UFO skeptics say they can't be alien ships because the vast distances of space are just too much.

But a university professor in the US thinks he's solved that problem - they're not from other planets at all. They're piloted by humans, but from the future.

"We know we're here. We know humans exist," professor of biological anthropology at Montana Technological University told space.com last week.

"We know that we've had a long evolutionary history on this planet. And we know our technology is going to be more advanced in the future.

"I think the simplest explanation, innately, is that it is us. I'm just trying to offer what is likely the most parsimonious explanation."