New simulations using a 3D general circulation model (GCM) have expanded the habitable zone potentially pushing some already known exoplanets inside the region where liquid water could be present on the surface of rocky planets. The group at the University of Chicago have analysed the surface temperatures of rocky planets with varying rotation rates. The rotation rate effects the atmospheric dynamics and hence large scale cloud properties of planets. The distribution, thickness and altitude of clouds greatly effects the surface temperature of rocky planets.
The group found that slowly rotating planets have a lower surface temperature than an otherwise similar planet which rotates faster. In the case of rapidly rotating planets (like the Earth), the atmospheric dynamics is banded and split in to several regions. On Earth we get large cloud cover at the inter tropical convergence zone (ITCZ) which is important for reflecting solar radiation and keeping the planet cool. As solar radiation increases (move the planet closer to the star), the equator-pole dynamics (Hadley Cell) weakens causing a reduction in cloud cover in this region and therefore causing warming of the surface.
On the other hand, slowly rotating planets tend to have ‘planet-scale’ dynamics and exhibit large scale ascent on the dayside and descent on the nightside. This large scale ascent on the dayside atmosphere produces large amount of cloud. As the stellar radiation increases, the aerial extent of this ascent region increases, therefore producing a greater cloud coverage and a negative feedback process.
The authors of this study point out an interesting application of this relationship between rotation rate and the habitable zone. The results suggest that an Earth-like planet placed around a Sun-like star with the orbital distance and planetary rotation of Venus then the planet would be habitable despite a near doubling of the insolation.
This provides interesting insights to the past evolution of the climate of Venus. There is evidence of Venus being able to support an ocean which has subsequently been lost via a runaway greenhouse state. This means that if the oceans were lost early in its history, then the rotation period of Venus must have been a few weeks, or if it happened more recently a few months. This is a much more rapid rotation than the current rotation rate of 243 days. As an alternative, the water could have been lost from the atmosphere via hydrodynamic escape, negating the need of a slowing rotation.
In summary, this work pushes the inner edge of the habitable zone closer to the star for slowly rotating planets via a cloud-albedo feedback process. However, one must be careful, as it is not just the current rotation rate but also the past rotation rates which are important.
The paper by Yang et al. is available here: http://iopscience.iop.org/2041-8205/787/1/L2/pdf/2041-8205_787_1_L2.pdf