Towards the Minimum Inner Edge Distance of the Habitable Zone

TL;DR

This study determines that the inner edge of the habitable zone for hot desert worlds can be as close as 0.38 AU from a solar-like star, expanding the potential for habitable exoplanets.

Contribution

It introduces new estimates for the inner edge of the habitable zone considering various atmospheric parameters, suggesting closer habitable zones than previously thought.

Findings

Inner edge can be as close as 0.38 AU for hot desert worlds.

Water loss timescale depends on atmospheric CO2 levels.

Transmission spectra of hot desert worlds resemble Earth-like planets.

Abstract

We explore the minimum distance from a host star where an exoplanet could potentially be habitable in order not to discard close-in rocky exoplanets for follow-up observations. We find that the inner edge of the Habitable Zone for hot desert worlds can be as close as 0.38 AU around a solar-like star, if the greenhouse effect is reduced ($\sim$ 1% relative humidity) and the surface albedo is increased. We consider a wide range of atmospheric and planetary parameters such as the mixing ratios of greenhouse gases (water vapor and CO$_2$), surface albedo, pressure and gravity. Intermediate surface pressure ($\sim$1-10 bars) is necessary to limit water loss and to simultaneously sustain an active water cycle. We additionally find that the water loss timescale is influenced by the atmospheric CO$_2$ level, because it indirectly influences the stratospheric water mixing ratio. If the CO$_2$ mixing ratio of dry planets at the inner edge is smaller than 10$^{-4}$, the water loss timescale is $\sim$1 billion years, which is considered here too short for life to evolve. We also show that the expected transmission spectra of hot desert worlds are similar to an Earth-like planet. Therefore, an instrument designed to identify biosignature gases in an Earth-like atmosphere can also identify similarly abundant gases in the atmospheres of dry planets. Our inner edge limit is closer to the host star than previous estimates. As a consequence, the occurrence rate of potentially habitable planets is larger than previously thought.