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 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.

The role of sea-ice albedo in the climate of slowly rotating aquaplanets

“…1). The difference in the GMST between our iceOFF simulations and Yang et al (2014) is partly caused by the higher CO 2 concentration in our study, with 800 ppmv in comparison to only 400 ppmv in Yang et al (2014). Another factor that contributes to the difference in GMST is the global-mean planetary albedo that is higher in the simulations by Yang et al (2014) (Fig.…”

“…However, at each P rot , the global-mean surface temperature is substantially higher when sea ice is not included in our sensitivity simulations iceOFF and in the simulations performed by Yang et al (2014). For similar rotation periods, the GMST in our iceOFF simulations is around 10 K higher than the one in Yang et al (2014) (Fig.…”

“…And a synchronously rotating land-planet exhibits a stronger diurnal contrast in surface temperature than a synchronously rotating aquaplanet (Joshi 2003). In sensitivity simulations with the orbital parameters of Venus, Yang et al (2014) showed that the GMST in an aquaplanet configuration is 11 K higher than in an Earth-like continental configuration. Therefore, our results could be sensitive to the surface configuration chosen.…”

“…This is especially apparent in the simulations with P rot between 64 and 300 Earth-days. Over ice-free surfaces, Yang et al (2014) are in grey the response of the SST to the energy from the incoming solar radiation is slow due to the large thermal inertia of the mixed-layer ocean. Thus, along the ice-free equatorial band, the highest surface temperature is shifted towards the east of the substellar meridian at 90 • W-towards the afternoon time (Fig.…”

Surface flux patterns on planets in circumbinary systems and potential for photosynthesis

“…Shields et al 2014;Yang et al 2014) then these surface flux and darkness patterns will play a crucial role in determining both atmospheric physics and oceanic circulation (and the subsequent interactions thereof).…”