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Drake's equation
Copyright : Paris Observatory / UFE

Drake's equation takes a scientific position on the problem. It was introduced by the American astronomer Frank Drake in 1960.


We restrict this study to our galaxy. Indeed, even at the speed of light (300000 km/s), it is not possible to go to another galaxy, nor to communicate (by radio) with the possible inhabitants. The nearest galaxies are at distances of hundreds of thousands of light-years.

In our galaxy, Drake wrote a simple equation to compute the number of civilizations with which we could communicate today (Nciv). This equation proposed by Drake in 1961 is  :

Nciv = Fét x Ppla x Npla x Pvie x Pint x Pcom x T

Fét is the rate of stellar formation in the galaxy. It is equal to the number of stars in the galaxy divided by the age of the galaxy, knowing that the current number of stars corresponds approximately to the total number of stars having existed.
P... is the probability or the fraction of stars satisfying a particular condition (from 0 to 100%, ie from 0 to 1)
Ppla is the probability for a star to host planets
Npla is the mean number of habitable planets per star. This assumes that the star has "good" characteristics, that the mass of the planet is "correct", and that the distance between the planet and the star is "correct".
Pvie is the probability for life to emerge on a habitable planet.
Pint is the probability for intelligence to appear on a planet on which life has emerged.
Pcom is the probability for intelligent life to develop communication tools with other worlds.
T is the length during which such a communication can be detected. Therefore this is the life time of the communicating civilization.

It is important to note that this scientific statement breaks down the required factors, and thus allows us to study them. However, this statement does not provide any answers. We can not estimate precisely the different terms. Actually, Drake's equation is an excellent way to measure our ignorance … and our progress.