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