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# 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.

By
Jason Marshall, PhD
Episode #295

## Planetary Probabilities

How about potential life-hosting planets? In our solar system, there are give-or-take 10 worlds that might possibly support life. Over the last decade, astronomers have found that planets are common around nearby stars. So if we assume 10 worlds per star system is typical, then there may be up to 500 billion potential life-hosting planets in our galaxy.

There may be up to 500 billion potential life-hosting planets in our galaxy.

Thus far we’ve confirmed that 1/10 of the worlds in our solar system have actually given rise to life (that’d be us). If that number is typical, then we would estimate that there are 1/10 • 500 billion = 50 billion planets with some sort of life (not necessarily intelligent) in the galaxy. On the other hand, if whatever happened in our solar system 4-ish billion years ago to give rise to life was an incredibly improbably event, then perhaps we are entirely alone in the galaxy … and maybe even the universe.

So this bit of probabilistic thinking puts the number of planets with life in our galaxy somewhere between 1 (so just us) and all the way up to 50 billion—which, obviously, is quite a range. Can we pin this number down a bit better? Scientists are working on it. And given that life seems to have started quickly on Earth (perhaps implying life is a universal phenomena when the conditions are right), and that life on Earth is surprisingly hardy (it shows up in the most unlikely and challenging places), the scales might be tilted towards the larger number. Maybe.

## Alien Probabilities and the Drake Equation

For fun, let’s go with the upper end of the range we just estimated and assume that there are billions of independent strains of life in our galaxy. Doing so gives us the freedom to contemplate a perhaps even more intriguing question: How many of these strains of life are intelligent? And perhaps even technologically advanced?

In truth, there’s just no way for us to pinpoint precise answers questions like these—there’s still way too much we don’t know. But that doesn’t mean we can’t ponder them. And that’s exactly what an astronomer named Frank Drake did in 1961 when he introduced what’s now called the Drake equation to the world.

This equation is a wonderful example of the type of back-of-the-envelope calculation we’ve talked about before. Although the details are a bit involved, the basic idea is fairly simple:

• If we multiply the number of stars in our galaxy that join the ranks of stars capable of giving rise to life each year by the fraction of those stars that have planets around them, and then we multiply this number by the number of planets per star that are actually capable of giving rise to this life, then we obtain an estimate of the number of planets in our galaxy that become capable of hosting life each year.
• If we then multiply this number by the fraction of the planets on which life isn't just theoretically possible but actually does emerge, and then we multiply this by the fraction of these planets on which not just any life but intelligent life emerges, and then finally multiply this by the fraction of these planets on which technologically advanced life emerges, then we obtain an estimate of the number of technologically advanced civilizations that come into being in our galaxy each year.
• Finally, if we want to know the number of technologically advanced civilizations that exist in our galaxy right now, we just need to multiply the number we just obtained by the average number of years that such a civilization exists before it goes extinct (either from an outside influence or due to its own actions).

Sounds like a piece of cake to figure out, right? OK, perhaps not.