Spin-orbit coupling for tidally evolving super-Earths

TL;DR

This study explores how spin-orbit resonances influence the orbital decay and eccentricity damping of close-in super-Earths, combining numerical simulations and analytical models to understand their rotational and orbital evolution.

Contribution

It introduces a detailed analysis of spin-orbit resonance captures in super-Earths, highlighting their impact on orbital evolution and applying the findings to observed exoplanet systems.

Findings

Resonant captures can accelerate orbital decay and eccentricity damping.

Some super-Earths may currently be in non-synchronous spin-orbit resonances.

Planets can avoid chaotic rotation by evolving through resonant trapping.

Abstract

We investigate the spin behavior of close-in rocky planets and the implications for their orbital evolution. Considering that the planet rotation evolves under simultaneous actions of the torque due to the equatorial deformation and the tidal torque, both raised by the central star, we analyze the possibility of temporary captures in spin-orbit resonances. The results of the numerical simulations of the exact equations of motions indicate that, whenever the planet rotation is trapped in a resonant motion, the orbital decay and the eccentricity damping are faster than the ones in which the rotation follows the so-called pseudo-synchronization. Analytical results obtained through the averaged equations of the spin-orbit problem show a good agreement with the numerical simulations. We apply the analysis to the cases of the recently discovered hot super-Earths Kepler-10 b, GJ 3634 b and 55 Cnc e. The simulated dynamical history of these systems indicates the possibility of capture in several spin-orbit resonances; particularly, GJ 3634 b and 55 Cnc e can currently evolve under a non-synchronous resonant motion for suitable values of the parameters. Moreover, 55 Cnc e may avoid a chaotic rotation behavior by evolving towards synchronization through successive temporary resonant trappings.