By Pallab Ghosh
Science correspondent, BBC News
The possibility of alien life has provoked excitement for centuries
The chance of discovering life on other worlds is greater than ever, according to Britain's leading astronomer.
Lord Rees, the president of the Royal Society and Astronomer Royal, said such a discovery would be a moment which would change humanity.
It would change our view of ourselves and our place in the cosmos, he said.
His comments come as scientists gather in London for an international conference to discuss the prospect of discovering extra-terrestrial life.
Scientists have been scanning the skies for radio broadcasts from intelligent life for 50 years, and so far they have only heard static.
But the chances of discovering life now were better than ever, Lord Rees said.
He said: "Technology has advanced so that for the very first time we can actually have the realistic hope of detecting planets no bigger than the earth orbiting other stars.
"(We'll be able to learn) whether they have continents and oceans, learning what type of atmosphere they have.
"Although it is a long shot to be able to learn more about any life of them, then it's tremendous progress to be able to get some sort of image of another planet, rather like the earth orbiting another star."
The recent deployment of space telescopes capable of detecting earth-like planets around distant stars now make it possible to focus the search.
"Were we to find life, even the simplest life, elsewhere that would clearly be one of the great discoveries of the 21st Century.
"I suspect there could be life and intelligence out there in forms that we can't conceive.
"And there could, of course, be forms of intelligence beyond human capacity, beyond as much as we are beyond a chimpanzee," he added.