Free Astronomy Magazine January-February 2015

ASTROBIOLOGY dwarfs have had the lion's share in several obser- vational cam- paigns, among which the most important was that conducted with the Kepler Space Telescope, which produced some candidate habitable planets tied precisely to red dwarfs. The fact that these small stars are characterized by intense magnetic surface activity, especially in their youth, did not seem to discourage too much the researchers: exuberant as they may be, their very high number left how- ever hope of finding many cases with fa- vourable conditions. It must be considered that about 80% of all existing stars are red dwarfs, and it is estimated that 40% of them could host at least one Earth-sized planet in the habitable zone. The step from habitable zone to habitable planet seems to be only a short one, but apparently it is not so and a clear hint of this has already been had last June, with the publication on The Astrophysical Jour- nal of an article by O. Cohen et al. (CfA), which showed that the stellar winds and the streams of high-energy radiation en- countered by the planets located in the habitable zone of red dwarfs are strong enough to strip away their atmospheres. T he drawing on the side shows the size of the habitable zones (in green) of bigger, hotter and brighter stars than the Sun (top), of Sun-like stars (centre) and smaller and cool- er stars (below). Among the latter are included red dwarfs, whose habitable zone can be even smaller than that shown here. In the illustration below, we see how the mighty surface activity of a red dwarf can cause the dis- persion of the atmosphere of a nearby planet. [NASA]