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

TL;DR

This study tackles the challenge of characterizing temperate exoplanet atmospheres by pairing laboratory experiments with 0D photochemical modeling to explore out-of-equilibrium chemistry in H$_2$-dominant gas mixtures. By varying CH$_4$, CO, and CO$_2$ across a wide range of C/O and metallicities, the authors identify key formation pathways for reduced hydrocarbons and oxidized organics, revealing that methane greatly promotes hydrocarbon growth while CO and CO$_2$ modulate oxygen incorporation and chemical diversity. The combined MS-IR diagnostics and Monte Carlo-driven simulations show that CH$_4$-rich, CO- or CH$_4$/CO/CO$_2$-mixed atmospheres favor hydrocarbons and oxidized products such as H$_2$CO, CH$_3$OH, and CH$_3$CHO, whereas CO$_2$-rich environments suppress hydrocarbon production due to oxidative losses. These results illuminate the role of non-equilibrium chemistry in temperate exoplanet atmospheres and provide concrete predictions for the detectability of photoproducts with JWST and future ground-based facilities. The work underscores the value of integrating experiments, modeling, and observations to interpret exoplanetary atmospheres and assess their potential prebiotic chemistry.

Abstract

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. Complementary approaches combining laboratory experiments and photochemical modeling are essential for constraining atmospheric chemistry and interpreting observations. 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. Our approach combines experimental and numerical simulations on H2-rich gas mixtures representative of sub-Neptune atmospheres, spanning a wide range of CH4, CO, and CO2 mixing ratios. A cold plasma reactor simulates out-of-equilibrium upper-atmospheric chemistry. A 0D photochemical model reproduces reactor conditions, guiding interpretation of key pathways and abundance trends. We observe the formation of both reduced and oxidized organic compounds. In CH4-rich mixtures, hydrocarbons form efficiently through methane chemistry, correlating with CH4 concentration and agreeing with models. In more oxidizing environments, particularly CO2-rich mixtures, hydrocarbon formation is inhibited by complex reaction networks and oxidative losses. Oxygen incorporation enhances chemical diversity and promotes 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. Out-of-equilibrium chemistry plays a key role in the diversification and organic complexification of temperate exoplanet atmospheres.

Figures (12)

• Figure 1: Scheme of the plasma reactor PAMPRE.
• Figure 2: Mass spectrum of a gas mixture of 95% H$_2$ + 5% CH$_4$ before (gold) and during irradiation (blue).
• Figure 3: InfraRed features of the reduced organics.
• Figure 4: InfraRed features of the oxidized organics.
• Figure 5: Mass spectra acquired for different gas mixtures while plasma is ON.