In 1961 the president of SETI, Frank Drake, developed an equation to calculate the probability of contacting other civilizations of the Milky Way. It took into account astronomical, biological and technological factors. Its formula is:
N = R * x Fp x Ne x Fl x Fi x Fc x L.
R * It is the number of stars that are born in our galaxy each year. Only those who live long enough to develop life count. Giants are excluded, because they consume their fuel quickly and die soon. Nor do dwarfs of very low mass count, since they do not usually generate habitable areas. They are about 200,000 million stars a year.
Fp It's the fraction of those stars with planets. It is common for a star to have planets in orbit. Let's put between 100 and 150,000 million stars. The planets are formed by accumulation of heavy elements. Near the galactic disk there are more heavy elements and more planets, but also more supernovae that compromise their survival. Far from the disk the number of planets is smaller, but their chances of survival increase. The number of stars with planets in the optimal area of the galactic disk is reduced to 25,000 million.
Ne it is the number of planets located in the ecosphere, that is, in the optimal zone for life. It is where there can be liquid water, basic element for life. Gaseous planets are discarded. We assume that there can only be life on the rocky planets, although we may be wrong. There are 10,000 million planets around 5,000 million stars.
Fl It is the fraction of these planets that can develop life. Where there is enough organic matter, carbon, liquid water and energy sources for life. The figure is reduced to 25 million planets.
Fi It is the fraction of planets where intelligent life evolves. And Fc the planets where intelligent life reaches a technological development that allows interstellar communication. They are the most difficult data to predict, since they are mere speculations. It is believed that, once life appears, its evolution is inevitable. Although maybe not. Or, even if it is, it does not evolve technologically just like us. Estimating an average of 1%, 250,000 planets remain.
L It is persistence. That is, the time that a civilization with that technological level survives. Civilizations are born, survive for a brief cosmic period and become extinct. The probability that two civilizations coincide in time is very small. And although they coincide, the interstellar distances are so great that the probability of communication during their existence is almost nil.
N It is the end result. Being optimistic, the probability of contacting other civilizations of the Milky Way does not reach one hundred. In the worst case, N = 1. We would be alone in our galaxy. There may or may have been many other civilizations in other galaxies or in ours. Or maybe we are the first. Whether we are alone or not, the probability of contacting other extraterrestrial civilizations is practically nil.
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