What Is the Probability that Alien Life Exists?

Are we alone in the universe? If so, why? If not, where is everybody? Thankfully, math can help us with these astronomically profound questions. Keep on reading to learn all about the probability of extraterrestrial life.

Jason Marshall, PhD
6-minute read
Episode #295

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In 1967, a graduate student named Jocelyn Bell discovered something strange emanating from a region of the sky known as the Summer Triangle: pulses of radio waves repeating every 1.3373 seconds over and over again like a clock ticking slightly-slowly. 

It’s not every day that you find a nearly perfect clock ticking at you from the sky, so Bell and her colleagues half-jokingly called the source of the signal “LGM1”—the “LGM” being short for “Little Green Men.” We now know that this signal wasn’t from little green anythings, but was instead from a previously undiscovered type of object known as a pulsar. This was exciting, although perhaps not quite as exciting as finding aliens.

A decade later, an even stranger signal fell upon a different radio telescope. This signal was unusual in many ways: it was very strong, it didn’t look like any known naturally occurring radio signal, and it seemed to be coming from “out there” (and not here on Earth). The signal was so striking that its discoverer famously wrote the word “Wow!” on the margin of a printout of the data. After ruling out obvious Earthly origins, most astronomers are convinced that the signal came from somewhere beyond Earth. The question—which still remains unanswered—is where? And from what or whom? Was the signal produced by a natural phenomena? Or might it have been broadcast by some alien intelligence?

The answer to the last question is: Probably not; but maybe. Although whether or not the “Wow!” signal was alien in origin, many scientists do think that alien life is likely to be out there. And they kind of have some math to back them up. Why only kind of? Because as we’re about to find out, at this point questions about extraterrestrial life lead to back-of-the-envelope calculations and a hefty dose of probabilities—and those probabilities are anything but certain.

Stellar Statistics

So what exactly does math say about the odds that we share the universe with other intelligent life? Let’s begin with the statistics of stars and planets. All life on Earth originated on a planet, and scientists think that most other independent origins of life (if they exist) probably did the same. For various reasons, I’ll let Ask Science explain someday, scientists think that life usually gets going on planets orbiting not-too-big-and-not-too-small stars similar to the Sun.

Astronomers estimate that there are roughly 100 billion of these Sun-like Goldilocks stars in our galaxy. Something like half of these live their lives orbiting one or more other stars, which makes it far less likely that they’ll have planets around them (since those planets get flung out of the system by the stars). This cuts the number of hospitable stars in half, but this is still roughly 50 billion potential life-hosting stars in our galaxy!


About the Author

Jason Marshall, PhD

Jason Marshall is the author of The Math Dude's Quick and Dirty Guide to Algebra. He provides clear explanations of math terms and principles, and his simple tricks for solving basic algebra problems will have even the most math-phobic person looking forward to working out whatever math problem comes their way.