Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars

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Science  06 Feb 2015:
Vol. 347, Issue 6222, pp. 632-635
DOI: 10.1126/science.1258686

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Can't keep hot sides hot and cold sides cold?

On Earth, we're accustomed to cycles of day and night, which drive a complex thermal relationship between the ground and atmosphere. Some exoplanets very close to their stars aren't so lucky and are generally assumed to be locked by tides into hemispheres of unchanging day and night. The thick atmosphere of Venus is thought to keep it from the same fate, but Leconte et al. present models showing that even a much sparser atmosphere may suffice for lower-mass stars. The non-instantaneous thermal response of the planet (when the hottest time of day is after noon) may speed up or slow down the planet. Such thermal tides may be important in assessing potential extraterrestrial habitable zones.

Science, this issue p. 632


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 to 0.7 solar mass. Thus, many recently discovered terrestrial planets could exhibit asynchronous spin-orbit rotation, even with a thin atmosphere.

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