Astrobiology and SETI
The Search for Extra Terrestrial Intelligence (SETI) is a bit like String Theory - a theory without any experimental evidence - or, more precisely, with only negative experimental evidence. When SETI was first proposed, it was far from clear that we would ever have any tools to learn about extraterrestrial biology if it wasn't smart enough to talk back at us. That's no longer the case, and consequently a lot of attention has been turned to the more basic question of extraterrestrial life. Since it seem likely that extraterrestrial life would have to precede extraterrestrial intelligence, maybe we should think about that problem first. That's the subject of astrobiology - another science that still lacks a subject.
Suppose we take a look at a modified form of the first few terms of the Drake Equation, first developed for SETI, but look just at those concerned with the development of life. When the equation was first written down, a big question was what fraction of stars had planets, and what fraction of those planets were habitable. We now know that a very large fraction of stars (perhaps most) have planets, and it seems likely that many of them will be at least in the habitable zone of their star. Other uncertainties abound. Is there liquid water? Does a planet need a big moon like Earth has? Is a Jupiter needed to collect and disperse threatening asteroids?
The monster in the closet is the probability of life developing on an otherwise habitable planet. Here we have exactly one data point - Earth. We have increasingly detailed and plausible speculations about how life developed here, but huge and hard to bridge gaps remain.
There seem to be at least several locations in the solar system that have, or recently had, liquid water: Mars, Europa, and Enceladus. Moreover, new soon to be launched instruments, promise at least some capability to identify signatures of life on extrasolar planets.