The trouble with counting aliens

Enlarge / The moon is seen behind an antenna on the site of the radiotelescope of Nancay on October 03, 2019, near Vierzon, Central France. (credit: Guillaume Souvant | Getty Images)

In the search for extraterrestrial intelligence, there’s really only one big question: Where is everybody? This question has haunted alien hunters ever since the Nobel-winning physicist Enrico Fermi posed it to some colleagues over lunch 70 years ago. There are billions of sun-like stars in our galaxy, and we now know that most of them host planets. But after decades of searching, astronomers haven’t found any that appear to host life. This is the so-called Fermi paradox: Our galaxy seems like it should be teeming with alien civilizations, but we can’t find a single one.

Researchers working on the search for extraterrestrial intelligence, or SETI, have proposed a number of solutions to the Fermi paradox over the years. But the most persuasive answer is also the most obvious: Perhaps intelligent life is just far more rare than we thought.

How rare? Many scientists have attempted to answer this notoriously tricky question. Based on their conclusions, there are between zero and 100 million extraterrestrial civilizations in the Milky Way. That is not an especially helpful range of estimates, so a pair of physicists in the UK recently took another stab at it and arrived at a remarkably specific conclusion. As detailed in a new paper published this week in the Astrophysical Journal, the duo calculated there should be at least 36 communicating extraterrestrial civilizations in our galaxy.