Stephen Hawking: aliens 'almost certain to exist,' could invade Earth

April 25, 2010 After three years working on a new television series for the, Stephen Hawking concludes that are "almost certain to exist" and could even be dangerous.

Hawking says that it is rational to assume that intelligent life exists somewhere in the, and mathematically unlikely that life is unique to the , given the existence of a hundred billion galaxies, each of them containing hundreds of millions of stars.

Hawking imagines nomadic aliens, having exhausted their home's resources, could attempt to take control of other planets, invading with "massive ships", and draws a comparison with discovering America, resulting in similar devastating consequences. With this in mind he advises "intelligent life might develop into something we wouldn't want to meet." However, he believes that the most likely forms of life would be microbes or simple animals on planets, in the centre of stars or drifting through space.

His declaration comes in the month of the 20th anniversary of the Hubble Space Telescope's launch into space.

The, created in the 1960s, estimates the probability of extraterrestrial civilizations; feeding in modern research gives a high likelihood. The contradiction between this and actual discovery is known as the.

Sir, the Astronomer Royal, supports efforts to search for transmissions that might be artificial in origin. "Even if we couldn't make much sense of it, we'd have learnt that 'intelligence' wasn't unique to the hardware inside human skulls, and had emerged elsewhere," he said.

Into the Universe with Stephen Hawking will begin on May 9 on the Discovery Channel.

Stephen Hawking, 68, retired as Lucasian Professor of Mathematics at the University of Cambridge in 2009. Known for his research into cosmology, quantum gravity and black holes, he became a household name following the publication of his 1988 book A Brief History of Time, which remained on the Sunday Times bestseller list for a record-breaking 237 weeks.