Tracing the inner edge of the Habitable Zone with exo-Venuses

The Venus—Earth dichotomy inspires our understanding of the inner edge of the liquid-water Habitable Zone (HZ), yet, multiple theories exist to define the HZ inner edge and Venus's own climate history is debated. Theories of the HZ inner edge can be tested provided we can observationally distinguish Earth-like planets with liquid water oceans, from Venus-like planets with dry planetary surfaces. Dry planetary surfaces can potentially be identified by observing atmospheric sulfur dioxide (SO2), which is otherwise scrubbed from the atmospheres of Earth-like planets via wet deposition. However, SO2 in the atmospheres of Venus-like planets can be efficiently destroyed by photochemistry. We here demonstrate how the photochemical behaviour of SO2 can allow us to observationally identify dry planetary surfaces, but uniquely around M-dwarf stars. We propose a statistical comparative planetology study that can constrain the location of the inner edge of the habitable zone around M-dwarf stars in the near future using exo-Venuses rather than exo-Earths.