Extra-terrestrial aliens may not make contact with the Earth for another 1,500 years, according to a team of astronomers.
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 that while there was no reason to think we are alone in the universe, getting in contact may take some time.
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.
A number of logical reasons for the lack of contact have been suggested. These include the possibility that intelligent extra-terrestrial 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 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," Solomonides pointed out. "We simply claim that it is somewhat unlikely that we will not hear anything before that time."
But other astronomers have cautioned against the Earth taking such a cavalier approach to getting in contact with aliens.
Several prominent scientists, including Stephen Hawking, have cautioned in the past against humanity broadcasting its presence to intelligent life on other planets as aliens could then visit the planet with malicious intent.
In March, a pair of New York astronomers suggested that powerful lasers could be pointed at a star where aliens might live at the exact time of the year that the Earth disrupts the light passage from our Sun in order to prevent aliens from discovering Earth.