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 re-examine in particular the question of whether the Earth can dynamically survive the red-giant phase in the evolution of the Sun. Subject headings: Planets and Satellites: General — Solar System: General — Stars: Planetary Systems — Sun: Solar-terrestrial Relations