Hawking, arguably the most famous living scientist, speculates that trying to interact directly with an ET might pose a huge risk for earthlings in a new Discovery Channel documentary, "Into the Universe With Stephen Hawking."
"To my mathematical brain, the numbers alone make thinking about aliens perfectly rational," he says. "The real challenge is to work out what aliens might actually be like."
But other scientists who are open to the idea of life existing on other planets take a different approach.
"Hawking is concerned about the possibility of betraying our presence, that, if they were to come here, it might not be good for us," says Seth Shostak, senior astronomer at the SETI Institute in Mountain View, Calif.
"It's hard for me to believe that they actually have [hostile] intentions on us."
Shostak likens the search for aliens to listening to a radio station: "When you tune in your favorite DJ, he doesn't know you've tuned him in -- so, there's no danger in SETI," or the search for extraterrestrial intelligence.
In the four-part series, Hawking suggests that most alien life might be simple microbes or animals, but there could also be intelligent ETs that may set their sights on Earth for their own purposes. "I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced ... nomads [would look to] conquer and colonize whatever planets they can reach."
Putting this series together was a daunting task, John Smithson, one of the executive producers, told AOL News from his London office. "It was pretty formidable because you're dealing with one of the most brilliant people in the world, and you're dealing with some of the most mind-numbingly complex things imaginable," he says.
"There's so much we know about the universe and there's so much we don't know. That's why it's good there can be vigorous debate, because we simply don't know the answers to many of these questions," Smithson says.
The possibility of life outside of our earthly confines has increased in recent years with the discovery of more than 450 planets in orbit around faraway stars.
It wouldn't be a real stretch of science or the imagination to consider that life may be abundant in the universe. After all, we now know there are billions of galaxies out there in the cosmos, with each galaxy made up of hundreds of millions of suns.
Hawking doesn't dismiss the importance of earthlings venturing out into space, Smithson says. "He's a very philosophical guy. I know he is totally for the continuation of space exploration. He believes it's absolutely vital for the human race to keep on exploring our universe."
And yet, Hawking is still pessimistic about trying to make friends with an alien species. "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."