Atmospheric Evolution on Low-gravity Waterworlds

Abstract: Low-gravity waterworlds ($M\lesssim 0.1 M_{\oplus}$) are of interest for their potential habitability. The weakly bound atmospheres of such worlds have proportionally larger radiative surfaces and are more susceptible to escape. We conduct a unified investigation into these phenomena, combining analytical energy balance and hydrodynamic escape with line-by-line radiative transfer calculations. Because outgoing radiation is forced to increase with surface temperature by the expansion of the radiative surface, we find that these worlds do not experience a runaway greenhouse. Furthermore, we show that a long-lived liquid water habitable zone is possible for low-gravity waterworlds of sufficient mass. Its inner edge is set by the rate of atmospheric escape, because a short-lived atmosphere limits the time available for life to evolve. In describing the physics of the parameter space transition from "planet-like" to "comet-like", our model produces a lower bound for habitability in terms of gravity. These results provide valuable insights in the context of the ongoing hunt for habitable exoplanets and exomoons.