So here's a question. Let's say we find a planet that has its own carbon-based life. Even if it was within the habitable range for humans, could humans ever live on that planet? Or would the threat of microbial nasties that we had no immunity to make that possible?
Or would our presence inevitably disrupt the existing ecosystem there? Would it be possible for us to live there without bringing our own microbial nasties?
I've never really thought about that--I've always assumed that landing on an "M-class" planet wouldn't quite as cavalier as Star Trek, but that humans would more or less be able to survive there. Is this a foolish assumption?