My beef with the Drake Equation

I’ve got some issues with the Drake Equation.

For those of you who aren’t familiar with it, the Drake Equation is a mathematical statement designed to estimate the number of detectable intelligent civilisations in our galaxy. It was devised in 1961 by Frank Drake, Emeritus Professor of Astronomy and Astrophysics at the University of California, and it assumes the following form:

N = R* • fp • ne • fl • fi • fc • L


N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
R* =The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life bearing planets on which intelligent life emerges.
fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.

When Drake first proposed the equation, he and his colleagues calculated that there were ten intelligent civilisations in the Milky Way at any given point in time. Since then the Drake Equation has been one of the cornerstones of speculation on extraterrestrial life, both in fiction and non-fiction.

But here’s the problem – many of its variables cannot be adequately estimated based on our current knowledge of the universe. In other words, the Drake Equation is an exercise in guessing.

Oh, sure, we have a pretty good idea when it comes to several of its variables. NASA and the ESA currently estimate that approximately seven new stars are born in our galaxy every year, and the ever-increasing discovery of extrasolar planets indicates that the number of stars with planetary systems may be as much as 60%. These figures are pretty solid; like many things in the world of science they can never be known with precision, but they can be considered close approximations of reality.

After here, things get a little hazy. In order to determine how many of those planets are suitable for life, we need to understand the conditions under which life might arise. Having never encountered life outside of our own humble little planet, our understanding of it is very Earth-centric – hence why scientists have previously relied on concepts such as the ‘Goldilocks Zone’, the invisible strip of space surrounding a star where the temperature is ‘just right’ for life to form. But our own Solar System demonstrates that the right atmospheric conditions are just as important in determing planetary habitability – otherwise Venus, Mars, the Moon and Ceres would host life as well. And what about the icy moons of the outer solar system like Europa, which supposedly contain oceans of warm, salty water underneath protective crusts of ice?

And let’s not forget the possibility of non-carbon based life. By assuming that life must adhere to the strict conditions under which it arose on Earth, we inadvertantly reverse the causality of evolution. If life is as tenacious and widespread as it seems, it may very well be able to arise within, and adapt itself to, environments we on Earth would consider hostile.

We know even less about the remaining parameters. Is intelligent life an inevitable conclusion of evolution, or merely one of an infinite number of branches? What does it even mean for a life form to be intelligent? Biologists and psychologists are still divided as to what the term entails. And is the development of some form of broadcasting technology a necessary part of the technological growth of a civilisation? As Frank Drake himself said after several decades of (relatively) fruitless searching by SETI, “All we know for sure is that the sky is not littered with powerful microwave transmitters.” Intelligent life may very well be out there, but it might not be a fan of HBO.

Finally, estimating the lifetime of an extraterrestrial civilisation is so problematic that it hardly warrants mention. Any figure we come up with is going to be so horribly Earth-centric – indeed, homo sapiens-centric – that it will be pracatically useless. Some scientists have attempted to quantify this variable by calculating the average duration of historical civilisations, but this is inadequate for two reasons: firstly, the rise and fall of intra-species civilisations are different from the survival of a species as a whole; and secondly, there is no way of knowing that intelligent extraterrestrial life would be based upon similar intra-species divisions. The Roman, Chinese and Mayan Empires represent historically significant clusters of culture, religion, warfare, politics and economics – concepts which may be wholly ‘alien’ to extraterrestrial life.

In other words, asking a scientist in the 21st Century to come up with a meaningful figure using the Drake Equation would be like asking an Ancient Greek to calculate the number of humans on the planet at any given time. (S)he might approach the problem thus:

P = (nc * npc)/(fs + ff + fw + fd)


P = The global population.
nc =The number of continents.
npc = The average number of people living on each continent.
fs = The fraction of people expected to die of seasonal variations in temperature in any given year.
ff = The fraction of people expected to die of famine in any given year.
fi = The fraction of people expected to die in war in any given year.
fl = The fraction of people expected to die of disease in any given year.

The Greeks were clever folk – one of them even calculated the circumference of the Earth almost two thousand years before it was circumnavigated by Magellan – but their limited knowledge would have made it impossible to accurately estimate most of the variables. Their world was comprised of three continents surrounded by a vast ocean – Europe as far north as Britain, Africa as far south as Ethiopia, and Asia Minor as far east as India. They knew war and agriculture very well, but their knowledge of diseases and their causes was slim.

Nevertheless, from our current 21st century vantage point, it is possible to accurately calculate the world’s population. Herein may lie the potential future value of the Drake Equation, assuming our knowledge of the universe ever reaches a point where its variables can be estimated with greater certainty. For now, however, it is far more useful as a heuristic device than as a scientific measurement.



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