There's a whole field of science, known as Seti (The Search for Extra-Terrestrial Intelligence) dedicated to listening for signals from intelligent beings elsewhere in the Universe. But Hawking cautioned against trying to actively hail any alien civilisations that might be out there.
In 2010, he told the Discovery Channel that aliens might simply raid Earth for resources and 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.
"We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."
