<P> Astronomers for many years ruled out red dwarfs as potential abodes for life . Their small size (from 0.08 to 0.45 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 locking . One side of the planet would eternally face the star, while the other would always face away from it . The only ways in which 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, or if there was a gas giant in the habitable zone, with a habitable moon, which would be locked to the planet instead of the star, allowing a more even distribution of radiation over the planet . It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing photosynthesis . </P> <P> This 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 CO and H O) 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 . Further research--including a consideration of the amount of photosynthetically active radiation--suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants . </P> <P> 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 . 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 . </P> <P> 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 . Such variation would be very damaging for life, as it would not only destroy any complex organic molecules that could possibly form biological precursors, but also because it would blow off sizeable portions of the planet's atmosphere . </P>

What kind of life would we expect to be in this habitable zone around other stars