  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 HD209458b 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.
