AS MANY as 38,000 alien civilisations could exist in our galaxy, all with the potential to make contact with Earth, scientists have claimed.
Scottish researchers have predicted there are likely to be at least 361 inhabited planets in the Milky Way, but say there could be more than 100 times that number.
And they say humans are likely to start detecting alien civilisations within the next few hundred years.
However, rather than resembling little green men or characters from science fiction movies, the aliens could be completely unrecognisable to us, according to the scientists behind the research.
Duncan Forgan, from the University of Edinburgh, who led the study, said: "I don't think it's quite like Star Trek, where they all look like human beings and have pointy ears."
Instead he said they might be so different to us that we may be unable to communicate with them.
And despite having the intelligence to reach out to our planet, they are likely to be so far away it would take them 30,000 years to make contact, he said.
However, he added: "It's still entirely possible we end up getting a TV broadcast from somewhere else."
Mr Forgan said: "Most of the other planets we have looked at are older than our own – so I would expect to see more advanced civilisations than ours existing.
"But that makes us ask why we have not already seen them – although some would say we have."
The scientists built a computer model of the galaxy and used certain criteria to decide whether life was likely on the planets within it.
They came up with a number of likely civilisations that ranged from 361 to 37,964, depending on the criteria used.
One factor taken into account was the distance of a planet from its star. When this is just right for life to be sustained, such as the distance of the Earth from the sun, the planet is said to be within the "Goldilocks zone".
Other criteria included the likelihood of life surviving long enough to develop into an intelligent civilisation, which the researchers defined as creatures that are biologically complex and potentially able to communicate with life on other planets.
However, Professor Keith Horne, an astronomer from the University of St Andrews, said the scientists had underestimated the "uncertainties" involved in that type of prediction.
"It's interesting to speculate but that's what it remains – speculation," he said.
"It's more likely the number is lower rather than higher.
"If they are widespread it's likely they are more advanced than we are and, if so, we would have better evidence for them than we do."
The study, funded by the Science and Technology Facilities Council and Scottish Universities Physics Alliance, is published in the International Journal of Astrobiology.
13.2 billion - Age of the galaxy, in years.
100,000 - The width of the Milky Way, in light years.
400 billion - Estimated number of stars in the Milky Way.
38,000 - Edinburgh scientists' estimate of the number of civilisations in the galaxy
1 - Number of planets where life has been proved to exist – Earth.
0.000003 - Percentage of the Milky Way that is visible to the naked eye from Earth.
250 million billion - Distance you would have to travel from Earth to get to the black hole at the centre of the galaxy, in kilometres.