The characterization of temperate sub-Neptunes like K2-18 b, orbiting within the habitable zone of M-dwarf stars, provides a unique opportunity to study the diversity of exoplanetary atmospheres. However, recent JWST observations have revealed apparent inconsistencies between instruments and highlight the complexity of interpreting these data.

Previous analyses combining HST and JWST/NIRISS–NIRSpec spectra indicate a H₂-rich, CH4-bearing atmosphere with possible CO₂, favoring high metallicity and a super-solar C/O ratio under non-equilibrium chemistry. Yet, the MIRI LRS spectra exhibit features nearly twice as large as those at shorter wavelengths, challenging these high-metallicity, CH₄-rich interpretations.

In this study, we explore the atmospheric composition of K2-18 b through an extensive grid of non-equilibrium chemical models using FRECKLL, complemented by atmospheric retrievals that incorporate aerosol absorption and scattering. We systematically vary metallicity, C/O ratio, and vertical mixing (Kzz), and test the influence of laboratory-derived photochemical haze analogues. Our analysis shows that while non-equilibrium models favor high metallicities (~100–300x solar) and C/O ≥ 2, the inclusion of absorbing aerosols—particularly CH₄-dominated, nitrogen-poor tholins—can reproduce both NIRISS and MIRI features while lowering the inferred metallicity by over an order of magnitude. These hazes explain the NIRISS spectral slope through scattering and the MIRI absorption near 7 um via C–H bending modes.

Our findings demonstrate that degeneracies between metallicity, molecular abundances, and aerosol properties play a key role in interpreting sub-Neptune spectra. They further emphasize the necessity of non-equilibrium and aerosol-inclusive models to avoid biased retrievals based on constant-abundance assumptions. Future JWST/NIRSpec G395H and ELT/ANDES observations, combined with laboratory haze studies, will be crucial to disentangling these effects and refining the prospects for habitability in temperate sub-Neptune atmospheres.

Yassin Jaziri, LATMOS.

Location
LATMOS
Guyancourt
Room 2202

Online
https://cnrs.zoom.us/j/91943565767?pwd=vtI6AYKgkBWufqWMCRLDpnV6joJHpn.1