2007A&A...461.1185L

To describe the evaporation status of extrasolar planets, we consider an energy diagram in which the potential energy of the planets is plotted versus the energy received by the upper atmosphere. Here we present a basic method to estimate these quantities. For the potential energy, we include the modification of the gravity field by the tidal forces from the parent stars. This description allows a rapid estimate of both the escape rate of the atmospheric gas and the lifetime of a planet against the evaporation process. In the energy diagram, we find an evaporation-forbidden region in which a gaseous planet would evaporate in less than 5 billion years. With their observed characteristics, all extrasolar planets are found outside this evaporation-forbidden region. The escape rates are estimated to be in the range 10⁵g/s to 10¹²g/s, with a few cases above 10¹¹g/s. The estimated escape rate for HD209458b is consistent with the lower limit of 10¹⁰g/s obtained from interpretation of the Hi Lyman-α observations. This diagram suggests possibilities for the nature of the recently discovered Neptune-mass planets. We find that GJ436b, 55Cnce and HD69830b cannot be low mass gaseous planets. With a density that must be above 0.5g/cm3 to survive evaporation, these planets must contain a large fraction of solid/liquid material. We find that GJ876d must have a density greater than ∼3g/cm3 to survive the strong EUV energy flux from its nearby parent star. GJ876d must contain a large fraction of massive elements.