Aliens are probably the most common topic of science fiction. They are typically an extension of our hopes and fears. Wise parental figures, evil enemies, noble savages, fierce predators. They are often physically quite similar to us, with a bipedal gait, opposable thumbs,etc. We dream of life on other worlds. Reaching out to the stars and meeting an alien intelligence. But is that likely, or even possible.
The difficulty with that question is that we currently only have one example of life in the universe, and that is us, the commonly descended family of life on Earth. Beyond that, there is a quite a bit of guesswork. It is at this point that the Drake equation is often invoked.
The Drake equation is often interpreted as a way to calculate the number of intelligent civilizations in the galaxy. It was originally proposed at the first SETI conference by Frank Drake in 1961 as a way to stimulate discussion. Drake did not intend it as a prediction of the correct value, but more as a “what if” to consider.
The equation itself is basically a product of the rate at which stars form in our galaxy, how many stars have planets, how many planets they typically have, what fraction are habitable, what fraction of habitable planets form life, how many form intelligent life, then civilizations, and how long those civilizations last.
When it was first proposed there weren’t any known extrasolar planets. We now know that planets are quite common, and stars are more likely to have planets than not. Current estimates calculate that about 1 in 3 Sun-like stars have terrestrial planets in their habitable zone. That means there may be 100 billion potentially habitable planets in our galaxy alone.
Of course “potentially habitable” doesn’t mean “has life”, only that it has the right temperature and orbits a stable star. This is where we have to start speculating. For life similar to ours there needs to be liquid water on the planet. Earth has liquid water, and we know Mars had liquid water in its youth, but we don’t know how likely it is for a potentially habitable planet to have liquid water.
If there is liquid water on a planet of the right temperature, how likely is it that life will appear? On Earth we know that life appeared quite early in its history. This hints that life is fairly likely to appear, but with only an example of one, we can’t read too much into it. Early Mars had liquid water, and even if life does or did exist on Mars it is not robust, which could indicate early life is quite fragile. Again we’re faced with a lack of information.
When life appears on a planet, how often does intelligent, technological life appear? It did on Earth, but does that mean civilizations are a near certainty, or are we the product of extraordinary circumstances. And how long does a technological species survive? How long do you think human civilization will survive? Decades? Millenia? Eons?
So while habitable planets appear to be common, we don’t know anything about how common life might be. If we assume that Earth is a relatively average terrestrial planet, then it would imply that life exists on billions of worlds. One would expect at least some of them lead to technological civilizations, so there could easily be hundreds or thousands of civilizations in our galaxy alone.
This leads to a bit of a puzzle, since one would assume that a technological civilization would eventually start exploring the stars. Assuming humanity survives for a million years, it would seem likely that we will explore at least a portion of our galaxy. If not ourselves, then through our robotic proxies. If we are typical, then there are civilizations a million years behind us technologically, and ones a million years ahead. So if civilizations are common, then why haven’t they made contact with us? (Yes, there are those who think they have, and apparently are highly interested in our body cavities, but there’s no evidence for that.)
This is known as Fermi’s paradox. If intelligent life is common, then why don’t we see it? Several solutions have been proposed. Perhaps civilizations have a very short lifespan. Once they are capable of space travel they nuke themselves, or pollute their planet, or form an idiocracy and go extinct. Maybe there is a vast galactic civilization, but contact with Earth is forbidden until we’ve proved our worth. Maybe we’re in the cosmic equivalent of the outback, and no one has happened to stop by.
Or maybe we’re the only civilization in the universe. Perhaps Earth is extraordinarily rare. Perhaps the appearance of life, much less intelligent life, requires such an improbable chain of events that Earth may be the only example in the universe.
As it stands, we have only a single example of life in the universe. Only one planet in the universe with an extraordinary diversity of creatures. One example in a galaxy of billions of stars, in a universe of billions of galaxies.
If we were to find a distant planet like Earth we would be awed by its complexity and beauty. We would long to communicate with its intelligent species, and learn about its diverse culture.
Look around, make contact.
Miss the beginning of the series? You can find it here.