Climates of Warm Earth-Like Planets III: Fractional Habitability from a Water Cycle Perspective

TL;DR

This study uses climate modeling to explore how insolation and rotation affect the habitable surface fraction of Earth-like planets, highlighting conditions that could support complex land life and informing future observational targets.

Contribution

It introduces a novel assessment of fractional habitability based on a water cycle perspective, linking climate parameters to habitable land area on Earth-like planets.

Findings

Earth-like planets can be 'superhabitable' with larger habitable areas under certain conditions.

Slowly rotating, highly irradiated planets exhibit unique hydrological regimes unlike Earth.

Moderately slowly rotating planets with Earth-like insolation are promising for surface characterization.

Abstract

The habitable fraction of a planet's surface is important for the detectability of surface biosignatures. The extent and distribution of habitable areas is influenced by external parameters that control the planet's climate, atmospheric circulation, and hydrological cycle. We explore these issues using the ROCKE-3D General Circulation Model, focusing on terrestrial water fluxes and thus the potential for the existence of complex life on land. Habitability is examined as a function of insolation and planet rotation for an Earth-like world with zero obliquity and eccentricity orbiting the Sun. We assess fractional habitability using an aridity index that measures the net supply of water to the land. Earth-like planets become ``superhabitable'' (a larger habitable surface area than Earth) as insolation and day-length increase because their climates become more equable, reminiscent of past warm periods on Earth when complex life was abundant and widespread. The most slowly rotating, most highly irradiated planets, though, occupy a hydrological regime unlike any on Earth, with extremely warm, humid conditions at high latitudes but little rain and subsurface water storage. Clouds increasingly obscure the surface as insolation increases, but visibility improves for modest increases in rotation period. Thus, moderately slowly rotating rocky planets with insolation near or somewhat greater than modern Earth's appear to be promising targets for surface characterization by a future direct imaging mission.