The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The equation was written in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI).

The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life. It is more properly thought of as an approximation than as a serious attempt to determine a precise number.

The Drake equation is:

- N = the number of civilizations in our galaxy with which communication might be possible.
- R∗ = the average rate of star formation in our galaxy.
- fp = the fraction of those stars that have planets.
- ne = the average number of planets that can potentially support life per star that has planets.
- fl = the fraction of planets that could support life that actually develop life at some point.
- fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations).
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
- L = the length of time for which such civilizations release detectable signals into space.

As many observers have pointed out, the Drake equation is a very simple model that omits potentially relevant parameters, and many changes and modifications to the equation have been proposed. One line of modification, for example, attempts to account for the uncertainty inherent in many of the terms.

Others note that the Drake equation ignores many concepts that might be relevant to the odds of contacting other civilizations. For example, David Brin states: “The Drake equation merely speaks of the number of sites at which ETIs spontaneously arise. The equation says nothing directly about the contact cross-section between an ETIS and contemporary human society”.

- R∗ = 1 yr−1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative).
- fp = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets).
- ne = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life).
- fl = 1 (100% of these planets will develop life).
- fi = 1 (100% of which will develop intelligent life).
- fc = 0.1 to 0.2 (10–20% of which will be able to communicate).

L = 1000 to 100,000,000 years (which will last somewhere between 1000 and 100,000,000 years).

Inserting the above minimum numbers into the equation gives a minimum N of 20. Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that N ≈ L, and there were probably between 1000 and 100,000,000 planets with civilizations in the Milky Way galaxy.

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