Evaporation rate of hot Jupiters and formation of Chthonian planets

Among the hundred of known extrasolar planets, about 15% are closer than 0.1 AU from their parent stars. But there are extremely few detections of planets orbiting in less than 3 days. At this limit the planet HD 209458b has been found to have an extended upper atmosphere of escaping hydrogen. This suggests that the so-called hot Jupiters which are close to their parent stars could evaporate. Here we estimate the evaporation rate of hydrogen from extrasolar planets in the star vicinity. With high exospheric temperatures, and owing to the tidal forces, planets evaporate through a geometrical blow-off. This may explain the absence of Jupiter mass planets below a critical distance from the stars. Below this critical distance, we infer the existence of a new class of planets made of the residual central core of former hot Jupiters, which we propose to call the " Chthonian " planets. Following the recent discovery of a significant escape of atomic hydrogen from the planet orbiting HD 209458 (Vidal-Madjar et al. 2003), we propose to evaluate the escape flux from the upper atmosphere of hot Jupiters under the influence of the strong tidal forces from their parent stars. This escape flux had previously been estimated (Guillot et al. 1996), but the conclusion that the mass loss is not significant was based on two hypotheses which need to be revisited. First, the black body radiative equilibrium used to calculate the temperature of the upper atmosphere (Schneider et al. 1998) is inappropriate because it does not apply to the low density upper atmosphere. Observations in the Solar System show that the temperature of a planetary upper atmosphere (thermo-sphere, exosphere) is much higher than the effective temperature of the bottom atmosphere. For example, whereas the temperatures in the Earth and Jupiter are around 200 K and 120 K respectively at the level of the tropopause, they reach 1000 K in the thermosphere of these two planets. Although these observed high temperatures in the planets of our own Solar System remain unexplained, there are some clues that a combination of the extreme and far ultraviolet fluxes with the Solar wind is responsible for the heating. The second important hypothesis is tidal force, which modifies the gravity of hot Jupiters. The common hypothesis is to neglect that effect as for isolated planets far from their star. However, tidal forces have a significant influence on the density distribution in …