If you ask me, focusing on colonizing planets is the completely wrong long term survival strategy anyway. What would be much better would be finding a way to survive in space, just orbiting a star in some kind of craft. I mean, if you can't grow food on it anyway than a planet is just a really big asteroid with an unreasonably large gravity well to deal with. If and when we eventually work out a way to viably travel interstellar distances (my money is on ditching biological bodies), every star would be a viable colonization target.