Continuing our summer infographic series, BBC Future commissioned Information is Beautiful to create a fascinating interactive graphic for the Drake Equation. It's completely embedded on the BBC website so click on the image to go directly to the interactive or click here.

The Drake Equation, for those unfamiliar with it, is a mathematical equation devised in 1961 by Prof. Francis Drake from the University of California - Santa Cruz as an attempt to obtain a scientifically informed estimate of the number of intelligent alien civilizations that may exist in our galaxy. This isn't some woo-woo concept - it's the standard model used by astronomy researchers at SETI and elsewhere to grapple with the question of how common life - and in particular, intelligent life - may be in the universe. Carl Sagan was quite fond of discussing the Drake Equation in his books and in the famous TV series, Cosmos.

The equation is based on extrapolations, from reasonably well-understood numbers (at least to an order of magnitude), the estimated percentage of solar systems in our galaxy that have planets; the number of those planets that are in the "Goldilocks Zone" of habitability; the percentage of those habitable planets that happen to develop life; the number of those that develop advanced intelligent life.... and so on.

As a result of new astronomical observations carried out since 1961, we have a much better handle on some of these numbers - and in particular the number of extrasolar planets orbiting other suns. For that, you can thank NASA's Kepler Space Telescope - an amazing piece of astronomical space engineering. Kepler has already detected the presence of thousands of extrasolar planets, and a January 2012 analysis of Kepler data suggests that "planets are at least as numerous as the stars in the Milky Way", and that each star of the 100 billion or so in our Milky Way galaxy is estimated to contain "on average ... at least 1.6 planets."

Ultimately, the Drake Equation is informed speculation - a guesstimate. Obviously no one should take the numbers too seriously. But it's an interesting exercise, and one I believe is important in order to have some scientifically sound method of calculating - at least to ballpark numbers - such an important question.

Embedded in the equation, as you will note from the interactive graphic, is a number that estimates the number of years an advanced civilization may be able to persist before it becomes extinct. This reflects an understanding of the Fermi Paradox - the evident contradiction between estimates of the apparent number of probable extraterrestrial civilizations and the fact that we have an utter lack of evidence for their existence. Put another way, assuming there are other advanced civilizations and also assuming, reasonably, that our civilization is somewhere in the middle of the bell curve of technological progress, that leaves a lot of more advanced civilizations out there in our galaxy. And if that's so, why haven't they contacted us to say "hey, we're here!"?

That's the Fermi Paradox. The estimates you use to plug in numbers to the Drake Equation heavily determine the results you get. And that, perhaps as tellingly as anything, reflects how optimistic or pessimistic you are about the ability of a civilization to be sophisticated and moral enough to avoid the pitfalls of social and technological evolution. The more pessimistic you are, the more easily the Fermi Paradox is to understand.

And yet you would be forgiven for hoping that there's something screwy in one or other equation somewhere, such that whatever extraterrestrial civilizations out there that may exist are just keeping their radio silence because they're shy and not because they killed themselves.

That wouldn't bode too well for us.

Image: BBC Future and Information is Beautiful

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