How Life could Evolve in a Red Dwarf Star System
On Earth it is believed that life originated or could have originated in caves or round Hydrothermal vents. If life evolved similarly in a Red dwarf star system life could adapt over millions of years to cope with stellar flares. Scientists disagree about whether life could exist in red dwarf star systems. See Proxima Centauri where two scientists are cited, http://www.liebertonline.com/doi/abs/10.1089/ast.2006.0127 A Reappraisal of The Habitability of Planets around M Dwarf Stars and Impact on Expected Magnetospheres of Earth-Like Exoplanets in Close-In Habitable Zones. See also Habitability of red dwarf systems and Aurelia.
The text below assumes those scientsts who think red dwarfs are habitable are correct.
Life in caves
Life in caves could gradually adapt to exploit environments nearer to the cave mouth. In the course of this flares would increasingly become a problem. Life could gradually evolve defences. For example living organisms could evolve opaque protective shells. When they detect that a flare is starting they could retreat into their shells. Probably different organisms would evolve different mechanisms. Resistant organisms could colonize habitats where more susceptible competitors would be killed by flares. Later when the protective mechanisms are sufficiently strong living organisms could live out in the open and evolve Photosynthesis.
Life round hydrothermal vents
Life round hydrothermal vents would face greater difficulties. Most hydrothermal vents are in deep oceans where there is no intermediate habitat between the vents and the ocean surface. On Earth in places like Hawaii and Iceland geological activity typical of hydrothermal vents happens in shallow water. In such places life could also gradually evolve protection against flares. Resistant organisms could colonize habitats where more susceptible competitors would be killed by flares. As resistance progressively evolved life could move progressively into shallower water where there is more light even between flares. Then again photosynthesis could evolve.
Small life and Microscopic life
A smaller living organism has a larger surface area to volume ratio. Therefore smaller plants and animals would need to use proportionately more energy to build protective shells and to carry such shells around if they moved. Small organisms could live in soil in mud, in the shadows of mountains where flares could not reach them or in deep water. Small organisms could also exist in rough terrain where there are places to shelter during flares. Small organisms could live on the surface of mud and soil as well provided they could burrow downwards during flares. Small life could live on the surface of water provided it could dive during flares. Small life, like tardigrades on Earth, could also survive radiation from flares by dehydrating their bodies, and remaining static for sustained periods of time.
Currently scientists believe that small life and microscopic life is probably more common than large life. Such life would be restricted to areas where at least some organisms could survive flares. Additionally, small life that is out in the open would need life chemistry that reacts strongly to the infrared and red light that a red dwarf star emits in large amounts so that it is protected, and/or able to utilise energy, efficiently from those long-wavelengths of light. At the same time it must not react fatally with the more intense light emitted during flares. The author is not a biochemist and does not know if this is possible.
Large organisms may if they are motile retreat into shells. They may also run and find shelter. A large animal or plant with a protective shell may spend its life in open terrain and the shell would protect it during flares. Would their young be large or small? There are two possibilities.
- The first possibility is that young would be large enough to have protective shells as soon as they are independent from their parent or would grow fast after independence. Many animals and plants may produce a few large young rather than many small young. They may have shells at birth or on hatching. Perhaps small dependent young would retreat into their parent's shell during a flare. Perhaps parents would make a shell or a protective burrow for small young before leaving them to fend for themselves. The young may alternatively grow shells after hatching, some young would die if a flare happened before they had grown a shell but sufficiently many young would live until they had a shell before their first flare so the species could continue. (Proxima Centauri flares every few days according to  but that site could be controversial This could happen with red dwarf stars that flare less often.)
- The second possibility is that parents would lay eggs or deposit live young in muddy places or other places where a small animal can survive. When the young animals were bigger and had protective shells they would migrate to open terrain.
On Earth mammals have a layer of dead epidermis on the surface of their skin. Also mammalian hair is dead. The sun of the Solar System does not flare and therefore a thick dead layer is not needed on Earth. On a planet orbiting a red dwarf star thick opaque layers of dead skin or scales could protect the living parts of an animal from radiation during flares without restricting movement.
It is quite possible that in a red dwarf ecology larger plants and animals with protective shells or protective outer skins would fill many ecological niches where small organisms are on Earth.
Older Red Dwarf stars
Astronomers believe that red dwarf stars eventually stop flaring. Red dwarfs last for very long periods of time and age very slowly. Proxima Centauri is as old as our sun but is still young by red dwarf standards and is flaring frequently. Life could evolve after the star has stopped flaring provided geological conditions on the planet are favourable for life. Some scientists believe that life requires a geologically active planet like Earth. Barnard's Star is much older than Proxima Centauri and it is believed that this star flared during the 1990's.
Paradoxically there might be a new threat to life when flares become less frequent. While flares are common mechanisms that enable organisms to survive them will remain sharp. Natural selection will weed out organisms with defective mechanisms to resist flares. What will happen if a red dwarf star passes through a stage when it flares on average once every ten thousand years or once every million years? If that happens most organisms may be unable to resist flares when they happen. Organisms that happened to be in places that are sheltered from the flare would survive but plants in those places would not get energy for photosynthesis afterwards.
On Earth many plants can survive if all the parts that are above ground are killed by, for example a forest fire or a plague of locusts. Also seeds below soil level can survive such disasters. Gardeners know that some weeds grow again even after all the parts that are above ground have been removed. If a red dwarf flares rarely plant life could survive similarly.
- Solar flares on dwarf stars could threaten alien life This is relevant, I do not know how to assess it.