Understanding WASP-12b

The orbital period of the hot Jupiter WASP-12b is apparently changing. We study whether this reflects orbital decay due to tidal dissipation in the star, or apsidal precession of a slightly eccentric orbit. In the latter case, a third body or other perturbation would be needed to sustain the eccentricity against tidal dissipation in the planet itself. We have analysed several such perturbative scenarios, but none is satisfactory. Most likely, therefore, the orbit really is decaying. If this is due to a dynamical tide, then WASP-12 should be a subgiant without a convective core as previous studies have suggested. We have modelled the star with the MESA code. Satisfactory main-sequence models can be found if the effective temperature is at the upper end of reported estimates, but subgiant models are in greater tension with current observational constraints.