The Drake Equation

Two days ago I blogged about the exciting discovery of a planet orbiting Proxima Centauri. One point that I had intended to make but forgot to mention was what impact it may have on the Drake equation.

The Drake equation is a thought experiment created by Frank Drake, an astronomer from the US. He has always had an interest in the search for extra-terrestrial life and was one of the founding members of the SETI institute. His equation is not complicated, it is just a series of parameters multiplied together to give a probability for intelligent life to exist in our Milky Way galaxy. It isn’t meant to give an actual figure, it is more designed to get us thinking about the sorts of questions we would need to answer to study the subject properly.

The equation is as follows:


Now don’t panic, this is no more complicated than knowing that the chance of rolling two sixes (or any two numbers you choose) on a six sided die is 1/6 x 1/6 = 1/36.

The number of civilisations that we could communicate with is N. This is calculated by multiplying together the average rate of star formation, R*, in our galaxy; the fraction of stars, fp, that have planets; the average number of those stars that have planets that can support life, ne; the fraction of those planets, fl, that actually develop life; the fraction of planets bearing life on which intelligent, civilised life, fi, has developed; the fraction of these civilisations that have developed communications, fc, i.e., technologies that release detectable signs into space; and the length of time, L, over which such civilisations release detectable signals into space.

Put slightly more succinctly, it’s the chance that there is an alien with a big radio transmitter that we can detect. Our detecting the signal is just as crucial as the signal itself existing. For example, here on earth there is (reasonably) intelligent life. Not long after the invention of radio and tv we were firing out very high power signals into space, though not deliberately. We were trying to beam them out sideways across the planet but the also went up and out into space. As our technology improved we stopped having such large, land-based transmitters and started beaming much weaker signals from orbiting satellites back down to the surface. Anyone on Proxima B that enjoyed tuning in to the World Service last century might be wondering if our civilisation has died given that we’re not really broadcasting much anymore.

The equation, devised in 1961, could probably do with some updating. For example, we now know that even if a planet is unsuitable for life, like our own Jupiter, all is not lost as that planet may have moons that are just right for life, like Europa.

The more we have learnt about our galaxy in the last 55 years the more we have been able to plug realistic numbers into the equation. We are still a long way from a proper answer, especially so long as we have an n of 1 when it comes to the crucial fraction of habitable planets that actually go on to develop life, fl.

What we do know is that stars do tend to have planetary systems around them, they’re not rare. Finding a planet around the star that just happens to be nearest to us exemplifies that.

For what it’s worth, I think that life will be fairly ubiquitous throughout the universe. Not necessarily intelligent life that we could detect but microbe-style life that seems to be able to thrive everywhere we look, at least here on earth. If I was top boffin at NASA, ESA, Roscosmos or anywhere else that had the funds to do so, my highest priority would be a mission to the moons of Jupiter and Saturn capable of finally putting to bed the question of whether there is life elsewhere in our own solar system. I can’t imagine a more profound discovery if it came to pass, a more humbling revelation to our egocentric minds.

I will close with the classic speech about the pale blue dot given by Carl Sagan in 1994 for no other reason than everybody should read it at least once per week.


“We succeeded in taking that picture, and, if you look at it, you see a dot. That’s here. That’s home. That’s us. On it, everyone you ever heard of, every human being who ever lived, lived out their lives. The aggregate of all our joys and sufferings, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilizations, every king and peasant, every young couple in love, every hopeful child, every mother and father, every inventor and explorer, every teacher of morals, every corrupt politician, every superstar, every supreme leader, every saint and sinner in the history of our species, lived there – on a mote of dust, suspended in a sunbeam.

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that in glory and in triumph they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of the dot on scarcely distinguishable inhabitants of some other corner of the dot. How frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the universe, are challenged by this point of pale light.

[…] To my mind, there is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly and compassionately with one another and to preserve and cherish that pale blue dot, the only home we’ve ever known.”


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