  Planetary rotation rate is a key parameter in determining atmospheric
circulation and hence the spatial pattern of clouds. Since clouds can exert a
dominant control on planetary radiation balance, rotation rate could be
critical for determining mean planetary climate. Here we investigate this idea
using a three-dimensional general circulation model with a sophisticated cloud
scheme. We find that slowly rotating planets (like Venus) can maintain an
Earth-like climate at nearly twice the stellar flux as rapidly rotating planets
(like Earth). This suggests that many exoplanets previously believed to be too
hot may actually be habitable, depending on their rotation rate. The
explanation for this behavior is that slowly rotating planets have a weak
Coriolis force and long daytime illumination, which promotes strong convergence
and convection in the substellar region. This produces a large area of
optically thick clouds, which greatly increases the planetary albedo. In
contrast, on rapidly rotating planets a much narrower belt of clouds form in
the deep tropics, leading to a relatively low albedo. A particularly striking
example of the importance of rotation rate suggested by our simulations is that
a planet with modern Earth's atmosphere, in Venus' orbit, and with modern
Venus' (slow) rotation rate would be habitable. This would imply that if Venus
went through a runaway greenhouse, it had a higher rotation rate at that time.
