So, we have equations estimating the number of planets that exist in a habitable zone within our galaxy. And from that, we've extrapolated a relatively large number.
But the other equation is, from a perfectly habitable planet, what's the chances of life evolving? There require a lot things to come completely in alignment for life to occur. Who knows, a day later and the earth might have missed out on life entirely. My suspicion is that the chances for life occurring are extremely low. Maybe not as low as 500,000 to 1, but probably lower than the average person would seem to think. Then from that number, what's the chances of life evolving to such a level that they can even develop the means to envision space travel or communication? It's only happened once on earth, after all.
People think that just because there are a lot of planets that there should be lots of aliens. But I think that there are a lot of big equations left to work out. We have a very huge number which is probably countered by a number of very small numbers. I could easily imagine that we eventually find that the chances of life occurring on a "habitable" planet are less than 1 billion to 1, which would make the chances of life occurring elsewhere in our galaxy fairly remote.
Then again, I'm no expert. I'm just trying to bring up the other big questions we have yet to really tackle... at least as far as I'm familiar with.