Habitability of red dwarf systems

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Determining the habitability of red dwarf systems could help reveal how likely extraterrestrial life is to exist, as red dwarfs make up a majority of all the stars in the galaxy. Critical factors assumed to be impediments to habitablity include relatively little energy output and thus reduced habitable zones, the probability of tidally locked planets, and high stellar variation. Positive factors include the ubiquity and longevity of red dwarfs.

Brown dwarfs are likely more numerous than red dwarfs. However, they are not generally classified as stars, and could never support life as we understand it, since what little heat they emit quickly disappears.

1 Red dwarf characteristics
2 Research
3 See also
4 Notes and references

[edit] Red dwarf characteristics

Red dwarfs are the smallest and coolest type of star and by far the most common. Estimates of their abundance range from 70% to more than 90% of all stars. (The term "dwarf" technically applies to all stars on the main sequence, including the Sun.) 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 colour). Given their small size, red dwarfs are never visible by the unaided eye from Earth; the closest solitary red dwarf, Barnard's star, is nowhere near visual magnitude.

[edit] Research

[edit] Light emission and tidal lock

Relative star sizes and photospheric temperatures. Any planet around a red dwarf such as the one shown here would have to huddle close to achieve Earth-like temperatures, likely inducing tidal lock. See Aurelia.

Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.1 to 0.6 solar masses) means that their nuclear reactions proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU for a star like Proxima Centauri^[1] (such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would cause tidal lock. Whether or not any planet is habitable may depend partly on whether the planet's atmosphere causes a Greenhouse effect and how strong any greenhouse effect is.^{[citation needed]} The daylight side of the planet would eternally face the star, while the night-time side would always face away from it. The only way potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing photosynthesis.

The pessimism has been tempered by research. 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 mbs, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side.^[2] This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of Greenwich Community College, has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Geothermal heat might help keep the lower parts of any ocean liquid.^{[citation needed]} Further research—including a consideration of the amount photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.^[3]

Size is not the only factor in making red dwarfs potentially unsuitable for life, however. On a red dwarf planet, photosynthesis on the night side would be impossible, since it would never see the sun. On the day side, because the sun does not rise or set, areas in the shadows of mountains would remain so forever. Photosynthesis as we understand it would be complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on the Earth the process depends on visible light. There are potential positives to this scenario. Numerous terrestrial ecosystems rely on chemosynthesis rather than photosynthesis, for instance, which would be possible in a red dwarf system. See Cave biology. Hydrothermal vents also have life not dependent on photosynthesis. Even if true this would not prevent life though it could limit any biosphere. Many scientists suspect that the earliest life on Earth was in places like caves and hydrothermal vents. Photosysthesis evolved later. If happens in the case of a planet orbiting a Red dwarf star fife would have billions of years to evolve ways of photosysthesizing the star's light.^{[citation needed]} A static primary star position removes the need for plants to steer leaves toward the sun, deal with changing shade/sun patterns, or change from photosynthesis to stored energy during night. Because of the lack of a day-night cycle, including the weak light of morning and evening, far more energy would be available at a given radiation level.^{[citation needed]}

[edit] 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, while at other times they emit gigantic flares that can double their brightness in a matter of minutes.^[4] Such variation would be very damaging for life, as it might destroy any complex organic molecules that could possibly form biological precursors. If life first evolves in places like caves and hydrothermal vents there would later be thoudands of millions of years during which life life could gradually evolve protection from flares.^{[citation needed]}Flares might also blow off sizeable portions of the planet's atmosphere. 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. However, a tidally locked planet rotates only very slowly, and so allegedly cannot produce a geodynamo at its core. Another scientist disagrees.

"No one found any showstoppers to habitability," says Gibor Basri of the University of California, Berkeley. One concern was that because M dwarfs frequently produce flares, the resulting torrents of charged particles could strip the atmosphere off any nearby planet. If the planet had a magnetic field, though, it would deflect the particles from the atmosphere. And 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. [1]

However, the violent flaring period of a red dwarf's lifecyle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidelock, and then migrates into the star's habitable zone after this turbulent initial period, it is possible that life may have a chance to develop.^[5] Proxima Centauri at age=4.85 × 10⁹ is a very active flare star. Barnard's Star is still a flare star despite being a very old star. Flares on Barnard's star are rare. The text is unclear how violent these flares are.

Other scientists disagree that Red Dwarf stars could sustain life. See Rare Earth hypothesis.

[edit] Abundance

There is, however, one major advantage that red dwarfs have over other stars as abodes for life: they live a long time. It took 4.5 billion years before humanity appeared on Earth, and life as we know it will see suitable conditions for as little as half a billion years more.^[6] Red dwarfs, by contrast, could live for trillions of years, because their nuclear reactions are far slower than those of larger stars, meaning that life both would have longer to evolve and longer to survive. Further, while the odds of finding a planet in the habitable zone around any specific red dwarf are slim, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around sun-like stars given their ubiquity.^[7] The first possibly rocky extrasolar planet with the mass of a Super-Earth that was found at the "warm" edge of the habitable zone of its star is Gliese 581 c, and its star, Gliese 581, is indeed a red dwarf.