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The Cometary Tail of Giant Extrasolar Planets at Small Orbital Distance

We estimate the radius of the exosphere of giant planets at small orbital distance from their parent star. It can be larger than the planet's Roche lobe, thus giving rise to a signiicant evaporation of the planet. The stellar wind of the parent star transforms this evaporating material into a giant permanent cometary-like tail. We discuss some possible observable consequences of the presence of this tail, in particular for 51 Peg. 1. The evaporation rate of giant planets in close orbits After the probable discovery of four giant extrasolar planets very close to their parent star (Mayor & Queloz 1995, Butler et al. 1997), their internal structure and atmosphere has been investigated, but without considering the innuence on the planet of the gravitational eld of the star. We investigate hereafter how this gravitational eld modiies the connguration of the system. The equilibrium temperature of the giant extrasolar planets, deduced from the balance of the energy budget between the incoming radiation received from the star and the thermal radiation of the planet, is given by T eq = T (1 ? A) 1=4 R 2a Pl 1=2 (1) where A is the planet albedo, T and R the star's temperature and radius and a Pl the planet's orbital distance to the star. Here we neglect greenhouse eeects.