The chances of finding life around an M dwarf star have just been increased, as researchers suggest that the habitable zone is much larger than originally thought.
The habitable zone is the region at a particular distance from a star where a delicate dance between a planet’s atmosphere and the radiation received from the star can permit the existence of liquid water. This zone needs to be not too hot, not too cold, but just right which sometimes earns it the nickname the Goldilocks Zone.
But just how large is the area where it is “just right”? Much larger than we originally thought, at least around M dwarf stars – a type of red dwarf – according to astronomers Manoj Joshi and Robert Haberle.
M dwarf stars are more plentiful than solar type stars, which means there is a good chance of finding a life bearing planet around an M dwarf star. They are much cooler than the Sun and they emit light at much longer wavelengths, which could result in the habitable zone being extended by up to 30 per cent. At longer wavelengths snow and ice reflect less light, and this reflectivity is known as albedo.
Light that would be reflected by ice and snow on a planet orbiting a Sun like star is now absorbed when the source is an M dwarf star. If the albedo is low, then less of the essential heat needed for life is being reflected away from the planet. Thus the outer edge of the habitable zone, which was previously “too cold”, now becomes “just right.”
The outer edge of the habitable zone is typically defined as the point where carbon dioxide starts to condense. Decreasing the albedo results in this point being pushed outwards. These new revelations have no impact on the inner edge of the habitable zone, which is the point where the oceans evaporate, as any ice and snow cover would be too small to influence the albedo significantly.
Joshi and Haberle looked at the stars Gliese 436 and GJ 1214 in order to calculate what the albedo would be if there was snow and ice present on terrestrial planets around these stars. If a planet was half covered in snow or ice, and the other half was land or ocean, then the albedo is reduced significantly. Reducing the albedo will warm the planet, which in turn can cause some of the snow and ice to melt in what is known as a feedback effect. However, this temperature is likely to stabilise after some time.
A typical concern of potential habitable planets around M dwarfs is that they are orbiting so close to their relatively cool parent star that they become tidally locked. This means that one side of the planet always faces the star and the other side is in constant darkness. The extension of the habitable zone by up to 30 per cent doesn’t negate this effect however. “The tidal lock radius is quite a way outside the outer edge of the habitable zone,” Joshi tells Skymania News. “I don’t think that planets on the outer edge of the HZ would be significantly less likely to be tidally locked.”
However this is not all bad news because if the atmosphere was thick enough then it is possible for heat to be transported around the planet to the cool side, giving better odds for the existence of life.