Orbital Decay of Short-Period Exoplanets via Tidal Resonance Locking

TL;DR

This paper investigates how tidal resonance locking can accelerate the orbital decay of short-period exoplanets, with implications for their survival and the evolution of their host stars.

Contribution

It introduces a detailed model of tidal resonance locking effects on exoplanet orbital decay, highlighting conditions under which it operates and its impact on planetary survival.

Findings

Resonance locking can cause rapid orbital decay for certain planets.

Low-mass planets may survive longer due to nonlinear damping effects.

Predicted decay timescales are comparable to stellar lifetimes.

Abstract

A large fraction of known exoplanets have short orbital periods where tidal excitation of gravity waves within the host star causes the planets' orbits to decay. We study the effects of tidal resonance locking, in which the planet locks into resonance with a tidally excited stellar gravity mode. Because a star's gravity mode frequencies typically increase as the star evolves, the planet's orbital frequency increases in lockstep, potentially causing much faster orbital decay than predicted by other tidal theories. Due to nonlinear mode damping, resonance locking in Sun-like stars likely only operates for low-mass planets ($M \lesssim 0.1 \, M_{\rm Jup}$), but in stars with convective cores it can likely operate for all planetary masses. The orbital decay timescale with resonance locking is typically comparable to the star's main-sequence lifetime, corresponding to a wide range in effective stellar quality factor ($10^3 \lesssim Q' \lesssim 10^9$), depending on the planet's mass and orbital period. We make predictions for several individual systems and examine the orbital evolution resulting from both resonance locking and nonlinear wave dissipation. Our models demonstrate how short-period massive planets can be quickly destroyed by nonlinear mode damping, while short-period low-mass planets can survive, even though they undergo substantial inward tidal migration via resonance locking.