Tidal excitation of the obliquity of Earth-like planets in the habitable zone of M-dwarf stars

TL;DR

This paper demonstrates that tidal interactions can excite and maintain high obliquities in Earth-like planets orbiting M-dwarf stars, potentially supporting habitable conditions by preventing obliquity damping.

Contribution

It reveals that spin-orbit resonance capture can lead to high, sustained obliquities, challenging the assumption that tides always damp obliquity to zero.

Findings

High obliquities (60-80 degrees) can be maintained over planetary lifetimes.

Tidal spin-orbit resonance trapping can excite obliquity rather than damp it.

Implications for habitability due to sustained temperate climates.

Abstract

Close-in planets undergo strong tidal interactions with the parent star that modify their spins and orbits. In the two-body problem, the final stage for tidal evolution is the synchronisation of the rotation and orbital periods, and the alignment of the planet spin axis with the normal to the orbit (zero planet obliquity). The orbital eccentricity is also damped to zero, but over a much longer timescale, that may exceed the lifetime of the system. For non-zero eccentricities, the rotation rate can be trapped in spin-orbit resonances that delay the evolution towards the synchronous state. Here we show that capture in some spin-orbit resonances may also excite the obliquity to high values rather than damp it to zero. Depending on the system parameters, obliquities of 60 to 80 degrees can be maintained throughout the entire lifetime of the planet. This unexpected behaviour is particularly important for Earth-like planets in the habitable zone of M-dwarf stars, as it may help to sustain temperate environments and thus more favourable conditions for life.