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A dollars 100m project to discover whether intelligent life exists on other planets will fail unless extraterrestrials are looking for us, an astronomer involved in the search said yesterday, writes Steve Connor.

The vast distances of space make it extremely unlikely that any extraterrestrial signals would reach us unless they were emitted from a giant radio-telescope, said Jill Tarter, a project scientist at the US space agency, Nasa.

'They would have to have invented radio-telescopes or we wouldn't be able to find them,' she said at the National Astronomy meeting at Leicester University.

The search for extraterrestrial intelligence, which began in earnest last October, is using some of the world's largest radio-telescopes to pan the sky for unnatural radio signals. Terrestrial signs of intelligent life - such as BBC broadcasts - are not powerful enough to penetrate deep space. Only signals from radio-telescopes can be detected halfway to the centre of our galaxy, Dr Tarter said.

The sign of an extraterrestrial is likely to be a continuous monotone in a frequency range of one to three gigahertz, which does not exist in nature. 'If we find it we'll either have found something produced by technology or a whole new branch of astrophysics,' she said.

The search is deliberately geared towards those frequencies 'where nature is relatively quiet' because of the risk of false alarms, she said. One potential source of false signals is terrestrial broadcasts bouncing off the Moon and back to Earth.

One false alarm was raised after astronomers working in France excitedly reported unnatural signals from a mysterious extraterrestrial source. Subsequent analysis showed it came from Paris airport.