I’m old enough to remember when we had nine planets in the Solar System, and zero outside. The news since then has been mixed. Here in our neighborhood we’re down to only eight planets; but in the wider galaxy, we’ve obtained direct evidence for about a thousand, with another several thousand candidates. [Thanks to Peter Edmonds for a correction there.] Now that we have real data, what used to be guesswork gives way to best-fit statistical inference. How many potentially habitable planets are there in the Milky Way, given some supposition about what counts as “habitable”? Well, there are about 200 billion stars in the galaxy. And about one in five are roughly Sun-like. And now our best estimate is that about one in five of them has a somewhat Earth-like planet. So you do the math: about eight billion Earth-like planets. (Here’s the PNAS paper, by Petigura, Howard, and Marcy.)
“Earth-like” doesn’t mean “littered with human-esque living organisms,” of course. The number of potentially habitable planets is a big number, but to get the number of intelligent civilizations we need to multiply by the fraction of such planets that are home to such civilizations. And we don’t know that.
It’s surprising how many people resist this conclusion. To drive it home, consider a very simplified model of the Drake equation.
x equals a times b. Now I give you a, and ask you to estimate x. Well, you can’t. You don’t know b. In the abstract this seems obvious, but there’s a temptation to think that if a (the number of Earth-like planets) is really big, then x (the number of intelligent civilizations) must be pretty big too. As if it’s just not possible that b (the fraction of Earth-like planets with intelligent life) could be that small. But it could be! It could be 10-100, in which case there could be billions of Earth-like planets for every particle in the observable universe and still it would be unlikely that any of the others contained intelligent life. Our knowledge of how easy it is for life to start, and what happens once it does, is pretty pitifully bad right now.
On the other hand — maybe b isn’t that small, and there really are (or perhaps “have been”) many other intelligent civilizations in the Milky Way. No matter what UFO enthusiasts might think, we haven’t actually found any yet. The galaxy is big, but its spatial extent (about a hundred thousand light-years) is not all that forbidding when you compare to its age (billions of years). It wouldn’t have been that hard for a plucky civilization from way back when to colonize the galaxy, whether in person or using self-replicating robots. It’s not the slightest bit surprising (to me) that we haven’t heard anything by pointing radio telescopes at the sky — beaming out electromagnetic radiation in all directions seems like an extraordinarily wasteful way to go about communicating. Much better to send spacecraft to lurk around likely star systems, à la the monolith from 2001. But we haven’t found any such thing, and 2001 was over a decade ago. That’s the Fermi paradox — where is everyone?
It isn’t hard to come up with solutions to the Fermi paradox. Maybe life is just rare, or maybe intelligence generally leads to self-destruction. I don’t have strong feelings one way or another, but I suspect that more credence should be given to a somewhat disturbing possibility: the Enlightentment/Boredom Hypothesis (EBH).
The EBH is basically the idea that life is kind of like tic-tac-toe. It’s fun for a while, but eventually you figure it out, and after that it gets kind of boring. Or, in slightly more exalted terms, intelligent beings learn to overcome the petty drives of the material world, and come to an understanding that all that strife and striving was to no particular purpose. We are imbued by evolution with a desire to survive and continue the species, but perhaps a sufficiently advanced civilization overcomes all that. Maybe they perfect life, figure out everything worth figuring out, and simply stop.
I’m not saying the EBH is likely, but I think it’s on the table as a respectable possibility. The Solar System is over four billion years old, but humans reached behavioral modernity only a few tens of thousands of years ago, and figured out how to do science only a few hundred years ago. Realistically, there’s no way we can possibly predict what humanity will evolve into over the next few hundreds of thousands or millions of years. Maybe the swashbuckling, galaxy-conquering impulse is something that intelligent species rapidly outgrow or grow tired of. It’s an empirical question — we should keep looking, not be discouraged by speculative musings for which there’s little evidence. While we’re still in swashbuckling mode, there’s no reason we shouldn’t enjoy it a little.