Exo-Milankovitch Cycles I: Orbits and Rotation States

TL;DR

This paper explores how orbital and rotational dynamics of exoplanets, especially secular spin-orbit resonances, cause obliquity variations that significantly impact their climates and habitability.

Contribution

It introduces a semi-analytic model to map obliquity variations across broad parameter spaces, highlighting the role of secular resonances in exoplanet climate dynamics.

Findings

Obliquity variations are mainly driven by secular spin-orbit resonances.

Test cases like Kepler-62 f show wide obliquity fluctuations.

Obliquity and orbital variations can dramatically affect exoplanet climates.

Abstract

The obliquity of the Earth, which controls our seasons, varies by only ~2.5 degrees over ~40,000 years, and its eccentricity varies by only ~0.05 over 100,000 years. Nonetheless, these small variations influence Earth's ice ages. For exoplanets, however, variations can be significantly larger. Previous studies of the habitability of moonless Earth-like exoplanets have found that high obliquities, high eccentricities, and dynamical variations can extend the outer edge of the habitable zone by preventing runaway glaciation (snowball states). We expand upon these studies by exploring the orbital dynamics with a semi-analytic model that allows us to map broad regions of parameter space. We find that in general, the largest drivers of obliquity variations are secular spin-orbit resonances. We show how the obliquity varies in several test cases, including Kepler-62 f, across a wide range of orbital and spin parameters. These obliquity variations, alongside orbital variations, will have a dramatic impact on the climates of such planets.