Extraterrestrial Life – Drake Equation

To estimate how many civilizations (able to communicate) could exist in our Milky Way, the astrophysicist F. D. Drake wrote the Drake Equation.

Today we know of the existence of general laws that apply to all points of the Universe. We can reasonably assume that a certain phenomenon, occurring at one point, may also occur in another, if the conditions are on average similar. In particular this applies also to the “intelligent life forms”: the Earth, a celestial body of infinitely small dimensions compared to those of the entire cosmos, in little more than 200,000 years has seen a technological civilization develop.

Although affected by an optimistic or pessimistic view, the acquired astrophysical knowledge makes it difficult for us to think of a total absence of extraterrestrial civilizations.
In this context, the difficulty consists in evaluating the various situations and the various processes, and then deriving a statistical probability from them.

To estimate how many civilizations (able to communicate) could exist in our Milky Way, the astrophysicist Frank Donald Drake wrote an equation that has now become famous, known as the Drake Equation or the Green Bank formula:

Where:

N  is the number of extraterrestrial civilizations present today in our Galaxy with which we can think of establishing a communication
R*  is the average annual rate with which new stars are formed in the Milky Way
fp    is the fraction of stars that possess planets
ne  is the average number of planets per planetary system in the condition of hosting life forms
fl    is the fraction of the planets on which life has actually developed
fi    is the fraction of the planets fl on which intelligent beings have evolved
fc    is the fraction of extraterrestrial civilizations able to communicate
L    is the estimate of the duration of these evolved civilizations

Current estimates of the parameters provided by the ESA and NASA space agencies can provide :
N = 7 × 0,5 × 2 × 0,33 × 0,01 × 0,1 × 10 000 = 23,1


The following is the “Probability Analysis” by Isaac Asimov
(from Extraterrestrial Civilizations, 1979)

In our Galaxy Milky Way there would be:
300,000,000,000: stars
280,000,000,000: planetary systems
75,000,000,000: planetary systems that revolve around stars similar to the Sun.
52.000.000.000: stars similar to the Sun that have a useful ecosphere
5,200,000,000: stars similar to the Sun, population I, 2nd generation, with useful ecosphere
2,600,000,000: stars similar to the Sun, population I, 2nd generation, with a useful ecosphere inside which you turn a planet
1,300,000,000: stars similar to the Sun, population I, 2nd generation, with a useful ecosphere inside which you turn a planet similar to the Earth
650,000,000: number of habitable planets
600,000,000: number of habitable planets that have generated life
433.000.000: number of habitable planets that have generated a multicellular life
416,000,000: number of habitable planets that have generated a rich earthly life
390,000,000: number of planets that have developed a technological civilization
530,000: number of planets in which there is today a technological civilization

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