TIDAL DECAY OF CLOSE PLANETARY ORBITS

The 4.2 day orbit of the newly discovered planet around 51 Pegasi is f ormally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the centr al star. Given our best current understanding of tidal interactions, a planet of Jupiter's mass around a solar-like star could have dynamica lly survived in an orbit with a period as short as similar to 10 hr. S ince radial velocities increase with decreasing period, we would expec t to find those planets close to the tidal limit first, and, unless th is is a very unusual system, we would expect to find many more. We als o consider the tidal stability of planets around more evolved stars, a nd we reexamine in particular the question of whether the Earth can dy namically survive the red giant phase in the evolution of the Sun.