Effects of Spin-Orbit Resonances and Tidal Heating on the Inner Edge of the Habitable Zone

TL;DR

This study uses advanced climate and tidal heating models to explore how spin-orbit resonances and tidal effects influence the habitability of ocean-covered exoplanets near the inner edge of their habitable zones.

Contribution

It is the first to incorporate tidal heating into a 3-D General Circulation Model to assess habitability across multiple resonant states.

Findings

Resonant state has minimal impact without tidal heating.

Strong tidal heating significantly affects habitability depending on rotational state.

Habitable conditions are confined within a narrow orbital range due to tidal and stellar heating.

Abstract

Much attention has been given to the climate dynamics and habitable boundaries of synchronously rotating planets around low mass stars. However, other rotational states are possible, particularly when higher eccentricity orbits can be maintained in a system, including spin-orbit resonant configurations. Additionally, the oscillating strain as a planet moves from periastron to apoastron results in friction and tidal heating, which can be an important energy source. Here, we simulate the climate of ocean-covered planets near the inner edge of the habitable zone around M to solar stars with ROCKE-3D, and leverage the planetary evolution software package, VPLanet, to calculate tidal heating rates for Earth-sized planets orbiting 2600 K and 3000 K stars. This study is the first to use a 3-D General Circulation Model that implements tidal heating to investigate habitability for multiple resonant states. We find that in the absence of tidal heating, the resonant state has little impact on the inner edge, because for a given stellar flux, higher-order states tend to be warmer than synchronous rotators, but for a given temperature, have drier upper atmospheres. However, when strong tidal heating is present, the rotational component implies a strong dependence of habitable conditions on the system evolution and rotational state. Since tidal and stellar heating both decrease with orbital distance, this results in a compact orbital width separating temperate and uninhabitable climates. We summarize these results and also compare ROCKE-3D to previously published simulations of the inner edge that used a modified version of the NCAR CAM4 model.