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Habitability of red dwarf systems
The habitability of red dwarf systems is determined by a large number of factors from a variety of sources. Although the low stellar flux, high probability of tidal locking, small circumstellar habitable zones, and high stellar variation experienced by planets of red dwarf stars are impediments to their planetary habitability, the ubiquity and longevity of red dwarfs are positive factors. Determining how the interactions between these factors affect habitability may help to reveal the frequency of extraterrestrial life and intelligence.
Intense tidal heating caused by the proximity of planets to their host red dwarfs is a major impediment to life developing in these systems. Other tidal effects, such as the extreme temperature differences created by one side of habitable-zone planets permanently facing the star and the other perpetually turned away and lack of planetary axial tilts, reduce the probability of life around red dwarfs.〔 Non-tidal factors, such as extreme stellar variation, spectral energy distributions shifted to the infrared relative to the Sun, and small circumstellar habitable zones due to low light output, further reduce the prospects for life in red-dwarf systems.〔
There are, however, several effects that increase the likelihood of life on red dwarf planets. Intense cloud formation on the star-facing side of a tidally locked planet may reduce overall thermal flux and drastically reduce equilibrium temperature differences between the two sides of the planet. In addition, the sheer number of red dwarfs, which account for about 85% of at least 100 billion stars in the Milky Way, increases the number of habitable planets that may be orbiting them; as of 2013, there are expected to be roughly 60 billion habitable red dwarf planets in the Milky Way.
==Red dwarf characteristics==
Red dwarfs〔The term ''dwarf'' applies to all stars in the main sequence, including the Sun.〕 are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% of stars in spiral galaxies to more than 90% of all stars in elliptical galaxies,〔van Dokkum, Pieter G. & Conroy, Charlie. "A substantial population of low-mass stars in luminous elliptical galaxies. Nature. 2010 468(7326):940〕〔"Discovery Triples Number of Stars in Universe" Yale University. Published December 1, 2010. Retrieved December 17, 2010, from http://www.sciencedaily.com/releases/2010/12/101201134158.htm〕 an often quoted median figure being 73% of the stars in the Milky Way (known since the 1990s from radio telescopic observation to be a barred spiral).〔Dole, Stephen H. ''Habitable Planets for Man'' 1965 Rand Corporation report, published in book form--A figure of 73% is given for the percentage of red dwarfs in the Milky Way.〕 Red dwarfs are either late K or M spectral type.〔the term is sometimes used as coterminus with M class. K class stars tend toward an orange color.〕 Given their low energy output, red dwarfs are never visible by the unaided eye from Earth; neither the closest red dwarf to the Sun when viewed individually, Proxima Centauri (which is also the closest star to the Sun), nor the closest solitary red dwarf, Barnard's star, is anywhere near visual magnitude.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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