There is no hard scientific evidence that intelligent life exists anywhere beyond Earth, yet when asked if there's anyone else out there, most people have a strong opinion one way or the other. When astronomers answer the question—typically in the affirmative—they are playing a numbers game. There are hundreds of billions of galaxies, each bearing hundreds of billions of stars and perhaps billions of planets, so even if intelligent life is rare, we can't be completely alone, the argument often goes.
The Drake Equation, named after its creator, radio astronomer Frank Drake, is an attempt to frame the question scientifically by assigning a value to all the relevant terms, from the number of stars born each year in our galaxy to the number of stars with planets, and so on. In this interactive version of the Drake Equation, see what Drake's current estimates are, then play around with the numbers yourself to see what you come up with for the value N—the number of communicating civilizations in the Milky Way.
To launch the interactive, click on the image at left.—Lauren Aguirre
