If human beings do manage to develop practical interstellar travel, it occurred to me that the ethical considerations of colonizing other planets are not completely dissimilar to those of colonizing other continents. I think that most of us will probably agree that invading and colonizing a planet already inhabited by intelligent life would be unethical. However, I don't see a ton of conversation space generally given to the ethics of colonizing a planet where there is life, but none that we recognize as "intelligent". This raises multiple questions, including how we define "intelligence", where we draw the line for ownership purposes, and also, even in the definite absence of intelligent life, is it ethical to colonize a pristine, unexploited ecosystem?
Further, why do we seem to assume that we have some sort of natural RIGHT to colonize other planets?