It might take another 1,500 years before extraterrestrial aliens make contact with the human race, a team of astronomers has predicted.

The scientists performed a calculation based on the likelihood of technological civilisations arising among the stars, and the probable length of time they had been transmitting signals across space.

They concluded there was no reason to think we are alone in the universe. We just need to exercise a little patience.

US researcher Evan Solomonides, a PhD student from Cornell University, said: "We haven't heard from aliens yet, as space is a big place - but that doesn't mean no one is out there.

"It's possible to hear any time at all, but it becomes likely we will have heard around 1,500 years from now.

"Until then, it is possible that we appear to be alone - even if we are not. But if we stop listening or looking, we may miss the signals. So we should keep looking."

Astronomers have long pondered why, given the number of stars in our galaxy, the Milky Way, there has been such a deafening silence from the cosmos, despite attempts by Seti (Search for Extraterrestrial Intelligence) scientists to eavesdrop on alien signals.

There are more than 200 billion stars in the Milky Way, almost half of which may host Earth-like planets, according to one estimate.

If life is not unique to the Earth and has evolved in a similar way elsewhere, the galaxy should be teeming with civilisations. Yet not one confirmed alien signal has ever been detected.

This puzzle was highlighted in 1950 by Italian physicist Enrico Fermi in what has become known as the "Fermi Paradox".

A number of logical reasons for the lack of contact have been suggested. These include the possibility that intelligent extraterrestrial life is very rare or non-existent, that all intelligent civilisations eventually destroy themselves before reaching out to the stars, that the aliens have decided inter-species communication is too dangerous, or that they are observing us from afar as part of a "hands off" experiment.

The new research suggests there is nothing contradictory about "Fermi's Paradox" and we should not have expected to make contact so soon.

The scientists' starting point is the fact that humans have only been broadcasting TV and radio signals into space for around 80 years. By now these signals should have reached more than 8,500 stars within 80 light years of the sun.

Although this seems a large figure, it is tiny compared with the number of stars in the Milky Way.

Adopting the so-called "Mediocrity Principle" that says there is nothing at all special about the Earth or its occupants, the team ruled out the likelihood of humans being among the first or last civilisations to develop radio technology.

As a result, the average length of broadcasting history in the galaxy was found to be the same as ours, roughly 80 years.

This figure was included in an equation that also measured the frequency of life arising on extrasolar planets.

The conclusion was that we could expect to start hearing from alien civilisations once their signals had spread across half the Milky Way.

And that could take another 1,500 years, said the scientists, who are to present their findings at the American Astronomical Society's annual meeting in San Diego, in the US.

"This is not to say that we must be reached by then or else we are, in fact, alone," Mr Solomonides pointed out. "We simply claim that it is somewhat unlikely that we will not hear anything before that time."

He added: "We are on the third planet around a tediously boring star surrounded by other completely normal stars about two-thirds of the way along one of several arms of a remarkably average spiral galaxy.

"The Mediocrity Principle is the idea that because we are not in any special location in the universe, we should not be anything special in the universe."