Why? Because we've gotten to the point that we can detect planets around other stars for a fair distance. AlphaCent is pretty massive (and there's still argument about whether it's POSSIBLE to have habitable worlds in systems like that, I think), so detecting an Earth-sized planet by the wobble or some other means will be difficult, but if they aren't sure now, they will be very soon.
I'm not sure how far out you SHOULD put your first alien species at this point. Actually, it occurs to me there's two separate "how fars" there. The first one is "how far out should it be so that someone reading it X years from now (say, 10) won't immediately be able to say 'bah, we know there aren't any habitable planets around any star THAT close'", and the second is "how far out should it be so that it would be believable we wouldn't KNOW there were any planets there by X years from now"; that is, if your story's set 35 years from today, how far out will we have expanded our search to the point that we can be sure there aren't any habitable worlds around any of the stars (or, alternatively, how far out will we be able to determine this, assuming that any selection of destination would have been studied?)
For *my* purposes, "habitable" is basically "appears to be somewhere human beings could live roughly as easily as we can in Antarctica or better", not "human beings can live there under domes, and maybe one day could terraform it to something as nice as Antarctica, if they could move under the gravity and survive the radiation, which they can't".