Photochemistry of Venus-Like Planets Orbiting K- and M-Dwarf Stars

TL;DR

This study investigates how stellar spectra influence the atmospheric photochemistry of Venus-like exoplanets orbiting K- and M-dwarfs, identifying sulfur gases as key observational markers to distinguish them from Earth-like planets.

Contribution

It systematically analyzes the impact of different stellar types on Venus-like planetary atmospheres, highlighting sulfur gases as potential indicators for exo-Venus detection.

Findings

SO2, OCS, and H2S survive above clouds around K- and M-dwarfs

Production of sulfuric acid decreases with cooler stars

Trace sulfur gases are promising observational markers

Abstract

Compared to the diversity seen in exoplanets, Venus is a veritable astrophysical twin of the Earth, however its global cloud layer truncates features in transmission spectroscopy, masking its non-Earth-like nature. Observational indicators that can distinguish an exo-Venus from an exo-Earth must therefore survive above the cloud layer. The above-cloud atmosphere is dominated by photochemistry, which depends on the spectrum of the host star and therefore changes between stellar systems. We explore the systematic changes in photochemistry above the clouds of Venus-like exoplanets orbiting K-Dwarf or M-Dwarf host stars, using a recently validated model of the full Venus atmosphere (0-115 km) and stellar spectra from the MUSCLES Treasury survey. SO2, OCS and H2S are key gas species in Venus-like planets that are not present in Earth-like planets, and could therefore act as observational discriminants if their atmospheric abundances are high enough to be detected. We find that SO2, OCS and H2S all survive above the cloud layer when irradiated by the coolest K-Dwarf and all seven M-Dwarfs, whereas these species are heavily photochemically depleted above the clouds of Venus. The production of sulfuric acid molecules that form the cloud layer decreases for decreasing stellar effective temperature. Less steady-state photochemical oxygen and ozone forms with decreasing stellar effective temperature, and the effect of chlorine-catalysed reaction cycles diminish in favour of HOx and SOx catalysed cycles. We conclude that trace sulfur gases will be prime observational indicators of Venus-like exoplanets around M-Dwarf host stars, potentially capable of distinguishing an exo-Venus from an exo-Earth.