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An M dwarf is main-sequence M-type star. The term red dwarf definitely includes M dwarfs, but sometime that term is also meant to include some or all main-sequence K-type stars (K dwarfs). M dwarfs are very common, comprising about three quarters of all main sequence stars, and it is estimated that about a quarter of them host a planet in their habitable zone. Despite being so plentiful, they are so dim that none can be seen with a telescope, including the closest star to the Sun, which is a red dwarf. Early M dwarfs (the hotter ones) are now considered likely places to observe extra-solar planets within a habitable zone. But given such stars' slow evolution, such planets are more likely to have experienced a period of high stellar activity (such as severe coronal mass ejections) long enough to remove the planet's atmosphere, less likely to leave the planet habitable. M dwarf examples include: