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How many civilizations can we expect to discover during the course of our search for extraterrestrial intelligence? No one can say for sure, but Frank Drake, who did the first systematic search for extraterrestrial life, has come up with an equation to provide an estimate of how many civilizations could be capable of sending signals into space. It's called the Drake Equation, and here it how it works:

"N" is the total number of civilizations in our galaxy that we can detect. You get "N" by multiplying all the factors on the other end of the equation together.

Here's what they are:

R* - this variable considers the rate of the formation of stars suitable for the development of life - some stars are too massive and burn out too quickly, while others are too small and don't generate enough energy.

fp - Of those stars, how many of them have planets around them? That is what this factor asks.

ne - This represents the number of those planets that are actually capable of supporting life. In our solar system, only one planet out of nine (so far as we know) is capable of that.

fl - Of those planets that can support life, how many do? That's the number that his variable represents.

fi - Mere life is not good enough - it has to be intelligent. This variable tracks that number.

fc - It's also not enough to be intelligent - the intelligent life on other planets has to develop technology that we can actually perceive: radio signals are those most obvious, but there may be others. This variabe takes that into consideration.

L - finally, for how long does a civilization that can broadcast it's existence so that we can perceive it actually do so? Time is of the essence, and what this variable is concerned about.

What "N" finally ends up being depends on the numbers for the variables, most of which we have no way of knowing. But consider that there are 200 billion stars in the galaxy. Let's suppose that only 1% of those stars could support life, and half of those had planets, and half of those had planets that supported life. Of those planets, let's say one in five actually does support life, and only one in a hundred of those support intelligent life. Among those intelligent species, one half are capable of sending signals into space, but only one in fifty is actually doing so right now.

Even after all that winnowing, we're still left with 10, 000 planets with intelligent life shouting out into the void. Now all that's left to do is find them. Maybe one day we will.

Courtesy - "The Rough Guide to the Universe, p. 212"