Life is pretty rare in the Universe Source? I'd be shocked if we didn't find life in the oceans of Jupiter's moons, if we ever bothered to go there.
Direct observation. Anything other than evidence based science is just you daydreaming. Besides, even if life was found elsewhere in this solar system that wouldn't be enough to change the general premise that life is pretty rare in the Universe. When all is said and done, as of 2015, life has only been observed on one planet, in one system, in one galaxy in the entirety of the Universe. You can't even point to evidence of past civilizations. But I guess it's only been 14 billion years give or take. Give them some time. They'll show. In all honesty though, if life was as common as you assert then shouldn't we see colonies and ships by now? It'd only take 1 civilization to decide to colonize our galaxy. Mathematically, they could have done it in 50 million years without FTL travel. So where are they?
Life is rare??? really? based on what?
Direct observation.
"Kill the Wabbit, Kill the Wabbit, Kill the Wabbit!" -- Looney Tunes, "What's Opera Doc?" (1957, Chuck Jones)