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Venus is astrophysically indistinguishable from Earth and has a global cloud layer that truncates spectral features in transmission spectroscopy. To distinguish an exo-Venus from an exo-Earth therefore requires unique observational indicators that survive above the cloud layer. However, the above-cloud atmosphere is dominated by photochemistry which depends on the spectrum of the host star, and therefore changes for different stellar systems. In this study we explore the systematic changes in photochemistry that occur 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 and stellar spectra from the MUSCLES Treasury survey. We find that the trace gases SO2, OCS, and H2S, all survive above the cloud layer when irradiated by the coolest K-Dwarf and all M-Dwarfs in our study, whereas these three species are heavily photochemically depleted above the clouds of Venus. Furthermore, the production of sulfuric acid molecules that form the cloud layer decreases for decreasing stellar effective temperature. We conclude that these trace sulfur gases will be more observable on Venus-like exoplanets around M-Dwarf host stars, and are therefore potential observational indicators that can distinguish an exo-Venus from an exo-Earth.

Venus as an Exoplanet: Photochemistry around K- and M-Dwarfs