2026A&A...707A..53S

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.