Only recently has the idea of habitable planets around red dwarf stars taken hold. But it’s a fascinating one, especially if you take a look at the potential window for life to develop on such worlds. M-class red dwarfs live anywhere from 50 billion up to several trillion years, a vast stretch compared with our own Sun’s projected ten billion years. And with 75 percent of main sequence stars thought to be red dwarfs, the hunt for life can be expanded enormously if we add red dwarfs to the mix.

But getting a stable environment for that life to develop is another matter, for planets in the habitable zone around such stars would be close enough to their primaries to be tidally locked, with one side always in sunlight, the other in darkness. The thought of a frozen dark side and a scalded day side isn’t pretty. It wasn’t until the late 1990s that models of heat transport within the atmosphere developed that could even out these stark extremes. Now it looks as though habitable worlds could exist around M dwarfs, but just where do we look?

A new study tackles this question from the standpoint of ultraviolet light and its potential effects on stellar habitable zones. Last April we looked at a paper on this topic by the same team, led by Andrea Buccino (Instituto de Astronomía y Física del Espacio, Buenos Aires). Now Buccino and colleagues are looking at the UV emitted by solar flares on M dwarfs, using their model of an ultraviolet habitable zone to study three dwarfs in particular: GL 581, Gj 849 and Gl 876. Their data come from observations made by the International Ultraviolet Explorer (IUE) satellite.

Ultraviolet is a critical parameter because enough of it can inhibit photosynthesis and destroy DNA. In the right amount, however, it can be a significant energy source for synthesizing biochemical compounds. In the case of the three planets studied, however, the liquid water habitable zone and the ultraviolet habitable zone do not overlap, with the UV much weaker than needed to trigger the formation of complex molecules at distances where liquid water could exist on a planetary surface.

But get this: Solar flares could provide the needed trigger, and many M dwarfs are flare stars. From the paper:

In contrast to what it has been believed for a long time, the ?are activity in M stars could play an important role in the origin of life. Moderate ?ares could trigger the biogenesis processes while the effect of strong ?ares, that are less frequent, could be mitigated by the production of abiotic ozone and the fact that only one face of the hypothetical terrestrial planet within the [liquid water habitable zone] will receive the damaging UV radiation.

M dwarfs with moderate flare activity thus become our most likely candidates for habitable worlds around such stars. Indeed, red dwarfs with no flare activity would need alternate sources of energy to start biogenesis. That conclusion marks the continuing reshaping of the red dwarf model, since it used to be thought that such flares would effectively prevent life from ever forming. The paper is Buccino et al., “UV habitable zones around M stars,” submitted to Astronomy & Astrophysics and now available as a preprint online.