  To describe the evaporation status of the extrasolar planets, we propose to
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 quick 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
10^5 g/s to 10^{12} g/s, with few cases above 10^{11} g/s. The estimated escape
rate for HD209458b is found to be consistent with the lower limit of 10^{10}
g/s obtained from interpretation of the HI Lyman-alpha observations.
  Finally, this diagram suggests possibilities for the nature of the recently
discovered Neptune-mass planets. We find that GJ436b, 55Cnc_e and HD69830b
cannot be low mass gaseous planets. With density necessarily above 0.5g/cm3 to
survive evaporation, these planets must contain a large fraction of
solid/liquid material. Concerning GJ876d, we find that it must have a density
larger than ~3g/cm3 to survive the strong EUV energy flux from its nearby
parent star. GJ876d must contain a large fraction of massive elements.
