As far as powerful ideas go, the thought that we humans might not be alone in the universe is a pretty strong one. Specifically, what interests a lot of us is whether we’re alone not only in being alive, but in being able to look at the sky and wonder whether we’re alone.
The Drake equation was born from this curiosity, and the accompanying hope and dread of finding an answer. It isn’t really an equation in the sense of being able to get a useful solution, since it involves a lot of factors that we can only guess at. From the beginning, it was intended as more of a tool to spark discussion about those factors and help us think logically about the search for extraterrestrial buddies.
As you can see, the equation starts very broadly, with the Milky Way’s star formation rate, and progressively narrows down the number of possible civilizations we might communicate with, ruthlessly cutting out stars without planets, planets without good conditions, good conditions that don’t result in life, and so on. It ends with the number of years we can expect a civilization to last, giving us a time window to go with the first term.
You’ll notice that most of the terms are fractions, so they all have a maximum value of 1 (if all stars form planets, all Earthlike planets have life, etc.). You’ll also notice that we have no way of knowing what most of these numbers are.
There are a few ways to look at the information we have, which is based on a sample size of one planet and one intelligent species, assuming you accept that label for humans. (As Wikipedia entertainingly points out, “One of the few points of wide agreement is that the presence of humanity implies a probability of intelligence arising of greater than zero.”)
One point of view is we have evidence of life on Earth very soon, relatively speaking, after conditions became favorable for life as we know it. If we subscribe to the mediocrity principle in this case, we can argue that Earth is likely to reflect a common or average condition, so that life (and intelligence, and maybe communication) is very probable on planets where conditions are favorable.
On the other hand, the anthropic principle points out that of course the planet we know about is one with intelligent life because, well, this is where we live. Really, we can’t make any assumptions about the existence of life on habitable planets, or the development of intelligence and intent to communicate, when we only have our own history to go on. We can’t even really estimate the lifespan of signaling civilizations like ours, since our ability to attempt to contact other life is very new and sporadic at best.
(On this subject, Wikipedia adds another charming note that Earth isn’t exactly a random sample, since it’s been chosen “by the living organisms that already inhabit it (ourselves).” I’m glad they specified which organisms are doing the choosing so we wouldn’t get confused.)
It’s not all bad news, though—in some ways, we do understand more than we used to about the factors that might produce playmates for us in the great galactic sandbox. For one thing, we can come up with at least a rough average of how many stars form in the galaxy every year (about 1.5-3 currently, though obviously this formation rate won’t affect any people who might contact us anytime soon).
Thanks to ever-improving methods of detecting exoplanets, we also know more about planet formation, which is much more common than we once thought, so fp might be close to its maximum value of 1. Current observations also allow us to estimate the zone around each star in which liquid water might be possible. This probably isn’t the only condition for being hospitable to life, assuming it is a necessity, but it’s a start toward understanding ne, the number of habitable planets per star.
At this point, what’s still well beyond our grasp is the intricacy of life itself—how common it might be, how other civilizations might develop and choose to live, and how long we can all expect to be around. We Earthlings still have a lot to discuss among ourselves about the philosophical, ethical, and religious implications of our search for others, especially if it yields results one day. As usual, the only predictable thing about life at home or abroad is its unpredictability, and judging from our fascination with it, we wouldn’t have it any other way.