  Planets in the habitable zone of lower-mass stars are often assumed to be in
a state of tidally synchronized rotation, which would considerably affect their
putative habitability. Although thermal tides cause Venus to rotate
retrogradely, simple scaling arguments tend to attribute this peculiarity to
the massive Venusian atmosphere. Using a global climate model, we show that
even a relatively thin atmosphere can drive terrestrial planets' rotation away
from synchronicity. We derive a more realistic atmospheric tide model that
predicts four asynchronous equilibrium spin states, two being stable, when the
amplitude of the thermal tide exceeds a threshold that is met for habitable
Earth-like planets with a 1-bar atmosphere around stars more massive than
0.5-0.7Msun. Thus, many recently discovered terrestrial planets could exhibit
asynchronous spin-orbit rotation, even with a thin atmosphere.
