Understanding the chemistry of temperate exoplanet atmospheres through experimental and numerical simulations

Context. Characterizing temperate exoplanet atmospheres remains challenging due to their small size and low temperatures. Recent JWST observations provide valuable data, but their interpretation has led to diverging conclusions, highlighting the limitations of observations alone. Complementary approaches combining laboratory experiments and photochemical modeling are essential for constraining atmospheric chemistry and interpreting observations. Aims. This study investigates out-of-equilibrium chemistry in the upper atmospheres of H2-dominated temperate sub-Neptunes enriched in carbon-bearing species (CH4, CO, or CO2). We aim to identify chemical pathways governing the formation and evolution of neutral species and to assess their sensitivity to key parameters such as C/O ratio and metallicity. Methods. Our approach combines experimental and numerical simulations on H2-rich gas mixtures representative of sub-Neptune atmospheres and spanning a wide range of CH4, CO, and CO2 mixing ratios. We used a cold plasma reactor to simulate out-of-equilibrium upper-atmospheric chemistry. Chemical evolution was tracked by mass spectrometry and infrared spectroscopy (IR). A 0D photochemical model was used to reproduce reactor conditions, guiding interpretation of the key pathways and abundance trends. Results. We observed the formation of both reduced and oxidized organic compounds. In CH4-rich mixtures, hydrocarbons formed efficiently through methane chemistry, correlating with CH4 concentration and agreeing with models. In more oxidizing environments, particularly CO2-rich mixtures, hydrocarbon formation was inhibited by complex reaction networks and oxidative losses. We find that oxygen incorporation enhances chemical diversity and promotes the formation of oxidized organic compounds of prebiotic interest (H2CO, CH3OH, CH3CHO), especially in atmospheres containing both CH4 and CO2. Atmospheres containing CH4 and CO – which balance carbon and oxygen supply without excessive oxidative destruction – favor efficient production of hydrocarbons and oxidized compounds. Conclusions. Out-of-equilibrium chemistry plays a key role in the diversification and organic complexification of temperate exo-planet atmospheres. Combining laboratory experiments with photochemical modeling elucidates pathways to hydrocarbon and oxidized organic formation. Studying detectability of these photoproducts with JWST and new high-resolution ground-based instruments is an important focus for future studies.