Quote:
Originally Posted by CraigS
Aside from water, oxygen in the atmosphere would also seem to be important, although I read the other day about some bacteria which metabolises sulphur .. which could be present where there is no water, or oxygen … the diversity is almost limitless .. and this latter point seems to be consistently reinforced with more solar system and exo-solar system planetary/moon research  |
Free oxygen in an exoplanet's atmosphere would be an indication of possible oxygen breathing life. It should not be a criteria though, since the Earth's free oxygen only appeared 1.7Gy ago while life itself has been present here for at least 3.5Gy. The first life forms on Earth didn't breathe oxygen at all.
But it really depends on what question we want to answer? To search for 'intelligent', and supposedly large, life forms it seems likely that oxygen breathing would be a criteria. But if we want to know if some form of life arises simply when the conditions are favourable then we don't need to limit the search to oxygen-rich planets.