Stephen Hawking thinks alien life is "mathematically probable" in his new Discovery Channel documentary.
However, his speculation on how contact could pan out is interesting.
From the London Times:
"He suggests that aliens might simply raid Earth for its resources and then move on: 'We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach.'
He concludes that trying to make contact with alien races is 'a little too risky'. He said: 'If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.'"
Well put, Hawking.