Habitability of red dwarf systems From Wikipedia 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.[1][2] 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,[3] reduce the probability of life around red dwarfs.[2] 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.[2] 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.[4] In addition, the sheer number of red dwarfs, which account for about 85%[5] of at least 100 billion stars in the Milky Way,[6]statistically increases the probability that there might exist habitable planets orbiting some of them. As of 2013, there are expected to be tens of billions of super-Earth planets in the habitable zones of red dwarf stars in the Milky Way.[7] Red dwarf characteristics Red dwarf stars[8] 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,[9][10] 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).[11] Red dwarfs are either late K or M spectral type.[12] 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. Research Luminosity and spectral composition Relative star sizes and photospheric temperatures. Any planet around a red dwarf, such as the one shown here (Gliese 229A), would have to huddle close to achieve Earth-like temperatures, probably inducing tidal lock. See Aurelia. Credit: MPIA/V. Joergens. For years, astronomers ruled out red dwarfs, with masses ranging from roughly 0.08 to 0.45 solar masses (M☉), as potential abodes for life. The low masses of the stars cause the nuclear fusion reactions at their cores to proceed exceedingly slowly, giving them luminosities ranging from a maximum of roughly 3 percent that of the Sun to a minimum of just 0.01 percent.[13] Consequently, any planet orbiting a red dwarf would have to have a low semimajor axis in order to maintain Earth-like surface temperature, from 0.3 astronomical units (AU) for a relatively luminous red dwarf like Lacaille 8760 to 0.032 AU for a smaller star like Proxima Centauri, the nearest star to the Solar System.[14] Such a world would have a year lasting just six days.[15][16] Much of the low luminosity of a red dwarf falls in the infrared part of the electromagnetic spectrum, with lower energy than the visible light in which the Sun peaks. As a result, photosynthesis on a red dwarf planet would require additional photons to achieve excitation potentials comparable to those needed in Earth photosynthesis for electron transfers, due to the lower average energy level of near-infrared photons compared to visible.[17] Having to adapt to a far wider spectrum to gain the maximum amount of energy, foliage on a habitable red dwarf planet would probably appear black if viewed in visible light.[17] In addition, because water strongly absorbs red and infrared light, less energy would be available for aquatic life on red dwarf planets.[18] However, a similar effect of preferential absorption by water ice would increase its temperature relative to an equivalent amount of radiation from a Sun-like star, thereby extending the habitable zone of red dwarfs outward.[19] Another fact that would inhibit habitability is the evolution of the Red Dwarf stars; as such stars have an extended pre-main sequence phase, their eventual habitable zones would be for around 1 billion years a zone where water wasn't liquid but in its gaseous state. Thus, terrestrial planets in the actual habitable zones, if provided with abundant surface water in their formation, would have been subject to a runaway greenhouse effect for several hundred million years. During such an early runaway phase, photolysis of water vapor would allow hydrogen escape to space and the loss of several Earth oceans of water, leaving a thick abiotic oxygen atmosphere.[20] Tidal effects At the close distances that red dwarf planets would have to maintain to their stars in order to maintain liquid water at their surfaces, tidal locking to the host star is likely, causing the planet to rotate around its axis once for every revolution around the star; as a result, one side of the planet would eternally face the star and another side would perpetually face away, creating great extremes of temperature. For many years, it was believed that life on such planets would be limited to a ring-like region known as the terminator, where the star would always appear on the horizon. It was also believed that efficient heat transfer between the sides of the planet necessitates atmospheric circulation of an atmosphere so thick as to disallow photosynthesis. Due to differential heating, it was argued, a tidally locked planet would experience fierce winds with permanent torrential rain at the point directly facing the local star,[21] the subsolar point. In the opinion of one author this makes complex life improbable.[22] Plant life would have to adapt to the constant gale, for example by anchoring securely into the soil and sprouting long flexible leaves that do not snap. Animals would rely on infrared vision, as signaling by calls or scents would be difficult over the din of the planet-wide gale. Underwater life would, however, be protected from fierce winds and flares, and vast blooms of black photosynthetic plankton and algae could support the sea life.[23] In contrast to the previously bleak picture for life, 1997 studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO2 and H2O) need only be 100 millibar, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side, a figure well within the bounds of photosynthesis.[24] Research two years later by Martin Heath of Greenwich Community College has shown that seawater, too, could effectively circulate without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Additionally, a 2010 study concluded that Earth-like water worlds tidally locked to their stars would still have temperatures above 240 K (−33 °C) on the night side.[25] Climate models constructed in 2013 indicate that cloud formation on tidally locked planets would minimize the temperature difference between the day and the night side, greatly improving habitability prospects for red dwarf planets.[4] Further research, including a consideration of the amount of photosynthetically active radiation, has suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.[26] The existence of a permanent day side and night side is not the only potential setback for life around red dwarfs. Tidal heating experienced by planets in the habitable zone of red dwarfs less than 30% of the mass of the Sun may cause them to be "baked out" and become "tidal Venuses."[1] Combined with the other impediments to red dwarf habitability,[3] this may make the probability of many red dwarfs hosting life as we know it very low compared to other star types.[2] There may not even be enough water for habitable planets around many red dwarfs;[27] what little water found on these planets, in particular Earth-sized ones, may be located on the cold night side of the planet. In contrast to the predictions of earlier studies on tidal Venuses, though, this "trapped water" may help to stave off runaway greenhouse effects and improve the habitability of red dwarf systems.[28] Variability Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40% for months at a time. On Earth life has adapted in many ways to the similarly reduced temperatures of the winter. Life may survive by hibernating and/or by diving into deep water where temperatures could be more constant. More serious is that the oceans could perhaps freeze over during cold periods. After the cold has ended the planet’s albedo would be higher causing light from the red dwarf to be reflected, reducing planetary temperatures.