The Kepler mission has revolutionized the study of exoplanet statistics by increasing the number of known extrasolar planets and planet candidates by a factor of five, and by discovering systems with longer orbital periods and smaller planet radii than any of the prior exoplanet surveys. There is of course considerable interest in locating Earth-sized planets residing in the habitable zones of their stars, that is, having orbits producing surface temperatures that allow water to remain liquid – a prerequisite for the development of life.
It turns out that small stars, so-called M-dwarfs whose masses are about half a solar-mass and whose surface temperatures are less than about 4000K, are much more numerous than solar-type stars – about twelve times as common. Hunting for Earth-sized planets around M-dwarfs, therefore, is of particular interest. Although the idea of finding habitable planets around M-dwarfs had been discussed as early as fifty years ago, the possibilities were considered slight because of two concerns about these smaller stars. The first is that because the star is cooler and less luminous than the Sun, the planet needs to be closer for its surface temperature to be suitable, but then gravity will tidally lock it facing the star (much as the Moon is tidally locked facing the Earth). With one face perpetually toward (and one away from) the star, the planet’s surface might be either to hot or too cold. The second difficulty was that small stars tend to flare, perhaps affecting a planet’s atmosphere.
Read more at: Phys.org