New Estimate Calculates There Could be 30 Intelligent Civilizations Communicating Across the Milky Way

Over the years, scientific estimates of potential intelligent life in our galaxy have ranged widely. Some estimates say just one (only us Earthlings) to just a handful, to possibly thousands or even millions. A new study attempts to quantify the number of other worlds we could potentially talk to by estimating the number of intelligent civilizations within the Milky Way that are actively communicating.   

The number?

36, plus or minus a few dozen, depending on various assumptions. And the research team says this number is a lower limit, based on the assumption of how life arose and how long radio communications have been used here on Earth.  

To make their estimates, the research team from the University of Nottingham said they take into account various factors like star formation histories, the distribution of metal-rich stars (like the Sun) and the likelihood of stars hosting Earth-like planets in their habitable zones.

They call their assumptions the “Astrobiological Copernican Limit” and the limit is either weak or strong based on when intelligent life arises on the planet.  

“There should be at least a few dozen active civilizations in our Galaxy under the assumption that it takes 5 billion years for intelligent life to form on other planets, as on Earth,” said Christopher Conselice, Professor of Astrophysics at the University of Nottingham, who led the research. “The idea is looking at evolution, but on a cosmic scale.”

The Weak Astrobiological Copernican scenario is such that a planet forms intelligent life sometime after 5 billion years, but not earlier. The Strong Astrobiological Copernican scenario is where life forms between 4.5 and 5.5 billion years, as it did on Earth. Then the team bases the time of an average communicating civilization is 100 years, since we know that our own civilization has had radio communications for this amount of time and actively sending out signals into space, such as radio transmissions from satellites, and television.

 “The classic method for estimating the number of intelligent civilizations relies on making guesses of values relating to life,” said first author Tom Westby, “whereby opinions about such matters vary quite substantially. Our new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilizations in our Galaxy.”

The Drake Equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester

Astronomer Frank Drake gave us the Drake Equation in 1961 to help us understand the question of how many intelligent civilizations there might be, but that equation was meant to stimulate scientific dialogue, and does not provide numbers for quantifying the number intelligent civilizations. When various astronomers attempt to plug in numbers to the Drake Equation, the numbers have come out either quite optimistic or very pessimistic about having intelligent company in our galaxy.

But even if we do have a large number of talkative neighbors, there are a few caveats that make two-way communication seem unlikely. Other worlds are likely so far away – the Nottingham team estimates the average distance to these radio-active civilizations would be 17,000 light-years away – that detection and communication would be extremely difficult and unlikely, given our current technology.

SETI’s Allen Telescope Array monitor the stars for signs of intelligent life (

The researchers also estimate the likelihood is extremely small that the host stars for communicating, intelligent life are solar-type stars, and most would have to be M dwarfs, which may not be stable enough to host life over long timescales.

It is also possible that we are the only civilization within our Galaxy unless the survival times of civilizations like our own are long.

But like most theoretical research, the team says, it’s the journey, not the destination that counts.

“By searching for extraterrestrial intelligent life — even if we find nothing — we are discovering our own future and fate,” said Conselice.

Further info:
Paper published in The Astrophysical Journal
University of Nottingham press release