An interesting topic open to much speculation (sometimes it seems that topics not open to much speculation are less interesting). Basically, there is one single formula for calculating the probability of extraterrestrial life. You take the estimated number of stars and multiply it (assuming statistical independence between the many probabilities involved) with (among others)
the probability that planets exist for a star
the probability that planet orbits are stable for a very long time
the probability that the orbits are at a safe but warm distance
the probability that...

That's the idea in a nutshell. Now how do we know these figures? Actually, we don't, we make extremely rough guesses. These guesses don't differ just by 10% or so, they often differ by several powers of ten. Now what happens if you make the final calculation with one set of guesses and then with another set of guesses? You come to completely differing results.

I've read accounts from (a) so unlikely to have happened even once that life on earth is a proof for intelligent design to (b) the universe must be full of planets with life.

It is nice to know that for one of these probabilities there is now a better base for a good guess, but still the figures in the final calculation differ by several powers of ten depending upon the assumptions. As Grab has said, it is at this moment a question of emotion, not of intellect, what you believe.

Wolfgang