Life on other planets likely common; intelligence, probably not

To survive, we will have to boldly go where no one has gone before, says Professor Stephen Hawking.

Astrophysicist Stephen Hawking has been thinking a lot about the cosmic question, "Are we alone?" The answer is probably not, he says.

If there is life elsewhere in the universe, Hawking asks why has humanity not stumbled onto some alien broadcasts in space, maybe something like "alien quiz shows?"

Hawking's comments were part of a lecture at George Washington University on Monday in honour of Nasa's 50th anniversary. He theorised about possible answers to whether extraterrestrial life exists.

One option is that there likely is no life elsewhere. Or maybe there is intelligent life elsewhere, but when it gets smart enough to send signals into space, it also is smart enough to make destructive nuclear weapons.

Hawking said he prefers the third option: "Primitive life is very common and intelligent life is fairly rare," he then quickly added: "Some would say it has yet to occur on earth."

So should people worry about aliens? Alien abduction claims come from "weirdos" and are unlikely. However, because alien life might not have DNA like earthlings, Hawking warned: "Watch out if you would meet an alien. You could be infected with a disease with which you have no resistance."

The 66-year-old British cosmologist, who suffers from amyotrophic lateral sclerosis, or ALS, and must speak through a mechanical device, believes "if the human race is to continue for another million years, we will have to boldly go where no one has gone before."

Hawking compared people who do not want to spend money on human space exploration to those who opposed the journey of Christopher Columbus in 1492.

"The discovery of the New World made a profound difference to the old. Just think, we wouldn't have had a Big Mac or KFC."