I think it's extremely unlikely to be aliens, since each time in the past something like this has come up a non-aliens explanation has been found. That said, I

I think it's extremely unlikely to be aliens, since each time in the past something like this has come up a non-aliens explanation has been found. That said, I don't think distances/timescales is a great argument against aliens, since it makes a lot of assumptions. They could use robotic ships, or generation ships (no telling how long their generations might last.) If they're moving at a significant fraction of the speed of light, relativistic effects could be dramatically reducing their perceived mission duration. Maybe the biology of their species is such that cloning or suspended animation is straight forward.