Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets

The Habitable Zone (HZ) is the circumstellar region where a planet can sustain surface liquid water. Searching for terrestrial planets in the HZ of nearby stars is the stated goal of ongoing and planned extrasolar planet surveys. Previous estimates of the inner edge of the HZ were based on one dimensional radiative-convective models. The most serious limitation of these models is the inability to predict cloud behavior. Here we use global climate models with sophisticated cloud schemes to show that due to a stabilizing cloud feedback, tidally locked planets can be habitable at twice the stellar flux found by previous studies. This dramatically expands the HZ and roughly doubles the frequency of habitable planets orbiting red dwarf stars. At high stellar flux, strong convection produces thick water clouds near the substellar location that greatly increase the planetary albedo and reduce surface temperatures. Higher insolation produces stronger substellar convection and therefore higher albedo, making this phenomenon a stabilizing climate feedback. Substellar clouds also effectively block outgoing radiation from the surface, reducing or even completely reversing the thermal emission contrast between dayside and nightside. The presence of substellar water clouds and the resulting clement surface conditions will therefore be detectable with the James Webb Space Telescope. Climates of tidally locked and non-tidally locked terrestrial planets. (a) global-mean surface temperature (K), (b) stratospheric H2O volume mixing ratio at the substellar point, (c) planetary albedo and (d) global-mean greenhouse effect (K). The upper horizontal axis is the corresponding semimajor axis between an M-star with 2.3% solar luminosity and the planet. 1:1 denotes a tidally locked state, and 2:1 and 6:1 denote 2 or 6 rotations per orbit, respectively. For "no cloud" cases, all clouds are set to zero. The stellar spectrum is for an M-star or a K-star. Results for HD85512b are represented by a pentagram. The gray area denotes the HZ around an M-star with an inner edge of ~1200 W/m2 and an outer edge of ~270 W/m2 (not shown), obtained in a 1D model without clouds (Kopparapu et al., 2013).