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Also maybe what we consider to be habitable places an unnecessary restriction upon life as where we can expect to find it...maybe in other places is not so.
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Alex, I agree. Since we are now touching on my area (biology) I feel I am somewhat qualified to pass comment. All the talk of planets in the habitable zone, presence of water, oxygen, carbon etc is all very well if you are searching for Earth-like life. But, as we have seen on Earth life has been found to flourish in "uninhabitable" locations such as in the superheated, high pressure water around volcanic vents in deep ocean and even flourishing in pools of sulphurous "magma". The bacteria found near volcanic vents are called thermophiles (heat loving) and have been shown via their DNA to be related to all other life on Earth via the "single common ancestor".
So if life can survive in such places, who is to say that life has to occur only in certain places, on certain types of planets. It is possible that life may be possible on a wide variety of planets, either Earth or Jupiter sized, with an Earth like atmosphere or a Jovian etc etc etc like atmosphere, close to a sun-like star or farther away from a very different star.
However the current research on thermophiles indicates that they probably adapted to life in their current environments, rather than being the "original" life form themselves. If this is true, then maybe life is required to form on an Earth like planet before it can gradually evolve to life such as thermophiles.
Paul Davies (Arizona State University) has (or maybe by now that should be had??) a theory (or perhaps hypothesis would be more correct) that life on Earth developed and was wiped out on several occasions during the 'early period of heavy bombardment.
The suggestions that follow on from this is that, if life did form on Earth on several separate occasions then:
1. Life WILL form if it is at all possible for it do so.
and therefore
2. Life MUST exist elsewhere in the Universe.
Again, these are suggestions, but the point is that if something so apparently unlikely can happen several times over on one planet, then the drive for life to form from abiotic substrates would appear to be so strong as to make life almost a certainty. Or....would it???
The converse of this is that, if life did only form on Earth once, then maybe we truly are alone.
Or......are we?
Stuart