Simulating and Modeling to Understand Atmospheres of Temperate Exoplanets

The atmospheres of temperate exoplanets present a significant challenge to characterize, largely due to their diminutive size and relatively cool temperatures. Although recent data gathered by the James Webb Space Telescope (JWST) offers invaluable insights, the interpretation of this information has, in some cases, led researchers to differing conclusions. To resolve these discrepancies and gain a more complete understanding of atmospheric chemistry, a combined strategy utilizing both laboratory experiments and photochemical modeling is essential.

Researchers are focusing on identifying the chemical processes that govern the creation and development of neutral elements. They also aim to determine how sensitive these processes are to crucial factors like the carbon-to-oxygen (C/O) ratio and metallicity. Their methodology involves experimental work alongside numerical simulations performed on hydrogen-rich gas mixtures. These mixtures are designed to be representative of sub-Neptune atmospheres and encompass a broad spectrum of mixing ratios for methane (CH4), carbon monoxide (CO), and carbon dioxide (CO2).

To simulate the non-equilibrium chemistry of upper atmospheres, a cold plasma reactor is employed. Furthermore, a 0D photochemical model mirrors the conditions within the reactor, thus aiding in the interpretation of key reaction pathways and overall abundance trends. This setup has enabled the observation of both reduced and oxidized organic compounds forming within these simulated atmospheres.

The experiments reveal that in methane-dominant mixtures, hydrocarbons develop efficiently as a result of methane chemistry. This formation is directly related to the concentration of CH4, a finding that aligns with model predictions. That said, the reality is a bit more complicated. in more oxidizing environments, particularly those rich in CO2, the creation of hydrocarbons is hindered by intricate reaction networks and oxidative losses.

Incorporating oxygen into the mix boosts chemical diversity, fostering the production of oxidized organic compounds that are relevant to prebiotic chemistry. Examples include formaldehyde (H2CO), methanol (CH3OH), and acetaldehyde (CH3CHO). This effect is especially pronounced in atmospheres containing both methane and carbon dioxide. Atmospheres that feature both methane and carbon monoxide appear to strike a balance, delivering sufficient carbon and oxygen without excessive oxidative destruction. This balance favors the efficient generation of both hydrocarbons and oxidized compounds.

In summary, out-of-equilibrium chemistry is crucial in driving the diversification and organic complexification observed in temperate exoplanet atmospheres.

The study, "Understanding The Chemistry Of Temperate Exoplanets Atmospheres Through Experimental And Numerical Simulations," by O. Sohier, A. Y. Jaziri, L. Vettier, A. Chatain, T. Drant, and N. Carrasco, has been accepted for publication in *Astronomy & Astrophysics*. A preprint is available on arXiv:2512.16421.