# Milky Way Could Host Over 42,000 Alien Civilizations, Scientists Estimate

Scientists have used probability simulations to come to an estimate on the number of alien civilizations that might exist in our Milky Way galaxy, and the number they've arrived at is pretty specific: 42,777, give or take a few hundred.

The value was calculated by researchers Wenjie Song and He Gao from the Department of Astronomy at Beijing Normal University in China, who used something called a Monte Carlo simulation —a mathematical technique used to estimate possible outcomes of an uncertain event—to come to their conclusion.

Scientists have attempted to calculate the number of alien civilizations that might exist in our galaxy before. Most notable among these was Frank Drake, the astronomer who lends his name to the Drake equation that attempts to do this using factors such as average rate of star formation, the fraction of stars with planetary systems, the fraction of such planets which could host life, and so on.

The problem with the Drake equation and other calculations to estimate the likelihood of alien life is that there are so many uncertainties in the above factors. Thus, models are based on reasonable assumptions that might, or might not, produce plausible predictions about alien life.

The Drake equation goes back to the 1960s. Since then we have learned a great deal more about the cosmos, including the observation of thousands of exoplanets—planets outside of the solar system.

Using up-to-date information, Song and Gao simulated star formation in our galaxy taking into account metallicity and mass, the probability of terrestrial planets forming in stars' habitable zones, and the probability of life emerging and eventually becoming advanced enough to communicate with Earth.

Crucially, the astronomers also took into account the time scales involved in order to come to a conclusion on how long we as a species would need to survive in order to discover alien signals.

Their work yielded two main results: an optimistic one and a pessimistic one. In the optimistic situation, the researchers suggested the aforementioned 42,777 communicating extraterrestrial intelligent civilizations (CETIs) with an error margin of plus 267 and minus 369, and they would need to survive 2,000 years on average to communicate with us.

In the pessimistic situation, the researchers said there could be as few as 111 CETIS and that they would need to survive for almost 1 million years on average to communicate with us.

Song and Gao say their work may shed some light on the Fermi paradox, which asks why humans have not received any signals from alien life despite estimates for their existence.

"The reason why we have not received a signal may be that the communication lifetime of humans is not long enough at present," the study reads. "However, it has been proposed that the lifetime of civilizations is very likely self-limiting due to many potential disruptions, such as population issues, nuclear annihilation, sudden climate change, rogue comets, ecological changes, etc.

"According to our simulations, for the tail value of some optimistic situations, human beings still have hope of detecting a CETI signal."

The study was published in The Astrophysical Journal on April 1 this year.