Habitability of extrasolar planets and tidal spin evolution

TL;DR

This paper reviews how tidal forces from host stars influence the spin states and habitability potential of exoplanets, highlighting that tides can lead to non-synchronous rotation and loss of seasonal variations.

Contribution

It provides a detailed analysis of tidal effects on planetary spin evolution, emphasizing their role in habitability assessments beyond stellar radiation considerations.

Findings

Tides can drive planets into equilibrium spin states without necessarily causing synchronous rotation.

Tidal circularization leads to planets having rotation periods equal to their orbital periods.

Super-Earth Gl581d exhibits two rotations per orbit with eroded primordial obliquity.

Abstract

Stellar radiation has conservatively been used as the key constraint to planetary habitability. We review here the effects of tides, exerted by the host star on the planet, on the evolution of the planetary spin. Tides initially drive the rotation period and the orientation of the rotation axis into an equilibrium state but do not necessarily lead to synchronous rotation. As tides also circularize the orbit, eventually the rotation period does equal the orbital period and one hemisphere will be permanently irradiated by the star. Furthermore, the rotational axis will become perpendicular to the orbit, i.e. the planetary surface will not experience seasonal variations of the insolation. We illustrate here how tides alter the spins of planets in the traditional habitable zone. As an example, we show that, neglecting perturbations due to other companions, the Super-Earth Gl581d performs two rotations per orbit and that any primordial obliquity has been eroded.