On Aliens and Probability

Probability of Alien Life

Billions of Worlds

I call this the Big Universe argument: the Universe is so mind-destroyingly huge that there just has to be alien life out there somewhere. There just has to. Unfortunately this is wishful thinking. In science we must be careful to distinguish between wishful thinking and actual evidence. Read on before you send me an angry comment about what a buzzkill I am.

If we’re discussing alien life, a reasonable place to start is with an American astronomer named Frank Drake. In 1961 Drake created a now-famous equation. The Drake equation was unusual in that it wasn’t intended to give a definite solution, but to stimulate discussion about the possibility of intelligent civilizations on other worlds. It worked beautifully.

Drake Equation

In this formula, N represents the number of extraterrestrial civilizations in our galaxy with which radio communication might be possible. The rest of the factors are based on assumptions, and they become increasingly harder to estimate as you advance to the right side of the equation.

R* is the average rate at which stars are forming in our galaxy.
fp is the fraction of those stars that have planets.
ne is the average number of planets in each star system that are capable of developing life.
fl is the fraction of life-friendly planets that actually develop life at some point.
fi is the fraction of life-bearing worlds that go on to develop intelligent life.
fc is the fraction of extraterrestrial civilizations that release some sign of their existence (radio waves, perhaps) into space, and
L is the amount of time over which each broadcasting civilization releases their signals.

As long as you’re using the Drake equation as a tool to encourage critical thinking, I have no problem with it. If you try to solve it, however, you have misunderstood its purpose. Drake made no effort to adjust the importance of each factor in his equation. A functional equation for calculating the number of extraterrestrial civilizations in the galaxy would most likely contain exponents and logarithms and other scary math things that you haven’t thought about since high school.

Rather than evaluating each of the factors separately, I’d like to take the liberty of reducing the equation to two factors – sort of a Drake Lite, if you will.

Nl = P x np

In this shortened version, I am unconcerned with intelligent life. Nl represents the number of worlds in our galaxy that have life of any kind, from little green men to unicellular slime molds. P is the probability of life arising on a suitable world, and np is the number of life-friendly planets (and let’s be fair – moons) in our galaxy.

Let’s say that the more optimistic of these two memes is correct, and that there are 40 billion Earth-like planets. (By Earth-like, I assume we’re talking about a world with liquid water on its surface and a host of other factors that don’t automatically preclude the possibility of life.) Great…now solve for Nl!

And of course you can’t, because we still don’t know the value of P. P is a fraction somewhere between zero and one. If P is very close to zero, then Earth may well be the only planet in the galaxy that contains life. On the other hand, if P is very close to one, then there are billions of planets in our galaxy alone that support life.

So which is it? Is life on Earth a probabilistic fluke, not likely to be repeated elsewhere, or are life-bearing planets a dime a dozen?

To be fair, there are indications that life is not a fluke; after all, it got started on Earth just about as soon as it could, and once it got established, it took off in a big way. But I’d like to remind you that we’re still only dealing with one data point, and it’s dangerous to extrapolate from one data point to any valid conclusion about the universe.

So we don’t know, and that’s the most important message. We simply do not know. If we’re being scientifically honest, we must concede that the critical bit of data we need is frustratingly absent.

That’s not the same as saying that aliens definitely don’t exist. I would not presume to know either way. In fact, I kind of hope that aliens do exist; if not the Star Wars variety than at least some single-celled organisms. But to re-emphasize my initial point, it is very important to understand that wishful thinking is not the same as evidence. Perhaps one day the evidence will come in, and we can modify the Drake equation as needed and finally solve it. Until then, we must remain agnostic as far as extraterrestrial life is concerned.


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