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."
In the past, probes have been sent into space with engravings of human beings on board and diagrams showing the location of our planet.
Radio beams have been fired into space in the hope of reaching alien civilisations.
Prof Hawking said: "To my mathematical brain, the numbers alone make thinking about aliens perfectly rational.
"The real challenge is to work out what aliens might actually be like."
The programme envisages numerous alien species including two-legged herbivores and yellow, lizard-like predators.
But Prof Hawking conceded most life elsewhere in the universe is likely to consist of simple microbes.
In the recent BBC series Wonders of the Solar System, Professor Brian Cox, a physicist from the University of Manchester, also suggested life may exist elsewhere within our solar system.
He said organisms could be present under the ice sheet that envelops Europa, one of Jupiter's moons.
Professor Cox added: "Closer to home, the evidence that life could exist on Mars is growing.
"We will only know for sure when the next generation of spacecraft, fine-tuned to search for life, are launched to the moons of Jupiter and the arid plains of Mars in the coming decades."