Hawking: Aliens exist, but interaction would be risky

British astrophysicist Stephen Hawking says aliens are out there, but it could be too dangerous for humans to interact with extraterrestrial life.

Hawking claims in a new documentary that intelligent alien lifeforms almost certainly exist, but warns that communicating with them could be "too risky."

The 68-year-old scientist says a visit by extraterrestrials to Earth would be like Christopher Columbus arriving in the Americas, "which didn't turn out very well for the Native Americans."

He speculates most extraterrestrial life will be similar to microbes, or small animals - but adds advanced lifeforms may be "nomads, looking to conquer and colonize."

The Discovery Channel said Sunday it will broadcast "Stephen Hawking's Universe" in Britain next month.