strange story.

[no one]

I have a better theory: nobody can live long enough or have a powerful engine to make it that far. So think about the migration. They can move from planet to planet, their ships are big enough to hold their planet's population and it can move with some thing like 2 time faster than sound speed. It can use energy from some mineral which not very rare in space but we haven't notice yet to move on. So they are like locus, move from planet to planet, and after centuries, they finally reach earth.