Aliens almost certainly exist but humans should avoid making contact, Professor Stephen Hawking has warned.
In a series for the Discovery Channel the renowned astrophysicist said it was "perfectly rational" to assume intelligent life exists elsewhere.
But he warned that aliens might simply raid Earth for resources, then move on.
"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said.
Prof Hawking thinks that, rather than actively trying to communicate with extra-terrestrials, humans should do everything possible to avoid contact.
He explained: "We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."
War is a biological necessity of the first importance, a regulative element in the life of mankind which cannot be dispensed with, since without it an unhealthy development will follow, which excludes every advancement of the race, and therefore all real civilization.-Friedrich von Bernhardi
What can be said for a man who would allow his home to be invaded by strangers who demanded they be fed, clothed, housed and granted the rights of the first-born? What can be said for a ruling elite that permits this to be done to the nation, and who celebrate it as a milestone of moral progress?-Pat Buchanan