The Worlds of Red Dwarfs

A red dwarf is a small and relatively cool star on the main sequence. Red dwarfs range in mass from a low of 0.075 solar masses (the upper limit for a brown dwarf) to about 50% of the Sun and have a surface temperature of less than 4,000 K. So they are not very impressive but there are a lot of them. Using publicly available data from NASA's Kepler space telescope, astronomers at the Harvard-Smithsonian Center for Astrophysics estimate that six percent of red dwarf stars in the galaxy have Earth-size planets in the habitable zone, the range of distances from a star where the surface temperature of an orbiting planet might be suitable for liquid water.
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