Why the Drake equation is still important 65 years after its creation

It is the refrain of teachers everywhere preparing their students for standardised tests: “Make an educated guess.” An answer is always better than no answer, they say, and if you can apply what you know to the question to rule out some of the choices, even better. Of course, scientific research isn’t a multiple-choice exam, and its bedrock is the idea that an educated guess without solid evidence just isn’t good enough. But the concept is still a useful one, particularly when our questions outpace the technology we currently have to answer them. The key part of the phrase “educated guess” is the first word. Put another way, a guess based on nothing may be worse than no guess at all. “But Leah,” I hear you asking, “Do you have a framework, or a favourite example, for educated guesses?” And just because you are clamouring for it, and I’m the kind of person who has favourite equations, I will tell you about it. The Drake equation, which turns 65 years old this month, is an incredibly useful tool for framing our thinking about life in the cosmos – even though, at its core, it is simply a method for making an educated guess. Astrobiologist Frank Drake came up with his eponymous equation in 1961, before convening the first scientific conference on the search for extraterrestrial intelligence (SETI). According to an account written by his daughter, science journalist Nadia Drake, he sketched out the equation on a bit of a whim to get the few attendees of the meeting talking and is, to this day, astonished by how important it is now considered. Advertisement The equation has seven key terms, and when multiplied together, they provide an estimate of N: the number of intelligent civilisations in the Milky Way galaxy with whom we might someday be able to communicate. It looks like this: N = R* × fp × ne × fl × fi × fc × L The first few terms are relatively simple. R* is the rate of star formation in our galaxy, which has been measured at about 1 to 3 stars per year. The fraction of those stars with planetary systems is fp, and we now think that number is about 1 as well – most stars seem to have planets. The number of planets per star with an environment suitable for life is ne, and that is where things start to get more complicated. “Suitable for life” is a broad idea, and one that’s difficult to define. Even the most basic requirement – that the planet be in its star’s habitable zone, where temperatures allow liquid water on the surface – is required for Earth-like life, as far as we know, but it’s certainly possible that other lifeforms have different needs. In our own solar system, there are either 2 or 3 planets in the habitable zone, depending how you slice it – but Venus and Mars don’t seem particularly suitable for life, and there are several moons in the outer solar system that may well be. That is the first time the equation forces us to make an educated guess. The second comes hot on its heels with fl, the fraction of the habitable planets that go on to develop life. On this, we have an even smaller sample size. We haven’t found life anywhere but Earth, so this number could be very small, or it could be 1 – remember, this includes life that arises at any point in the history of the cosmos. But until we do find life elsewhere, it will be more of a philosophical argument than anything else. The same is true of the next two terms of the Drake equation: fi, which denotes the fraction of life that is intelligent – the definition of which is also wily – and fc, the fraction of intelligent lifeforms that eventually broadcast their existence into space and are thus capable of communicating with us. The final term is L, the length of time in which a civilisation of intelligent lifeforms sends detectable signals out into space. As the astronomer Carl Sagan pointed out, this depends heavily on whether such civilisations can avoid self-destruction, which isn’t something we can know for sure – but we make our best guess based on the only evidence we have: human history. If you combine all the lowest estimates, N could be one-trillionth or even lower, implying that we are totally alone in our galaxy. If you combine the highest estimates, there could be tens of millions of civilisations in the Milky Way that we could communicate with, which to me would say it’s only a matter of time before we find them. At Drake’s original SETI meeting, the researchers set N between 1000 and 100,000,000, and the lower end of that sounds about right to me – an estimate that, like those at the meeting, is based largely on my not-particularly-scientific opinions about how long our civilisation might last. But when it comes down to it, the number itself isn’t the important part of the equation; the important part is the process of estimating each of its terms. In doing so, we force ourselves to consider some of the biggest questions in science: How similar are other places in the universe to our own cosmic neighbourhood? How likely are those most-similar places to develop life? What is life, anyway? What does life truly need? What is intelligence? How can we search for it? And how long will we be capable of searching for it? None of those questions are answered definitively, but pondering them can help us direct our scientific research – and more than that, it gives us an idea of our place in the grand scheme of the universe. That’s why teachers tell their students to make an educated guess, and it’s why the Drake equation is still considered so important. The value in an educated guess is in taking account of what you know and what is still out there, waiting to be known.