STRONG DEPENDENCE OF THE INNER EDGE OF THE HABITABLE ZONE ON PLANETARY ROTATION RATE

Abstract

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