Astrophysicist Stephen Hawking thinks there are aliens out there. He also thinks it could be too dangerous for humans to interact with extraterrestrial life.

He says it's almost certain that intelligent alien lifeforms exist, but he warns against communicating with them because it could be "too risky."

68-year-Hawking thinks that If extraterrestrials visited Earth, it would be like Christopher Columbus arriving in the Americas, which he says "didn't turn out very well for the Native Americans."

He thinks most extraterrestrial life is similar to microbes or small animals but that advanced lifeforms might be "nomads, looking to conquer and colonize."

Hawking's comments were part of a new Discovery Channel documentary called "Stephen Hawking's Universe".

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