Long-time Centauri Dreams readers will be familiar with the work of Manoj Joshi and Robert Haberle. Back in the 1990s when both were at NASA Ames (Joshi is now at the University of Reading), the scientists went to work on the question of whether planets around red dwarf stars could be habitable, given the problem of close orbits and tidal lock. Simulating the atmosphere of such a planet, they found even a thin atmosphere would circulate globally, moving enough heat to prevent the air on the darkside from freezing out. The prospect of a planet with oceans and a climate mild enough to support life began to look more promising.
Joshi and Haberle have a new paper out that looks once again at planets around red dwarfs, this time extending the possible habitable zone to a greater distance from the star. M-class red dwarfs are smaller and cooler than G-class stars like the Sun, and emit a much larger fraction of their radiation at longer wavelengths where the reflectivity of ice and snow are lower. The effect is striking, as the paper notes:
The values for snow and ice for a planetary surface orbiting the Sun are 0.8 and 0.5 respectively, which are broadly consistent with the values that are used in climate models. Fresh snow and ice albedos on a planet receiving black body radiation from an object at 3300K are 0.6 and 0.3 respectively, which are significant reductions from the “solar” values.
The black body radiation is an idealised representation of an M-dwarf that is approximately 40% as massive as the Sun. The authors then go on to calculate the albedos for snow and ice on hypothetical planetary surfaces around the stars Gliese 436 and GJ 1214 and find them even lower. What this means is that more of the long-wave radiation emitted by an M-dwarf will be absorbed rather than reflected by an icy surface. The effect is to widen the habitable zone of a planet around this kind of star outwards by anywhere from 10 to 30 percent. What might have seemed a frigid world now looks more hospitable.
Allowing a habitable zone (defined here in terms of liquid water at the surface) to exist farther out from the parent star is significant, although there are no changes at the other end of the HZ:
The effect considered here should not move the inner edge of the habitable zone, usually considered as the locus of orbits where loss rates of water become significant to dry a planet on geological timescales (Kasting et al 1993), away from the parent M-star. This is because when a planet is at the inner edge of the habitable zone, surface temperatures should be high enough to ensure that ice cover is small. For a tidally locked planet this implies that ice is confined to the dark side that perpetually faces away from the parent star: such ice receives no stellar radiation, rendering albedo effects unimportant.
But planets with significant amounts of snow and ice will have higher surface temperatures and the outer edge of the habitable zone is extended. It’s an interesting thought because of the sheer ubiquity of red dwarfs — some estimates of their prevalence run as high as 80 percent of main-sequence stars, so seeing them as astrobiologically friendly would revise our estimates for extraterrestrial life. Just how close the nearest life-bearing planet might be affects our planet hunt astronomically as we choose targets and has significance for any future interstellar probes.
Other issues remain and are under active investigation, from the problem of flares to climate models involving the effects of clouds and water vapor. A good place to get an overview is Tarter et al., “A Reappraisal of The Habitability of Planets around M Dwarf Stars,” Astrobiology 7, pp 30-65 (2007). The Joshi and Haberle paper is “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone,” accepted by Astrobiology (preprint).