Aliens are out there, but they're unlikely to be friendly, says Stephen Hawking

• Alien life hopes "perfectly rational"
• Roaming universe in giant ships
• Likely to "conquer, colonise"

BRITISH astrophysicist Stephen Hawking says aliens are out there, but it could be too dangerous for humans to interact with extraterrestrial life.

Professor Hawking claims in a new documentary that intelligent alien lifeforms almost certainly exist, but warns that communicating with them could be "too risky".

With the universe made up of some 100 billion galaxies each containing hundreds of millions of stars, it was unlikely the Earth was the only place where life has evolved.

"To my mathematical brain, the numbers alone make thinking about aliens perfectly rational," he said.

"The real challenge is to work out what aliens might actually be like."

The 68-year-old scientist says a visit by extraterrestrials to Earth would be like Christopher Columbus arriving in the Americas, "which didn't turn out very well for the Native Americans".

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

Prof Hawking - who has spent much of his life in a wheelchair suffering from motor neurone disease and talking via a speech synthesiser - said he imagined the aliens were most likely roaming space in giant ships, having exhausted their planets of their resources.

He speculates most extraterrestrial life will be similar to microbes, or small animals, then added that advanced lifeforms may be "nomads, looking to conquer and colonise".

The Discovery Channel will broadcast Stephen Hawking's Universe in Britain next month.