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

Abstract

The atmospheric characterisation of temperate exoplanets is 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. We investigate K2-18 b using a self-consistent forward model to constrain its metallicity, composition, and thermal structure, with particular emphasis on the role of disequilibrium chemistry, photochemical hazes and clouds. For the first time in this context, we also assess the impact of photoelectrons on the atmospheric chemistry of an exoplanet. We explore a wide range of metallicities and intrinsic temperatures, evaluate haze and cloud formation, and compare the resulting transmission spectra with available JWST observations from multiple independent pipelines. We find that a high metallicity (200-400xsolar) H2-rich atmosphere consistently reproduces the observed transit spectra, independent of the data reduction pipeline used. The atmospheric composition is strongly shaped by disequilibrium chemistry, with CH4 dominating the spectrum alongside contributions from CO2 and OCS, and a potential contribution from C2H4 at mid-infrared wavelengths. Photoelectrons enhance the production of several disequilibrium species, particularly nitrogen-bearing molecules. Photochemical hazes play a key role in shaping the thermal structure, producing a temperature minimum near the 10-100 mbar range that enables efficient H2O condensation, suppressing its gaseous abundance. Under sufficiently strong haze cooling, NH4SH condensation provides a natural explanation for the apparent absence of NH3 in the observed spectra. No additional molecular species beyond those considered here are required to reproduce the observed spectra.

Figures (14)

• Figure 1: Transit spectrum of K2-18 b based on the detection with Kepler Montet15 and characterisation with HST/WFC Benneke17, Spitzer Benneke19, JWST/NIRISS &/NIRSpec Madhusudhan23Schmidt25 and JWST/MIRI Madhusudhan25Liu25. Different panels compare different pipelines for the NIRISS and NIRSpec analyses.
• Figure 2: Equilibrium abundances of H$_2$O, CO, CO$_2$ and CH$_4$ for a 300x solar metallicity case across different temperatures at 1000 bar. Different style lines show the implications of changing the C/O ratio.
• Figure 3: Thermal structure (top row) for the different metallicity cases considered. Blue (orange) lines correspond to the low (high) photochemical haze mass flux scenario, while solid and dashed lines correspond to the T$_{int}$ = 30 K and 60 K cases, respectively. The middle (high $\Phi_H$) and lower (low $\Phi_H$) panels present the corresponding disequilibrium composition for the high (solid) and low (dashed) T$_{int}$ cases, respectively.
• Figure 4: Particle size distributions including both photochemical hazes and H$_2$O clouds for different metallicity cases. The top row corresponds to T$_{int}$=30K and the bottom row to T$_{int}$=60K.
• Figure 5: Mean particle properties for hazes and H$_2$O clouds. The left panel present the mean density profiles for the 200x solar metallicity case for the different cases of T$_{int}$ (line styles) and $\Phi_H$ explored (line colours). The middle and right panels present the mean particle properties across different metallicities for hazes and clouds, respectively. For the haze we present results at 0.01 bar, while for the clouds the integrated mean values over the narrow altitude range of their existence.