atmospheres, and for constraining key parameters such as metallicity, C/O ratio, and vertical mixing (K_zz).

Aims. This study aims to comprehensively explore the parameter space of metallicity, C/O, and K_zz for K2-18 b using the non-equilibrium chemical model FRECKLL in conjunction with JWST observational data. By refining these constraints, we seek to improve our understanding of the planet’s atmospheric composition and to detect minor species in a temperate sub-Neptune within the habitable zone of an M-dwarf star.

Methods. Our approach involves running non-equilibrium chemical models across a three-dimensional parameter space (metallicity, C/O, and K_zz), generating the corresponding theoretical spectra, and comparing these spectra to JWST observational data to refine atmospheric constraints. This approach assumes a fixed P–T profile, which is sufficient to capture first-order chemical trends, though it introduces some uncertainty in the derived values.

Results. We retrieved a best-fit atmospheric model for K2-18 b favoring high metallicity (266₋₁₀₄⁺²⁹¹ at 2σ) and a high C/O value (C/O ≥ 2.1 at 2σ). CH₄ is robustly detected (log₁₀[CH₄] = −0.3_−1.7^+0.1 at 1 mbar), while CO₂ and other species remain uncertain due to observational noise and spectral overlap. The K_zz has no significant impact on the fit and remains unconstrained. Non-equilibrium models exceed 4σ confidence over a flat-line, which validates the presence of atmospheric features. Several minor species may exist at ppm levels, though their features are likely masked by dominant species.

Conclusions. We used non-equilibrium chemical models and JWST data to investigate the atmosphere of K2-18 b, revealing a high metallicity, a high C/O, and complex chemical composition. While CH₄ is robustly detected, CO₂ remains uncertain, and minor species (e.g., H₂O and NH₃) are likely present. A lower limit on C/O is constrained, though no upper limit is established. The high C/O also suggests a higher probability of aerosol formation. Our findings highlight the limitations of traditional retrievals with constant abundances and the importance of non-equilibrium models with combining exploration on a large range of metallicity and C/O values. Although equilibrium models can reproduce the data, physical conditions indicate that the atmosphere is in a non-equilibrium state, highlighting the limited constraints from current observations and the pressing need for improved data. Future observations with JWST NIRSpec G395H and ELT/ANDES will be key to refining atmospheric constraints and probing potential habitability.