The Milky Way Contains 36 Contactable Alien Civilizations, Scientists Estimate

Scientists estimated how many alien societies in the Milky Way can send interstellar messages. But just because they can talk does not mean we will hear them.
DSN Radio Telescope. Image: NASA​
DSN Radio Telescope. Image: NASA
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For hundreds of millions of years, Earth has nurtured a spectacular diversity of lifeforms, including humans, the only species known to develop advanced technologies. So, what’s the rest of the galaxy’s excuse? Are there intelligent alien civilizations out there in the Milky Way, and if so, how many?

The answer to that second question is 36, more or less, according to a study published on Monday in The Astrophysical Journal. This is only a statistical estimate, not an announcement that we have stumbled across three dozen civilizations in the galaxy, so there’s no need to pledge allegiance to any alien overlords yet.


But though its conclusions are speculative, the study incorporates new metrics and approaches in approximating how many alien societies within the Milky Way are capable of interstellar messaging (a group known as Communicating Extraterrestrial Intelligent civilizations, or CETI).

“One of the oldest questions that humans have asked is whether our existence—as an advanced intelligent species—is unique,” said authors Tom Westby and Christopher Conselice, who are astrophysicists at the University of Nottingham, in the study.

“Of course—from a statistical perspective—this is one of the most challenging problems in science, since all we can do is attempt to learn from a single known data point (ourselves), with no possible method of modelling the distribution of the potential population of civilizations across the Galaxy,” the team noted.

Westby and Conselice are far from the first scientists who have taken a shot at this challenging question by constraining the possible number of CETI worlds in the Milky Way. This tradition dates back to the Drake equation, pioneered by renowned astronomer Frank Drake in 1961. Drake’s probabilistic thought experiment outlines the conditions that might influence the galactic population of intelligent aliens, and factors in variables such as the galaxy’s star formation rate and the projected lifespan of a technologically advanced civilization.

Westby and Conselice present a revision of the Drake equation that loops in new findings from “a mixture of areas of contemporary astronomy,” according to the study. For instance, thousands of exoplanets have been detected in alien star systems over the past two decades, so Westby and Conselice included data about the odds that worlds orbit their stars within the habitable zone where liquid water can exist. The team also focused on the timescale of intelligent life’s emergence on Earth, a process that took about 4.5 billion years.


The pair’s results produced a range of possible CETI populations that currently exist in the Milky Way, with four at the low end, 211 as an upper limit, and 36 as the most likely figure based on the team’s assumptions.

These high numbers may sound like great news for alien enthusiasts, but Westby and Conselice caution that even if their estimate is correct, CETI worlds may be too far away from Earth to establish communication. If 36 contactable civilizations were scattered throughout the galaxy, they would be about 17,000 light years away from our planet on average, a distance that would require at least 34,000 years for a two-way conversation.

Some of these speculative civilizations may randomly end up closer to Earth, in which case it could be more feasible to strike up an interstellar chat. However, even if aliens were only 1,000 light years away, we would have to make sure that our own civilization survives another 2,000 years if we hope to exchange messages with it.

“If the average lifetime of civilizations is in fact less than 1,030 years, then their average separation becomes too great to allow any communication between neighbors before the species becomes extinct,” Westby and Conselice, citing their calculations.

“The lifetime of civilizations in our Galaxy is a big unknown within this and is by far the most important factor in the CETI equation we develop, as it was for the Drake equation,” they concluded.

In other words, if we humans truly hope to touch base with aliens some day, we should be as dedicated to maintaining Earth’s habitability for future generations as we are to seeking other inhabited worlds in the Milky Way.