(I am not a scientist) the assumotion is Earth is an average lifebearing plant until proven otherwise. If you find evidence of something, you are more likely to

(I am not a scientist) the assumotion is Earth is an average lifebearing plant until proven otherwise. If you find evidence of something, you are more likely to find it in the middle if the normal distribution. The problem though is choosing which population we are "average" in. Unfortunately we have a single sample for life. Its actually entirely possible we are literally the only intelligent life (or even Earth being the only planet with life) if you define the population as "life capable of wondering if they're alone in the universe". This is the anthropic principle [1] which is an observer bias implies the universe supports observers. Coupled with the fact a spacefaring civilization at 1% the speed of light could colonize the galaxy in about a million years (0.007% age of the universe) this is decently strong statistical evidence intelligent life is exceedingly rare (Fermi paradox). But to answer more directly, Carl Sagan convinced Nasa to look for life on earth. [2] We assume we are typical therefore we try and replicate this with observations of other planets. [1] https://en.m.wikipedia.org/wiki/Anthropic_principle https://en.m.wikipedia.org/wiki/Anthropic_principle [2] https://www.nature.com/articles/d41586-023-03230-z https://www.nature.com/articles/d41586-023-03230-z