The Drake Equation
Apr 2013 by Dale Alan Bryant
American astronomer, Frank Drake, proposed a simple algebraic equation in the early 1960’s for determining the number of communicating civilizations in our Milky Way galaxy, as represented by the number N. These would be civilizations that are both capable of, and are, communicating via the electromagnetic spectrum, at some radio wavelength or other EMS emission. These parameters could (and should) be applied to any spiral, or barred-spiral galaxy in the universe, comparable in size and age to the Milky Way.
Here is the equation in its entirety:
N = R*× fp × ne × fl × fi × fc × L
N = the number of galactic civilizations releasing detectable Electromagnetic Spectrum (EMS) signals into space.
R* = the average rate of star formation per year in our galaxy.
fp = the fraction of those stars that form planets.
ne = the average number of those planets that lie in the systems potential habitable zone.
fl = the fraction of the above planets that actually go on to develop life at some point.
fi = the fraction of the above life-bearing planets that go on to develop intelligent life.
fc = the fraction of planets harboring intelligent civilizations, who are able to develop a technology that releases detectable signs of the civilization's existence, through EMS emissions into space.
L = the length of time for which a given civilization releases detectable EMS signals into space.
In 1961, when the Drake Equation was introduced, it was thought that very few stars harbored planetary systems and the conservative value of N was placed at 36 million. As of 2013, it is known that more than half of all stars have orbiting planets (exoplanets: planets orbiting stars other than the Sun) with as many as six orbiting one star, and one planet orbiting four stars and more than 2,400 exoplanets are known to date. These planets all lie within the only sector analyzed so far by the Kepler Space Telescope - an area of sky smaller than the size of a postcard held at arm’s length, just east of the constellation Cygnus the swan (also known as the Northern Cross).
Though this sounds promising, only one signal-artifact to date was found back in 1977; it is known as the “Wow!” signal. It was neither a repeating sequence, nor was it ever deciphered, nor did it ever recur. All that is known of the “Wow!” signal is that it was not of terrestrial (Earth) origin.