Extrapolation. Life on this planet obviously originated. We now (approximately) the conditions necessary to have life originate. We know the (approx.) conditions that life needs to thrive in. We know that there are hundreds of billions or even trillions of planets in our own galaxy, and billions of years of windows in which some non-zero number of those planets would have the conditions and materials in which life COULD originate on them. We know that there are hundreds of billions or trillions of other galaxies out in the universe, all with the same chances.
Even leaving aside the non-zero probability that like from this planet could have been spread through impacts, that’s a huge number of chances for life to be in the universe during windows of time stretching billions of years.
It’s not a ‘conclusion’…it’s a belief, as was said in the subject of the OP. THAT is the basis by which ‘science’ posits that ‘extraterrestrial life is likely’.
Um, they HAVE told you how they reached the conclusions they reached. You may not agree, and since we don’t know the answer there is no way to determine who is right and who is wrong, but you can’t say no one has told you how they reached the conclusions they reached. There is no way to know a lot of things in science, so extrapolation and theory are used.
We have theories on how life might have originated, and we have experimentation that shows how it might have happened. We have theories and some data on what the conditions may have been like on the early Earth when life first originated (and even theories that life might have come here from somewhere else).
Because based on experimentation and theory into the conditions and possible methods that caused life to originate it seems ridiculously pessimistic. Also, I’ve never seen any scientists posit such a low probability for anything outside of something like magic or leprechauns spontaneously originating somewhere in the universe. Considering that humans are taking the first steps to creating life ourselves, your assertion seems extreme to me.
We have a sample size of one, no doubt. So what? I’ve seen theories that spring up about a single bone from a single species that lived 10’s of millions or 100’s of millions of years ago. That’s what science is all about…making theories and testing them. And this theory (that there is life in the universe) is certainly testable…heck, we are testing for it right here in our own solar system.
I’m not making a declaration…I’m saying that this is why ‘science’ (by which I presume the OP means a large number of interested or involved scientists) believe ‘that extraterrestrial life is likely’. They are making assumptions and extrapolating from data gathered about the Earth, life and the universe in general and they are theorizing that the probability of life in such a vast area with such a vast number of potential bodies and in such vast time frames is ‘likely’. Seems reasonable to me…certainly more reasonable than your ‘probability against it may be greater than the total number of subatomic particles in the universe’ based on what we know.
It’s a testable theory, and one that will probably be tested for as long as there are humans around with the ability and desire to test it. We may never know, considering the distances involved…or, we may find former life on Mars, or life on one of the various moons, which would pretty much move us to ‘what is the probability of INTELLIGENT life in the universe?’…