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Renowned physicist Stephen Hawking has repeated his long-held belief that intelligent aliens are likely to exist, and that a visit by them to present-day humanity would probably have unfortunate consequences for us.

Publicising a new documentary he has made for the Discovery Channel, the legendary boffin told the Times at the weekend:

The 68-year-old, who is paralysed by motor neurone disease, has been working on his documentary for the last three years, reportedly exercising firm editorial control. The Times reports that the show will feature speculative scenes of alien life in extraterrestrial habitats, including some - for instance the possible hot oceans of Europa, moon of Jupiter - in our own solar system.

Logically enough, Hawking reportedly considers that most alien life would be on the same general order as the life which has existed on Earth for almost all the time it has been a living planet - microbes or simple animal forms.

But with a hundred billion galaxies each with potentially a hundred billion stars, Hawking considers that intelligent life is entirely possible - even, perhaps, intelligent life with technology so advanced as to be able to travel across interstellar distances. He considers it an extremely unwise move for primitive present-day humanity to attract the attention of such voyagers - by such efforts as the controversial practice of beaming out "Active SETI" signals, for instance.

“We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet," he argues. "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.”

The idea of wandering alien raiders in massive starships - possibly "generation ships" making centuries-long hops from star to star at small fractions of the speed of light - is a common science-fictional one. Under some scenarios it might be possible for the ship and its technology to be primitive enough that the aliens might have a need for "resources" present on Earth - enriched uranium, perhaps, though there would probably be many easier ways of obtaining this than making an interstellar voyage.

Alternatively, in the case of aliens having originally evolved on an Earthlike world, it might simply be their goal to seize and colonise ours, as Prof Hawking says - regardless of the status of their homeworld, they might like to have another. The mere fact of their being able to get here across interstellar distances would tend to suggest they might be technically capable of overcoming humanity and exterminating or enslaving us - or confining limited numbers of us to reservations, if they were relatively kind aliens.

Given that those parts of humanity with resources to spare show no serious interest in developing space travel capabilities of our own*, it would seem that Prof Hawking is right and we'd do well to keep our heads down lest we draw the attention of some race a bit more interested in the universe around it.

Professor Hawking's views on aliens aren't new - he expressed much the same ideas to your correspondent during a discussion at the Cambridge Union, 20 years ago. The actual news today is the forthcoming documentary.

Stephen Hawking's Universe begins on the Discovery Channel on Sunday 9 May at 9pm. ®

*NASA, the best funded space agency of the human race, boasts a budget of less than $20bn - a small fraction of a single percentage point of US government spending, and a tinier proportion yet of US gross domestic product.

What you need to know about cloud backup