  What kind of environment may exist on terrestrial planets around other stars?
In spite of the lack of direct observations, it may not be premature to
speculate on exoplanetary climates, for instance to optimize future telescopic
observations, or to assess the probability of habitable worlds. To first order,
climate primarily depends on 1) The atmospheric composition and the volatile
inventory; 2) The incident stellar flux; 3) The tidal evolution of the
planetary spin, which can notably lock a planet with a permanent night side.
The atmospheric composition and mass depends on complex processes which are
difficult to model: origins of volatile, atmospheric escape, geochemistry,
photochemistry. We discuss physical constraints which can help us to speculate
on the possible type of atmosphere, depending on the planet size, its final
distance for its star and the star type. Assuming that the atmosphere is known,
the possible climates can be explored using Global Climate Models analogous to
the ones developed to simulate the Earth as well as the other telluric
atmospheres in the solar system. Our experience with Mars, Titan and Venus
suggests that realistic climate simulators can be developed by combining
components like a "dynamical core", a radiative transfer solver, a
parametrisation of subgrid-scale turbulence and convection, a thermal ground
model, and a volatile phase change code. On this basis, we can aspire to build
reliable climate predictors for exoplanets. However, whatever the accuracy of
the models, predicting the actual climate regime on a specific planet will
remain challenging because climate systems are affected by strong positive
destabilizing feedbacks (such as runaway glaciations and runaway greenhouse
effect). They can drive planets with very similar forcing and volatile
inventory to completely different states.
