Ever wonder if aliens have been spying on us? You're not alone. In a provocative new paper, a pair of astrophysicists consider the possibility that intelligent alien beings could have built a fleet of self-replicating space probes that use gravity to sling themselves from star to star. "We're saying that if aliens build self-replicating probes, they could explore the entire Milky Way in about ten million years," study co-author Dr. Duncan Forgan, a postdoctoral fellow at the University of Edinburgh’s Institute for Astronomy, told The Huffington Post in an email. Ten million years is a relatively short timespan, given that scientists estimate the universe is around 13.8 billion years old. via Huffington Post.