Falling Transiting Extrasolar Giant Planets

TL;DR

This paper demonstrates that most transiting extrasolar giant planets are unlikely to be in tidal equilibrium, suggesting their circular orbits and spin-orbit alignments are not caused by tidal dissipation but possibly by other mechanisms.

Contribution

It provides a revised understanding of tidal stability and evolution in transiting exoplanet systems, showing that only one such planet (HAT-P-2b) can reach tidal equilibrium.

Findings

Most transiting planets lack tidal equilibrium states.

Eccentricity and obliquity decrease only during final merger.

Orbital and rotational parameters evolve over the system's lifetime.

Abstract

We revisit the tidal stability of extrasolar systems harboring a transiting planet and demonstrate that, independently of any tidal model, none but one (HAT-P-2b) of these planets has a tidal equilibrium state, which implies ultimately a collision of these objects with their host star. Consequently, conventional circularization and synchronization timescales cannot be defined because the corresponding states do not represent the endpoint of the tidal evolution. Using numerical simulations of the coupled tidal equations for the spin and orbital parameters of each transiting planetary system, we confirm these predictions and show that the orbital eccentricity and the stellar obliquity do not follow the usually assumed exponential relaxation but instead decrease significantly, reaching eventually a zero value, only during the final runaway merging of the planet with the star. The only characteristic evolution timescale of {\it all} rotational and orbital parameters is the lifetime of the system, which crucially depends on the magnitude of tidal dissipation within the star. These results imply that the nearly circular orbits of transiting planets and the alignment between the stellar spin axis and the planetary orbit are unlikely to be due to tidal dissipation. Other dissipative mechanisms, for instance interactions with the protoplanetary disk, must be invoked to explain these properties.