Tracing the Inner Edge of the Habitable Zone with Sulfur Chemistry

TL;DR

This paper proposes using atmospheric sulfur gases as indicators to map the inner edge of the habitable zone around exoplanets, with detection feasibility depending on the host star's UV flux, enabling empirical habitable zone definition.

Contribution

It introduces a novel method to identify habitable exoplanets via sulfur chemistry, accounting for stellar UV flux variations, and suggests a population-level approach for empirical habitable zone mapping.

Findings

Sulfur gases can indicate dry planetary surfaces.

UV flux of host star affects sulfur gas observability.

Population studies can empirically define the habitable zone.

Abstract

The circumstellar liquid-water habitable zone guides our search for potentially inhabited exoplanets, but remains observationally untested. We show that the inner edge of the habitable zone can now be mapped among exoplanets using their lack of surface water, which, unlike the presence of water, can be unambiguously revealed by atmospheric sulfur species. Using coupled climate-chemistry modelling we find that the observability of sulfur-gases on exoplanets depends critically on the ultraviolet (UV) flux of their host star, a property with wide variation: most M-dwarfs have a low UV flux and thereby allow the detection of sulfur-gases as a tracer of dry planetary surfaces; however, the UV flux of Trappist-1 may be too high for sulfur to disambiguate uninhabitable from habitable surfaces on any of its planets. We generalise this result to show how a population-level search for sulfur-chemistry on M-dwarf planets can be used to empirically define the Habitable Zone in the near-future.