The atmosphere of K2-18 b: The role of hazes, clouds, and photoelectrons

The atmospheric characterisation of temperate exoplanets is now becoming accessible with JWST, providing a critical connection between Solar System planets and the more commonly observed hot-Jupiters. K2-18 b, a temperate sub-Neptune orbiting an M dwarf, has emerged as a benchmark case following extensive JWST observations and ongoing debate regarding its atmospheric composition [1-10].

We investigated the atmosphere of K2-18 b using a self-consistent forward model [11] in order to constrain its metallicity, composition, and thermal structure, with a particular emphasis on the role of disequilibrium chemistry, photochemical hazes, and clouds. For the first time in this context, we also assessed the impact of photoelectrons on the atmospheric chemistry of an exoplanet.

We employed a one-dimensional model that couples stellar energy deposition, disequilibrium gas-phase chemistry, and haze and cloud microphysics to generate physically consistent atmospheric scenarios. We explored a wide range of metallicities and intrinsic temperatures, evaluated haze and cloud formation, and compared the resulting transmission spectra with available JWST observations (NIRISS, NIRSpec, MIRI) reduced using multiple independent pipelines [1-5].

We demonstrate that a high metallicity (200-400×solar) H₂-rich atmosphere consistently reproduces the observed transit spectra of K2-18 b, largely independent of the data reduction pipeline used. The atmospheric composition is strongly shaped by disequilibrium chemistry, with CH₄ dominating the spectrum alongside significant contributions from CO₂ and OCS, and a potential contribution from C₂H₄ at mid-infrared wavelengths. Photochemical hazes play a key role in shaping the thermal structure, producing a temperature minimum near the 10–100 mbar level that enables efficient condensation of H₂O and suppresses its gaseous abundance in the region probed by transit observations. Photoelectrons enhance the production of several disequilibrium species, particularly nitrogen-bearing molecules, although their direct impact on the current transmission spectra remains limited. Under sufficiently strong haze cooling, condensation of NH₄SH provides a natural explanation for the apparent absence of NH₃ in the observed spectra.

In summary, our results indicate that the JWST observations of K2-18 b are best explained by a hazy, high-metallicity sub-Neptune atmosphere shaped by disequilibrium chemistry. The combined effects of photochemical hazes and cloud formation are essential for interpreting the current K2-18 b observations. While uncertainties remain regarding haze optical properties, no additional molecular species beyond those considered here are required to reproduce the observed spectra.

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