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."
The real challenge is to work out what aliens might actually be like
In the past probes have been sent into space with engravings of human 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.
![Afraid](http://www.digital-kaos.co.uk/forums/images/smilies/afraid.gif)
"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."
THE aliens are out there and Earth had better watch out, at least according to Stephen Hawking. He has suggested that extraterrestrials are almost certain to exist ? but that instead of seeking them out, humanity should be doing all it that can to avoid any contact.
The suggestions come in a new documentary series in which Hawking, one of the world?s leading scientists, will set out his latest thinking on some of the universe?s greatest mysteries.
Alien life, he will suggest, is almost certain to exist in many other parts of the universe: not just in planets, but perhaps in the centre of stars or even floating in interplanetary space.
Hawking?s logic on aliens is, for him, unusually simple. The universe, he points out, has 100 billion galaxies, each containing hundreds of millions of stars. In such a big place, Earth is unlikely to be the only planet where life has evolved.
?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.?
One scene in his documentary for the Discovery Channel shows herds of two-legged herbivores browsing on an alien cliff-face where they are picked off by flying, yellow lizard-like predators. Another shows glowing fluorescent aquatic animals forming vast shoals in the oceans thought to underlie the thick ice coating Europa, one of the moons of Jupiter.
Such scenes are speculative, but Hawking uses them to lead on to a serious point: that a few life forms could be intelligent and pose a threat. Hawking believes that contact with such a species could be devastating for humanity.
He suggests that aliens might simply raid Earth for its resources and then move on:
Hawking?s belief in aliens places him in good scientific company. In his recent Wonders of the Solar System BBC series, Professor Brian Cox backed the idea, too, suggesting Mars, Europa and Titan, a moon of Saturn, as likely places to look.
Similarly, Lord Rees, the astronomer royal, warned in a lecture earlier this year that aliens might prove to be beyond human understanding.
?I suspect there could be life and intelligence out there in forms we can?t conceive,? he said. ?Just as a chimpanzee can?t understand quantum theory, it could be there are aspects of reality that are beyond the capacity of our brains.?
Stephen Hawking's Universe begins on the Discovery Channel on Sunday May 9 at 9pm
source timesonline
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