Human beings are making it harder for extraterrestials to pick up our broadcasts and make contact, the world's leading expert on the search for alien life warned yesterday.

At a special meeting on the search for extraterrestrial intelligence (Seti), the US astronomer Frank Drake – who has been seeking radio signals from alien civilisations for almost 50 years – told scientists that earthlings were making it less likely they would be heard in space.

Astronomers assumed that a standard technique for any alien intelligence trying to pinpoint other civilisations in the galaxy would involve seeking signals from TV, radio and radar broadcasts, Drake told the meeting at the Royal Society in London.

Scientists on Earth have been using this method, without success so far, to find evidence of intelligent aliens. The theory is that elsewhere in the galaxy other civilisations would probably be doing the same.

An example of this interstellar eavesdropping is dramatised in the Jodie Foster film Contact. Based on a novel by the US astronomer Carl Sagan, it tells the story of an alien civilisation that makes contact after picking up TV broadcasts from Earth.

"The trouble is that we are making ourselves more and more difficult to be heard," said Dr Drake. "We are broadcasting in much more efficient ways today and are making our signals fainter and fainter."

In the past, TV and radio programmes were broadcast from huge ground stations that transmitted signals at thousands of watts. These could be picked up relatively easily across the depths of space, astronomers calculated.

Now, most TV and radio programmes are transmitted from satellites that typically use only 75 watts and have aerials pointing toward Earth, rather than into space.

"For good measure, in America we have switched from analogue to digital broadcasting and you are going to do the same in Britain very soon," Drake added. "When you do that, your transmissions will become four times fainter because digital uses less power."

"Very soon we will become undetectable," he said. In short, in space no one will hear us at all.

What is true for humans would probably also be true for aliens, who may already have moved to much more efficient methods of TV and radio broadcasting. Trying to find ET from their favourite shows was going to be harder than we thought, Drake said.

Most scientists at the meeting said they were sure that life existed on other worlds.

Lord Rees, president of the Royal Society and the astronomer royal, said it should soon be possible to detect planets no bigger than Earth orbiting other stars and determine whether they had continents and oceans.

"Although it is a long shot to be able to learn more about any life on them, then it's tremendous progress to be able to get some sort of image of another planet, rather like an Earth, orbiting another star. And 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."