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

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.

💡 Research Summary

This study presents a comprehensive analysis of the atmosphere of K2-18 b, a temperate sub-Neptune orbiting an M dwarf star, using observations from the James Webb Space Telescope (JWST). The research employs a sophisticated one-dimensional self-consistent forward model that couples radiative transfer, disequilibrium gas-phase chemistry, and haze/cloud microphysics to explore a wide range of atmospheric scenarios.

The core finding is that a high-metallicity (200 to 400 times solar), hydrogen-rich atmosphere consistently reproduces the available transit spectroscopy data from JWST’s NIRISS, NIRSpec, and MIRI instruments. This conclusion holds largely independent of the data reduction pipeline used, strengthening the case for K2-18 b being a world that has retained a significant primordial H2/He envelope.

The atmospheric composition is profoundly shaped by disequilibrium processes. Methane (CH4) dominates the observed transmission spectrum, with significant contributions from carbon dioxide (CO2) and carbonyl sulfide (OCS). The study also identifies a potential spectral contribution from ethylene (C2H4) at mid-infrared wavelengths. A novel aspect of this work is the first assessment of photoelectrons—electrons generated by stellar high-energy radiation—in the context of a temperate exoplanet. While their direct impact on the current spectrum is limited, photoelectrons are shown to enhance the production of several disequilibrium species, particularly nitrogen-bearing molecules.

Photochemical hazes play a pivotal role in shaping the atmospheric thermal structure. Their efficient infrared cooling creates a pronounced temperature minimum in the 10-100 mbar region. This cold trap enables efficient condensation of water vapor, forming H2O clouds and suppressing the gaseous H2O abundance in the atmospheric region probed by transit observations, explaining its apparent absence. Furthermore, under sufficiently strong haze-induced cooling, the condensation of ammonium hydrosulfide (NH4SH) clouds provides a natural, non-biological explanation for the non-detection of ammonia (NH3) in the spectra.

The research contrasts its findings with other proposed scenarios, such as the Hycean (water-world) model, and finds the hazy, high-metallicity sub-Neptune model more consistent with the full JWST dataset and planetary density constraints. The study concludes that the combined effects of disequilibrium chemistry, photochemical hazes, and cloud formation are essential for interpreting the current observations of K2-18 b. It demonstrates that the observed spectrum can be explained without invoking additional, more complex molecular species, providing a robust physical and chemical framework for understanding this benchmark temperate exoplanet.