I disagree with that one. If there really was a nice, juicy habitable planet next door, I doubt there would be much fuss about contamination. Oh sure, there may be some initial desire to preserve the 'pristine' environment for further study, etc, but given the opportunity to exploit a vast new resource with a minimum of trouble, it wouldn't take long before developers moved in.

Only slightly more thorny would be the scenario if the planet had intelligent life, but less technically developed then we are. What would happen? I think it would be inevitable that native species would cease to be, in time. It's not that we would kill them all in some kind of bloodthirsty war, but as our colonies grow and expanded, the natives would slowly retread and decline. It's what happens when an invasive new species takes hold of a new environment, even if it has the best of intentions...