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" The Drake Equation, written by astrophysicist Frank Drake in 1961, is a probabilistic equation to come up with an estimate of the number of intelligent, technological civilizations that should be in the Milky Way\u2014and by extension, the universe. It is the foundation for a number of statistical models that suggest intelligent alien life should be widespread throughout the galaxy. Here is the equation:   Here&amp;#039;s the breakdown variable by variable:  N is the number of intelligent civilizations R* is the rate of star formation in the galaxy f(p) is the fraction of those stars that have planets n(e) is the average number of planets that could support life f(l) is the fraction of those planets that actually develop life f(i) is the fraction of planets with life that develop intelligent life f(c) is the fraction of intelligent species that send signals out into space L is the amount of time that the average communicative intelligent species is broadcasting its existence  Of course, the values for most of these variables are extremely rough estimates and the subject of much debate. In 1961, Drake&amp;#039;s original estimate for the number of intelligent civilizations in our galaxy was between 20 and 50,000,000. As a new episode of PBS&amp;#039;s Space Time points out, we have significantly refined our estimates for the number of potentially habitable planets in the Milky Way thanks to the Kepler planet-hunting mission. (We think there are around 40 billion rocky planets orbiting within the habitable zone of their parent stars.)        What we still struggle with is pinning down the probability that life will spring from organic compounds, a process known as abiogenesis, and the probability that basic microbial life will eventually evolve into an intelligent species. To help dial in this estimate a bit more, astrophysicists Adam Frank and Woodruff Sullivan asked how small the intelligent life probability would need to be if we are in fact the only technologically advanced species in the entire universe.  They concluded that if only one intelligent civilization ever existed in the history of the known universe (humans, and nothing else ever before), then the probability that a habitable planet produces intelligent life would have to be less than 1 in 400 billion trillion, or 2.5 x 10^-24\u2014and, even if this were the case, there would still only be a 1 percent chance that no technological civilization ever existed other than humans. This is such a ludicrously small probability that astrophysicists are forced to conclude that we are not the only intelligent civilization to ever exist.  If we narrow the focus to just the Milky Way, then there is still only a 1 in 60 billion chance that a habitable planet produces an advanced civilization, assuming that we are the only such civilization to ever exist in the galaxy. Most people therefore conclude that there must be other intelligent civilizations in our galaxy, if not now then at some point in the past. But we have never detected any, and therein lies the Fermi paradox. Check out the Space Time video above to learn more about the likelihood that we are not alone in our galaxy, and be sure to stay tuned for the bonus question at the end of the episode. If you get it correct, PBS will send you a Space Time t-shirt free of charge.  "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/a23244/drake-equation-intelligent-alien-life-in-the-galaxy/>