Constraints on Climate and Habitability for Earth-like Exoplanets Determined from a General Circulation Model

TL;DR

This study uses a climate model to identify stable climate states and habitability limits for Earth-like exoplanets around different star types, considering the hydrological cycle and climate feedbacks.

Contribution

It provides a comprehensive analysis of climate stability and habitability boundaries for Earth-like planets using a general circulation model across various stellar fluxes.

Findings

Identified four stable climate states: snowball, waterbelt, temperate, and moist greenhouse.

Determined habitable temperature range up to ~355 K with water-loss timescales of about 1 Gyr.

Estimated maximum habitability durations around different star types due to stellar brightening.

Abstract

Conventional definitions of habitability require abundant liquid surface water to exist continuously over geologic timescales. Water in each of its thermodynamic phases interacts with solar and thermal radiation and is the cause for strong climatic feedbacks. Thus, assessments of the habitable zone require models to include a complete treatment of the hydrological cycle over geologic time. Here, we use the Community Atmosphere Model from the National Center for Atmospheric Research to study the evolution of climate for an Earth-like planet at constant CO2, under a wide range of stellar fluxes from F-, G-, and K-dwarf main sequence stars. Around each star we find four stable climate states defined by mutually exclusive global mean surface temperatures (Ts); snowball (Ts < 235 K), waterbelt (235 K < Ts < 250 K), temperate (275 K < Ts < 315 K), and moist greenhouse (Ts > 330 K). Each is separated by abrupt climatic transitions. Waterbelt, temperate, and cooler moist greenhouse climates can maintain open-ocean against both sea-ice albedo and hydrogen escape processes respectively, and thus constitute habitable worlds. We consider the warmest possible habitable planet as having Ts ~ 355 K, at which point diffusion limited water-loss could remove an Earth ocean in ~1 Gyr. Without long timescale regulation of non-condensable greenhouse species at Earth-like temperatures and pressures, such as CO2, habitability can be maintained for an upper limit of ~2.2, ~2.4 and ~4.7 Gyr around F-, G- and K-dwarf stars respectively due to main sequence brightening.