Habitability of Planets

What makes a planet suitable for life and its evolution? This question has 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. The climatic conditions allowing surface liquid water have been studied with global mean 1-D models which have defined the "classical habitable zone" (Kasting et al. 1993). Key issues are now to better understand the geophysical feedback that seems to be necessary to control atmospheric evolution and maintain a continuously suitable climate (is this unique to the Earth?), as well as exploring with accuracy climate regimes that could locally allow liquid water. For this last purpose a new generation of 3-D climate models based on universal equations and tested on bodies in the solar system is now available.