  In the past 15 years, astronomers have revealed that a significant fraction
of the stars should harbor planets and that it is likely that terrestrial
planets are abundant in our galaxy. Among these planets, how many are
habitable, i.e. suitable for life and its evolution? These questions have been
discussed for years and we are slowly making progress. Liquid water remains the
key criterion for habitability. It can exist in the interior of a variety of
planetary bodies, but it is usually assumed that liquid water at the surface
interacting with rocks and light is necessary for the emergence of a life able
to modify its environment and evolve. A first key issue is thus to understand
the climatic conditions allowing surface liquid water assuming a suitable
atmosphere. This have been studied with global mean 1D models which has defined
the "classical habitable zone", the range of orbital distances within which
worlds can maintain liquid water on their surfaces (Kasting et al. 1993). A new
generation of 3D climate models based on universal equations and tested on
bodies in the solar system is now available to explore with accuracy climate
regimes that could locally allow liquid water. A second key issue is now to
better understand the processes which control the composition and the evolution
of the atmospheres of exoplanets, and in particular the geophysical feedbacks
that seems to be necessary to maintain a continuously habitable climate. From
that point of view, it is not impossible that the Earth's case may be special
and uncommon.
