# Habitability of red dwarf systems

The theorized habitability of red dwarf systems is determined by a large number of factors. Modern evidence suggests that planets in red dwarf systems are unlikely to be habitable, due to high probability of tidal locking, likely lack of atmospheres, and the high stellar variation many such planets would experience. However, the sheer number and longevity of red dwarfs could likely provide ample opportunity to realize any small possibility of habitability.
Current arguments concerning the habitability of red dwarf systems are unresolved, and the area remains an open question of study in the fields of climate modeling and the evolution of life on Earth. Observational data and statistical arguments suggest that red dwarf systems are uninhabitable for indeterminate reasons. On the other hand, 3D climate models favor habitability and wider habitable zones for slow rotating and tidally locked planets.
A major impediment to the development of life in red dwarf systems is the intense tidal heating caused by the eccentric orbits of planets around their host stars. Other tidal effects reduce the probability of life around red dwarfs, such as the lack of planetary axial tilts and the extreme temperature differences created by one side of planet permanently facing the star and the other perpetually turned away. Still, a planetary atmosphere may redistribute the heat, making temperatures more uniform. However, it is important to bear in mind that many red dwarfs are flare stars, and their flare events could greatly reduce the habitability of their satellites by eroding their atmosphere (though a planetary magnetic field could protect from these flares). Non-tidal factors further reduce the prospects for life in red-dwarf systems, such as spectral energy distributions shifted toward the infrared side of the spectrum relative to the Sun and small circumstellar habitable zones due to low light output.
There are, however, a few factors that could 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 and equilibrium temperature differences between the two sides of the planet. In addition, the sheer number of red dwarfs statistically increases the probability that there might exist habitable planets orbiting some of them. Red dwarfs account for about 85% of stars in the Milky Way and constitute the vast majority of stars in spiral and elliptical galaxies. There are expected to be tens of billions of super-Earth planets in the habitable zones of red dwarf stars in the Milky Way. Investigating the habitability of red dwarf star systems could help determine the frequency of life in the universe and aid scientific understanding of the evolution of life.

## Background

Red dwarfs 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, 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 a barred spiral). Red dwarfs are usually defined as being of spectral 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 (+6.7) is visible to the naked eye only under ideal viewing conditions.

## Longevity and ubiquity

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 to 2.3 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, 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. 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, potentially allowing for the evolution of microbial or intelligent life in the future.

## Luminosity and spectral composition

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 0.60 solar masses (M$_☉$)) cause their nuclear fusion reactions to proceed exceedingly slowly, giving them low luminosities ranging from 10% to just 0.0125% that of the Earth's Sun. Consequently, any planet orbiting a red dwarf would need a low semi-major axis in order to maintain an Earth-like surface temperature, from 0.268 astronomical units (AU) for a relatively luminous red dwarf like Lacaille 8760 to 0.032 AU for a smaller star like Proxima Centauri. Such a world would have a year lasting just 3 to 150 Earth days.
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. Potential plants would likely adapt to a much wider spectrum (and as such appear black in visible light). 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. Additionally, some bacteria, such as purple bacteria, have pigments such as bacteriochlorophyll 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. 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.
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 a runaway 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. 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 a blue dwarf after exhausting its hydrogen supply. As this kind of star is more luminous than the previous 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 appear and evolve.

## Tidal effects

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. 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 the terminator, 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 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, the sub-solar point. In the opinion of one author this makes complex life improbable. 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.
In contrast to the previously bleak picture for life, 1997 studies by NASA's Ames Research Center have shown that a planet's atmosphere (assuming it included greenhouse gases CO$_2$ and H$_2$O) 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. 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. 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. 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.
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." 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.
Combined with the other impediments to red dwarf habitability, this may make the probability of many red dwarfs hosting life as we know it very low compared to other star types. There may not even be enough water for habitable planets around many red dwarfs; 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.
Note however that how quickly tidal locking occurs can depend upon a planet's oceans and even atmosphere, and it may mean that tidal locking fails to happen even after many billions of years. Additionally, 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.

## Variability

Red dwarfs are far more volatile than their larger, more stable cousins. Often, they are covered in starspots 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. Indeed, as more and more red dwarfs have been scrutinized for variability, more of them have been classified as flare 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. This caveat has proven difficult, however, since flares produce torrents of charged particles that could strip off sizable portions of the planet's atmosphere. Scientists who believe in the Rare Earth hypothesis doubt that red dwarfs could support life amid strong flaring. Tidal locking would probably result in a relatively low planetary magnetic moment. Active red dwarfs that emit coronal mass ejections (CMEs) would bow back the magnetosphere until it contacted the planetary atmosphere. As a result, the atmosphere would undergo strong erosion, possibly leaving the planet uninhabitable. 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.
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). This magnetic field should be much stronger compared to Earth's to give protection against flares of the observed magnitude (10–1000 G compared to Earth's ~0.5 G), which is unlikely to be generated. Mathematical models additionally conclude that, 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 by CME bursts and 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 the depletion of water oceans as well. Planets shrouded by a thick haze of hydrocarbons, such as the ones on primordial Earth or Saturn's moon Titan might still survive the flares, as floating droplets of hydrocarbon are particularly efficient at absorbing ultraviolet radiation.
Measurements reject the presence of relevant atmospheres in two exoplanets orbiting a red dwarf: TRAPPIST-1b and TRAPPIST-1c. The two planets are bare rocks, or have very thin atmospheres. The rest of the TRAPPIST-1 planets, all of whom other than the exceptions of TRAPPIST-1h or possibly TRAPPIST-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 as LHS 1140b or K2-18b have likely atmospheres.
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.

### Flare activity

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 a geodynamo 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 existence. If a planet forms far away from a red dwarf so as to avoid atmospheric erosion, and then migrates into the star's habitable zone after this turbulent initial period, it is possible for life to develop. However, observations of the 7 to 12-billion year old Barnard'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 intense stellar flare, showing that it is a flare star.
It has been found that the largest flares happen 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 previously thought.

## Methane habitable zone

If methane-based life is possible (similar to the hypothetical life 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. This zone would lie at 2.573 astronomical units (AU) for Lacaille 8760, to 0.379 AU for Proxima Centauri.

## Frequency of Earth-sized worlds around ultra-cool dwarfs

A study of archival Spitzer data gives the first idea and estimate of how frequent Earth-sized worlds are around ultra-cool dwarf stars: 30–45%. A computer simulation finds that planets that form around stars with similar mass to TRAPPIST-1 (c. 0.084 M$_⊙$) most likely have sizes similar to the Earth's.

## In fiction

* Ark: In Stephen Baxter's Ark, after planet Earth is completely submerged by the oceans a small group of humans embark on an interstellar journey eventually making it to a planet named Earth III. The planet is cold, tidally locked and the plant life is black (in order to better absorb the light from the red dwarf).
* Draco Tavern: In Larry Niven's Draco Tavern stories, the highly advanced Chirpsithra aliens evolved on a tide-locked oxygen world around a red dwarf. However, no detail is given beyond that it was about 1 terrestrial mass, a little colder, and used red dwarf sunlight.
* Nemesis: Isaac Asimov avoids the tidal effect issues of the red dwarf Nemesis by making the habitable "planet" a satellite of a gas giant which is tidally locked to the star.
* Star Maker: In Olaf Stapledon's 1937 science fiction novel Star Maker, one of the many alien civilizations in the Milky Way he describes is located in the terminator zone of a tidally locked planet of a red dwarf system. This planet is inhabited by intelligent plants that look like carrots with arms, legs, and a head, which "sleep" part of the time by inserting themselves in soil on plots of land and absorbing sunlight through photosynthesis, and which are awake part of the time, emerging from their plots of soil as locomoting beings who participate in all the complex activities of a modern industrial civilization. Stapledon also describes how life evolved on this planet.
* Superman: Superman's home, Krypton, was in orbit around a red star called Rao which in some stories is described as being a red dwarf, although it is more often referred to as a red giant.
* Ready Jet Go!: In the children's show Ready Jet Go!, Carrot, Celery and Jet are a family of aliens known as Bortronians who come from Bortron 7, a planet of the fictional red dwarf Bortron. They discovered Earth and the Sun when they picked up a "primitive" radio signal (Episode: "How We Found Your Sun"). They also gave a description of the planets in the Bortronian solar system in a song in the movie Ready Jet Go!: Back to Bortron 7.
* Aurelia: This planet, seen in the speculative documentary Extraterrestrial (also known as Alien Worlds), details what scientist theorize alien life could be like on a planet orbiting a red dwarf star.