Intelligent alien civilisations could number 36, new research from Nottingham University has found.
Although there is no evidence for intergalactic life which could communicate with humans, the calculation has put a number on how many different types scientists should be looking for.
Scientists also added that we would "not be surprised" by the alien life, and that it would likely be similar to humanity.
Researchers said the average distance to these civilizations would be 17,000 light-years, which with our technology would make the chances of encountering such life very slim. They worked out that humanity would likely have to survive another 6,120 years for two-way communication.
The paper, published in the Astrophysics Journal, is based on the assumption that intelligent life would evolve on other planets in the same way that it has done on Earth.
They used a theory called the Astrobiological Copernican Limit, which is when scientists apply evolution on a cosmic scale.
Their calculation is a refinement of one first thought up in 1961 by Francis Drake, who set seven criteria for alien life, ranging from number of stars in the galaxy to the presence of detectable electronic signals. However, estimates using this equation are far-ranging, from zero to many billions.
There are two Astrobiological Copernican Limits, which dictate that intelligent life forms in less than five billion years, or after about five billion years, based on the fact that on Earth a communicating civilisation formed after 4.5 billion years. Intelligent life would only form in this time for planets which are, like Earth, within a star's habitable zone and have the right distribution of elements.
First author of the paper Tom Westby said: “In the strong criteria, whereby a metal content equal to that of the Sun is needed (the Sun is relatively speaking quite metal rich), we calculate that there should be around 36 active civilizations in our Galaxy.”
Scientists estimated the number of Communicating Extra-Terrestrial Intelligent civilisations (CETI), which are civilisations similar to those on Earth, which broadcast radio signals into space, by using these limits.
Professor of Astrophysics at the University of Nottingham, Christopher Conselice led the research.
He said: “There should be at least a few dozen active civilisations in our Galaxy under the assumption that it takes five billion years for intelligent life to form on other planets, as on Earth.”
Another factor in the equation is how long the technology used by intelligent civilisations, such as televisions and satellites, broadcasts signals into space. Our technological civilisation is 100 years old, and if others are of a similar age then then there will be about 36 ongoing intelligent technical civilisations throughout our galaxy.
This is the lower limit, scientists said, meaning that if it is likely that our technological civilisation lasts far longer than 100 years, there are likely to be many alien communities out in space.
If all the civilisations were found, the scientists said, they could work out how long humanity is likely to survive.
The researchers said that if it was found that intelligent life was common, and there were many dozens of civilisations, we could expect humanity's technological civilisation to last a few hundred years. If it is not, then that does not bode well for the longevity of the human race.
Professor Christopher Conselice explained: “By searching for extraterrestrial intelligent life - even if we find nothing - we are discovering our own future and fate.”