3D photochemistry of Venus-like planets orbiting M-dwarf stars

Venus provides a unique natural laboratory for the study of rocky exoplanets close to their host stars, where intense irradiation may drive atmospheric conditions analogous to those of Venus. The population of rocky planets has grown significantly with technological advances in the detection of such bodies, and most of them have been discovered orbiting M-type stars. However, the atmospheric characteristics of this population remain an open question. In addition, the detection of Earth-like planets is a major challenge in the search for extraterrestrial life because the main characteristics of Earth-like planets are similar to those found on other terrestrial planets like Venus.

 

Venus and Earth are regarded as twin sisters due to the many physical characteristics they share (size, mass, density, etc.). However, the main difference lies in their atmospheres. Possible atmospheric constituents such as CO2 are now detectable with JWST [1] but most of species will be corroborated only with new generation of instruments on board the Extremely Large Telescope (ELT) (e.g. ANDES [2] or with the Large Interferometer for Exoplanets (LIFE) [3]). The study of rocky exoplanet atmospheres is strongly complicated by clouds and aerosols. Cloud layers can mask the deeper atmosphere in transmission and emission spectroscopy [4], [5], while cloudy CO₂–N₂ atmospheres may produce similar spectra for both CO₂- and N₂-dominated cases [6]. As a result, observations cannot easily distinguish between Venus-like and Earth-like planets based only on CO₂ and N₂ spectral signatures.

 

Thus, studying photochemistry is essential because it occurs primarily above the cloud layer, where atmospheric species remain observable. Photochemical processes can produce distinctive spectral signatures that may be detectable remotely and could therefore help distinguish between different types of planetary atmospheres, such as Venus-like and Earth-like worlds. The photochemistry of planetary atmospheres around M-type stars differs from that around the Sun due to a different stellar spectrum and thus different photolysis and photochemical reactions. [7] used 1D photochemical models to study the atmospheres of exo-Venuses orbiting K- and M-type stars. While on Venus, SO2 is heavily depleted in the cloud, on Venus-like planets receiving low UV flux (e.g. from M-dwarf colder stars) the photolysis would be inefficient to convert SO2 into H2SO4 clouds, allowing sulphur species to survive in the upper atmosphere. To date, no 3D studies have been carried out. Tidal-locked rocky planets could exhibit different spectral signatures between the solar and antisolar sides. However, the resulting atmospheric circulation on such planets could also redistribute these species and alter their abundances. Studying 3D photochemistry on Venus analogues around M-dwarf stars will enable us to narrow down the species that may be detected in the future by the JWST or the ELT.

 

To simulate the photochemistry of planets with physical characteristics similar to those of Venus orbiting M-dwarf stars, we used the Generic-PCM coupled with the photochemical module developed by [8]. This photochemical module is a chemical solver that tracks the evolution of chemical species linked through a network of chemical reactions and photodissociation processes. We included sulfur-bearing species as well as the main reactions involved in the production and loss of H2SO4, following the work of [9]. We adopted synthetic M-dwarf spectra from [10,11] to reproduce the photochemistry of Venus-like planets orbiting this type of star. In addition, we developed a simplified H2SO4-H2O condensation scheme to simulate the condensation of H2SO4-H2O droplets without specific microphysical processes similarly as [12]. This module includes condensation, re-evaporation and sedimentation processes with a fixed particle distribution.

We performed simulations from the surface up to 100 km altitude for Venus-like planets subjected to different M-dwarf stellar spectra. In this work, we investigated how sulfur-bearing species survive above the cloud deck at both the substellar and antistellar points, and how atmospheric circulation influences photochemistry and the spatial distribution of chemical species across the planet. We also evaluated the changes in H2O and H2SO4 production and loss between a Venus analogue orbiting an M-dwarf star and one orbiting the Sun, and finally, identified the chemical species above the cloud layer that could be detected on potential Venus-like planets.

 

References :

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[2] Palle et al. (2023), Experimental Astronomy, doi:10.1007/s10686-025-10000-4

[3] Konrad et al. (2022), Astronomy & Astrophysics, doi:10.1051/0004-6361/202141964

[4] Ehrenreich et al. (2012), Astronomy & Astrophysics, doi:10.1051/0004-6361/201118400

[5] Lustig-Yaeger et al. (2019), The Astrophysical Journal Letters, doi:10.3847/2041-8213/ab5965

[6] Benneke et al. (2012), The Astrophysical Journal, doi :10.1088/0004-637X/753/2/100

[7] Jordan et al. (2021), The Astrophysical Journal, doi:10.3847/1538-4357/ac1d46

[8] Jaziri (2021), doi:10.70675/e18a9ce7z543dz4540zba04zea34b71ab894

[9] Stolzenbach et al. (2023), Icarus, 10.1016/j.icarus.2023.115447

[10] Rugheimer et al. (2013), Astrobiology, doi:10.1089/ast.2012.0888

[11] Rugheimer et al. (2015), The Astrophysical Journal, doi:10.1088/0004-637X/809/1/57

[12] Dai et al. (2022), Journal of Geophysical Research: Planets, doi:10.1029/2021JE007060

Acknowledgments: This work is funded by the Project CNS2024-154576 funded by MCIU/AEI, PN2024 -Research Consolidation-State Subprogram for Training, Attraction and Retention of Research and Innovation Talent - State Program of Human Resources-PEICTI 2024-2027.”