It is absolutely unanswerable - we're on the same page, CM.
We know the probability is non-zero. We are here, after all. So the probability of life developing in the Universe is somewhere between 1 in 10^23 (rough estimate) and 1 (every planet would have life). We know it's not 1, though, since we know not every planet in our own solar system has life. Technically, I suppose we don't know it for certain, but there's no indication of life on Mercury or Venus, so those two planets alone rule out a probability of 1.
Many things are possible, though. We know that life can exist apart from a photosynthetic base. Chemosynthetic lifeforms and ecosystems have been discovered near black smokers in the mid-Atlantic ridge. They get their energy from heat and chemicals emitted from fumaroles.
From a purely energetic perspective, the Galilean moon Europa is subjected to intense tidal forces similar to Io (they are in a orbital resonance, so they both must be feeling the flexing). Imagine the volcanic world of Io covered in miles of water, topped with ice. The existence of chemosynthetic lifeforms here on Earth indicates that it would be possible for life to feed off the energy of Europan underwater volcanoes. For us, it's a mere matter of checking. Then the existence of two life-bearing worlds in one solar system would increase the lower bound of probability to 2 out of 10^23.
So the possibilities are endless, but the probabilities are not, and I for one want some hard numbers to crunch.
I suppose we could approach this from a purely physics perspective, too. The odds are that life has to be carbon based. No other element is as capable of making a wide variety of bonds with other elements that are energetically favorable for life. The possibility of silicon-based life is often raised, but silicon has fewer options for bonding and is energetically unfavorable (if silicon-based life existed, it would have to be at extreme pressure and temperature conditions, and would still probably be much less active and much simpler than carbon-based life).
We have confirmed the existence of a variety of extrasolar planets, all of which can be examined from an energetics perspective to calculate a probability for carbon-based life to exist there. Carbon-life has a particular range of conditions under which it can survive - outside of those conditions it is simply not possible. The bonds won't form or be stable, and the elements would be vaporized or frozen.
I suppose I could run through the extrasolar planet database and run some numbers in my spare time. I might find it amusing....
Edited by pavonis, 11 February 2014 - 02:22 PM.