When British cosmologist Stephen Hawking warned against contact with extraterrestrials in a new Discovery Channel documentary, he was repeating a well-worn argument. If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didnt turn out very well for the Native Americans.

But Hawkings reasoning is flawed on a number of counts. First, we can ask why the aliens would come here with guns blazing. What could they possibly want? Hawking suggests that Earths resources...