Tidal decay of close planetary orbits

The 4.2 day orbit of the newly discovered planet around 51 Pegasi is formally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the central star. Given our best current understanding of tidal interactions, a planet of Jupiter's mass around a solar-like star could have dynamically survived in an orbit with a period as short as ∼10 hr. Since radial velocities increase with decreasing period, we would expect to find those planets close to the tidal limit first, and, unless this is a very unusual system, we would expect to find many more. We also consider the tidal stability of planets around more evolved stars, and we reexamine in particular the question of whether the Earth can dynamically survive the red giant phase in the evolution of the Sun.