ORBITAL MIGRATION OF THE PLANETARY COMPANION OF 51-PEGASI TO ITS PRESENT LOCATION

The recent discovery(1) and confirmation(2) of a possible planetary co mpanion orbiting the solar-type star 51 Pegasi represent a breakthroug h in the search for extrasolar planetary systems. Analysis of systemat ic variations in the velocity of the star indicate that the mass of th e companion is approximately that of Jupiter, and that it is travellin g in a nearly circular orbit at a distance from the star of 0.05 AU (a bout seven stellar radii). Here we show that, if the companion is inde ed a gas-giant planet, it is extremely unlikely to have formed at its present location. We suggest instead that the planet probably formed b y gradual accretion of solids and capture of gas at a much larger dist ance from the star (similar to 5 AU) and that it subsequently migrated inwards through interactions with the remnants of the circumstellar d isk. The planet's migration may have stopped in its present orbit as a result of tidal interactions with the star, or through truncation of the inner circumstellar disk by the stellar magnetosphere.