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The theorizedhabitability of red dwarf systems is determined by a large number of factors. Modern evidence suggests that planets inred dwarf systems are unlikely to behabitable, due to high probability oftidal locking, likely lack ofatmospheres, and the highstellar variation many suchplanets would experience. However, the sheer number andlongevity of red dwarfs could likely provide ample opportunity to realize any small possibility of habitability.
As of 2025[update], arguments concerning the habitability of red dwarf systems are unresolved, and the area remains an open question of study in the fields ofclimate modeling and the evolution of life on Earth. Observational data and statistical arguments suggest that red dwarf systems are uninhabitable for indeterminate reasons.[1] In contrast, 3D climate models favor habitability[2] and wider habitable zones for slow rotating and tidally locked planets.[3]
A major impediment to the development of life in red dwarf systems is the intensetidal heating caused by the eccentric orbits of planets around host stars.[4][5] Other tidal effects reduce the probability of life around red dwarfs, such as lack of planetary axial tilt, and extreme temperature differences created when always one side of a planet faces a star and the other side faces away. Still, a planetary atmosphere may redistribute the heat, making temperatures more uniform.[6][5] However, many red dwarfs areflare stars, and flare events may greatly reduce the habitability of their satellites by eroding their atmosphere (though a planetary magnetic field could protect from flares).[7] Non-tidal factors further reduce the prospects for life in red-dwarf systems, such aselectromagnetic spectrum energy distribution shifted toward theinfrared end of the spectrum, relative to the Sun, and small circumstellar habitable zones due to low light output.[5]
However, a few factors may increase the likelihood of life on red dwarf planets. Intense cloud formation on the star-facing side of a tidally locked planet can likely reduce overall thermal flux andequilibrium temperature differences between the two sides of the planet.[2] Further, the vast number of red dwarfs statistically increases the probability that habitable planets may exist orbiting some of them. Red dwarfs form about 85% of stars in theMilky Way[8][9] and form the vast majority of stars in spiral and elliptical galaxies. In the Milky Way, an estimated tens of billions ofsuper-Earth planets occur in the habitable zones of red dwarf stars.[10] Investigating the habitability ofred dwarf star systems could help determine the frequency of life in the universe and aid scientific understanding of the evolution of life.
Red dwarfs[11] are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% of stars inspiral galaxies to more than 90% of all stars inelliptical galaxies,[12][13] an often quoted median figure being 72–76% of the stars in the Milky Way (known since the 1990s from radio telescopic observation to be abarred spiral).[14] Red dwarfs are usually defined as being ofspectral type M, although some definitions are wider (including also some or all K-type stars). Given their low energy output, red dwarfs are almost never naked-eye visible from Earth: the closest red dwarf to the Sun,Proxima Centauri, is nowhere near visual magnitude. The brightest red dwarf in Earth's night sky,Lacaille 8760 (magnitude +6.7) is visible to the naked eye only under ideal viewing conditions.
Red dwarfs' greatest advantage as candidate stars for life is their longevity. It took 4.5 billion years for intelligent life to evolve on Earth, and life as we know it will see suitable conditions for 1[15] to 2.3[16]billion years more. Red dwarfs, by contrast, could live for trillions of years, as their nuclear reactions are far slower than those of larger stars,[a] meaning that life would have longer to evolve and survive.
While the likelihood of finding a planet in the habitable zone around any specific red dwarf is slight, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around Sun-like stars, given their ubiquity.[17] Furthermore, this total amount of habitable zone will last longer, because red dwarf stars live for hundreds of billions of years or even longer on the main sequence,[18] potentially allowing for the evolution of microbial or intelligent life in the future.
For years, astronomers have been pessimistic about red dwarfs as potential candidates for hosting life. The low masses of red dwarves (from roughly 0.08 to 0.60solar masses (M☉)) cause theirnuclear fusion reactions to proceed exceedingly slowly, giving them lowluminosities ranging from 10% to just 0.0125% that of the Earth's Sun.[19] Consequently, any planet orbiting a red dwarf would need a lowsemi-major axis to maintain an Earth-like surface temperature, from 0.268astronomical units (AU) for a relatively luminous red dwarf likeLacaille 8760 to 0.032 AU for a smaller star likeProxima Centauri.[20] Such a world would have a year lasting just 3 to 150 Earth days.[21][22]
Photosynthesis on such a planet would be difficult, as much of the low luminosity falls under the lower energy infrared and red part of the electromagnetic spectrum, and would therefore require additional photons to achieve excitation potentials.[23] Potential plants would likely adapt to a much wider spectrum (and as such appear black in visible light).[23] However, 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.[24] Further, some bacteria, such aspurple bacteria, have pigments such asbacteriochlorophyll which absorb infrared light, making at least hotter red dwarfs potentially suitable for photosynthetic life.
In addition, because water strongly absorbs red and infrared light, less energy would be available for aquatic life on red dwarf planets.[25] 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.[26]
The evolution of the red dwarf stars may also inhibit habitability. As red dwarf stars have an extended pre-main sequence phase, their eventual habitable zones would be for around 1 billion years in a zone where water was not liquid but rather in a gaseous state. Thus, terrestrial planets in the actual habitable zones, if provided with abundant surface water in their formation, would have been subject to arunaway greenhouse effect for several hundred million years. During such an early runaway greenhouse 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.[27] Nevertheless, photolysis could be at least slowed down with a sufficient ozone layer.
Since the lifespan of red dwarf stars exceeds the age of the known universe, the further evolution of red dwarfs is known only by theory and simulations. According to computer simulations, a red dwarf becomes ablue dwarf after exhausting itshydrogen supply. As this kind of star is more luminous than the prior red dwarf, planets orbiting it that were frozen during the former stage could be thawed during the several billions of years this evolutionary stage lasts (5 billion years, for example, for a 0.16 M☉ star), giving life an opportunity to arise and evolve.[28]
For planets to retain significant amounts of water in the habitable zone of ultra-cool dwarfs, a planet must orbit very near to the star.[29] At these close orbital distances, tidal locking to the host star is likely. Tidal locking makes the planet rotate on its axis once 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 theterminator, where the star would always appear on or close to the horizon. It was also believed that efficient heat transfer between the sides of the planet necessitatesatmospheric circulation of anatmosphere 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,[30] thesub-solar point. In the opinion of one author this makes complex life improbable.[31] 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.[32]
In contrast to the formerly bleak picture for life, 1997 studies byNASA'sAmes Research Center have shown that a planet's atmosphere (assuming it included greenhouse gasesCO2 andH2O) need only be 100millibar, 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.[33] Subsequent research 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. Further, a 2010 study concluded that Earth-likewater worlds tidally locked to their stars would still have temperatures above 240 K (−33 °C) on the night side.[34] 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.[2]
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."[4] The eccentricity of over 150 planets found orbiting M dwarfs was measured, and it was found that two-thirds of these exoplanets are exposed to extreme tidal forces, rendering them uninhabitable due to the intense heat generated by tidal heating.[35]
Combined with the other impediments to red dwarf habitability,[6] this may make the probability of many red dwarfs hosting life as we know it very low relative to other star types.[5] There may be too little water for habitable planets around many red dwarfs;[36] what little water is on such planets, especially 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.[37]
How fast tidal locking occurs can depend on a planet's oceans and atmosphere. This may cause tidal locking to fail to occur even after many billions of years. Further, tidal locking is not the only possible end state of tidal dampening. Mercury, for example, has had sufficient time to tidally lock, but is in a 3:2 spin orbit resonance due to an eccentric orbit.[38]
Red dwarfs are far more volatile than their larger, more stable cousins. Often, they are covered instarspots that can dim their emitted light by up to 40% for months at a time. At other times, red dwarfs emit gigantic flares that can double their brightness in a matter of minutes.[39] Indeed, as more and more red dwarfs have been scrutinized for variability, more of them have been classified asflare stars to some degree or other. Such variation in brightness could be very damaging for life. Recent 3D climate models simulate flare events by altering the stellar flux received by any given planet. One study found that, should a tidally locked planet possess a sufficient atmosphere, cloud coverage and albedo increase monotonically with stellar flux, increasing the resilience of the planet to variations in radiation.[2] This caveat has proven difficult, however, since flares produce torrents of charged particles that could strip off sizable portions of the planet's atmosphere.[40] Scientists who believe in theRare Earth hypothesis doubt that red dwarfs could support life amid strong flaring. Tidal locking would probably result in a relatively low planetarymagnetic moment. Active red dwarfs that emitcoronal mass ejections (CMEs) would bow back themagnetosphere until it contacted the planetary atmosphere. As a result, the atmosphere would undergo strong erosion, possibly leaving the planet uninhabitable.[41][42][43]However, it was found that red dwarfs have a much lower CME rate than expected from their rotation or flare activity, and large CMEs occur rarely. This suggests that atmospheric erosion is caused mainly by radiation rather than CMEs.[44]
Otherwise, it is suggested that if the planet had a magnetic field, it would deflect the particles from the atmosphere (even the slow rotation of a tidally locked M-dwarf planet—it spins once for every time it orbits its star—would be enough to generate a magnetic field as long as part of the planet's interior remained molten).[45] This magnetic field must be much stronger than Earth's to protect against flares of the observed magnitude: 10–1000G versus Earth's ~0.5 G. This is unlikely to be generated.[46]Mathematical models further conclude that,[47][48][49] even under the highest attainable dynamo-generated magnetic field strengths, exoplanets with masses similar to that of Earth lose a significant fraction of their atmospheres by the erosion of the exobase's atmosphere byCoronal mass ejection (CME) bursts andextreme ultraviolet (XUV) emissions (even those Earth-like planets closer than 0.8 AU, affecting also G and K stars, are prone to losing their atmospheres). Atmospheric erosion could likely trigger depletion of water oceans also.[50] Planets shrouded by a thick haze ofhydrocarbons, such as the ones on primordial Earth or Saturn's moonTitan might still survive the flares, as floating droplets of hydrocarbon are particularly efficient at absorbing ultraviolet radiation.[51]
Measurements reject the presence of relevant atmospheres in two exoplanets orbiting a red dwarf:TRAPPIST-1b andTRAPPIST-1c. The two planets are bare rocks, or have very thin atmospheres.[52] The rest of theTRAPPIST-1 planets, all of whom other than the exceptions ofTRAPPIST-1h or possiblyTRAPPIST-1d are in the habitable zone, are unlikely to have atmospheres, but their existence is not entirely ruled out. Other potentially habitable planets orbiting red dwarfs, such asLHS 1140b[53][54] orK2-18b[55] have likely atmospheres. Calculations based in XUV fluences provide that five out of 49 planets below 1.8 Earth-radii orbiting red dwarfs within 50 parsecs would have retained an atmosphere.[56]
Another way that life could initially protect itself from radiation would be remaining underwater until the star had passed through its early flare stage, assuming the planet could retain enough of an atmosphere to sustain liquid oceans. Once life reached land, the low amount of UV produced by a quiet red dwarf means that life could thrive without an ozone layer, and thus never need to produce oxygen.[23]
For a planet around a red dwarf star to support life, it would require a rapidly rotating magnetic field to protect it from the flares. A tidally locked planet rotates slowly, and so may not be able to produce ageodynamo at its core. The violent flaring period of a red dwarf's life cycle is estimated to last for only about the first 1.2 billion years of its active life. If a planet forms far away from a red dwarf so as to avoid atmospheric erosion, and thenmigrates into the star's habitable zone after this turbulent initial period, it is possible for life to develop.[57] However, observations of the 7 to 12-billion year oldBarnard's Star showcase that even old red dwarfs can have significant flare activity. Barnard's Star was long assumed to have little activity, but in 1998, astronomers observed an intensestellar flare, showing that it is aflare star.[58]
It has been found that the largest flares occur at high latitudes near the stellar poles; if an exoplanet's orbit is aligned with the stellar rotation (as is the case with the planets of the Solar System), then it is less affected by the flares than formerly thought.[59]
Ifmethane-based life is possible (similar to hypotheticallife on Titan), there would be a second habitable zone further out from the star corresponding to the region where methane is liquid. Titan's atmosphere is transparent to red and infrared light, so more of the light from red dwarfs would be expected to reach the surface of a Titan-like planet.[60] This zone would lie at 2.573astronomical units (AU) forLacaille 8760, to 0.379 AU forProxima Centauri.
A study of archivalSpitzer data gives the first idea and estimate of how frequent Earth-sized worlds are aroundultra-cool dwarf stars: 30–45%.[61] A computer simulation finds that planets that form around stars with similar mass toTRAPPIST-1 (c. 0.084 M⊙) most likely have sizes similar to the Earth's.[62]
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