Observatoire de Paris
<-   The notion of a habitable zone   ->

Once accepted, the biochemical methodology described above allows us to determine on which type of planets we could expect to discover biological life. For the latter to thrive, some conditions are required. These conditions are in general accepted by astronomers, but some variants are possible.

The main condition is a liquid environment, which tremendously encourages the transport of materials necessary for biochemical activity.

A priori, water is a promising liquid, since it is one of the most abundant liquids in the universe (the others being for instance alcohol, methane, and ammonia, all of which can also be liquid but at much lower temperatures). Furthermore, water has the advantage of being one of the best solvents, which favours biochemical exchanges and reactions.

In the frame of traditional thermodynamic concepts, another universal condition is a "worthy" energy source (that is in a non thermal shape) with a very low entropy. In addition, it must be permanent since its break would lead to the destruction of organisms. The best known permanent energy source, abundant and with a low entropy, is the radiation of stars.

The best known identified place to find both liquid water and a permanent and intense source of light is a planet located at a distance from its star such that the temperature is about 300 K. Moreover, it must be sufficiently massive in order to prevent water from escaping from the planet, but not too massive, otherwise the water is confined in deep layers without the light of a hydrogen atmosphere (this last point is, however, under discussion). Therefore, we first search for biological life on a planet with a mass between 1 and a few terrestrial masses, and located at a distance between 0.2 AU (for M type stars) and 1.5 AU (for F type stars) from its star (even if satellites of giant planets rich in water and heated by tidal effects like Europa are possible). This critical distance, dependent on the type of the star, defines what we call the habitable zone of the star.