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.
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; both the closest red dwarf star to the sun when viewed individually, Proxima Centauri, and the closest solitary red dwarf, Barnard's star, are nowhere near visual magnitude.
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 (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. 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. 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. 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.
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 to less extreme reduced temperatures for months at a time during 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. This could cause conditions similar to Snowball Earth so cold could last millions of years. At other times Red dwarf stars emit gigantic flares that can double their brightness in a matter of minutes. Such variation would be very damaging for life, as it might destroy any complex organic molecules that could possibly form biological precursors. Flares might also blow off sizable 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.
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.
Life could initially protect itself from radiation by remaining underwater until the star had passed through its early flare stage. Once it reached onto land, the low amount of UV produced by a quiescent red dwarf means that life could thrive without an ozone layer, and thus need never produce oxygen.
Other scientists disagree that red dwarf stars could sustain life. See Rare Earth hypothesis. Tidal-locking would likely result in a relatively low planetary magnetic moment. Active red dwarfs that emit coronal mass ejections 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.
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. 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. 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.