Exoplanets beyond the Conservative Habitable Zone: I. Habitability

The Habitable Zone (HZ) is defined by the possibility of sustaining liquid water on a planetary surface. In the Solar System, the HZ for a conservative climate model extends approximately between the orbits of Earth and Mars. We elaborate on earlier HZ models and apply an analytical climate model of the temperature distribution on tidally-locked planets to extend the HZ. We show that planets orbiting M- and K-dwarf stars may maintain liquid water on their night side, significantly closer to their host star than the inner border of the conservative HZ. We calculate the extended borders of the HZ in the flux-effective temperature diagram. This extension may explain the presence of water vapor and other volatile gases in the transmission spectra of warm Super-Earth-sized exoplanets closely orbiting M dwarfs, recently detected by JWST. We also mention the HZ extension outwards, due to subglacial liquid water in the form of intra-glacial lakes or subglacial melting.