Cosmological aspects of planetary habitability
Shchekinov et al
The habitable zone (HZ) is defined as the region around a star where a planet can support liquid water on its surface, which, together with an oxygen atmosphere, is presumed to be necessary (and sufficient) to develop and sustain life on the planet. Currently, about twenty potentially habitable planets are listed. The most intriguing question driving all these studies is whether planets within habitable zones host extraterrestrial life.
It is implicitly assumed that a planet in the habitable zone bears biota. However along with the two usual indicators of habitability, an oxygen atmosphere and liquid water on the surface, an additional one -- the age --- has to be taken into account when the question of the existence of life (or even a simple biota) on a planet is addressed. The importance of planetary age for the existence of life as we know it follows from the fact that the primary process, the photosynthesis, is endothermic with an activation energy higher than temperatures in habitable zones. Therefore on planets in habitable zones, due to variations in their albedo, orbits, diameters and other crucial parameters, the onset of photosynthesis may take much longer time than the planetary age.
Recently, several exoplanets orbiting Population~II stars with ages of 12--13 Gyr were discovered. Even though these stars have low metallicity, they can still form protoplanetary clouds where the abundance of metals can be enhanced due to the action of physical fractionation. Masses of protoplanets in such conditions can, in principle, be within Earth to super-Earth range. These planets had enough time to develop necessary chains of chemical reactions and may thus carry life provided they are within a habitable zone.