Princeton University Extrasolar Planet Discussion Group

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 non-linear mode damping, resonance locking in Sun-like stars likely only operates for low-mass planets (M <= 0.1MJup), but in stars with convective cores it can likely operate for all planetary masses. The resonance locking orbital decay time scale is typically comparable to the star’s main sequence life time, corresponding to a wide range in effective stellar quality factor (10^3 < Q′ < 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 non-linear wave dissipation. Our models demonstrate how short-period massive planets can be quickly destroyed due to non-linear mode damping, while short-period low-mass planets can survive, even though they undergo substantial inward tidal migration via resonance locking.