Roger, I'm not gonna deliver the whole panoply of biochemistry, planetology and evolutionary science that supports the statistic. I can only say that it's founded enough, and is largely constructed on the scientific fact that Earth can NOT be the only living planet in the universe. Yet this is very conservatively applied within the statistical calculation, as far as space technology is concerned. We're all being careful here.
Edgar, your
resembles Enrico Fermi's paradox "If they exist, then where are they?"
The study of ufology is layered with solid evidence, backed up by the most credible witnesses, that several ufos can not be terrestrial, hence are extraterrestrial. Which solves the Fermi paradox.
It is rather our lack of universality that keeps us numb in this topic, not the lack of proof. It's advisable to do some
research of your own, instead of bootstrapping opinions out of uninformed ignorance. This topic slightly outranks your regular chat session.