What is the probability that an exoplanet shelters a form of life? This question was posed long before the discovery of the first exoplanets. In the 1970s, the American astronomers Frank Drake and Carl Sagan tried to express the problem in figures. If N is the number of planets in the Galaxy that currently shelter some form of life, we may write:
Ncivil = N* • Fpl • Fhabit • Flife • Fcivil--j*1--(9.2)
In this equation, known as the Drake Equation, N* is the number of stars in our galaxy; Fpl is the percentage of stars with planets; Fhabit is the fraction of these planets that are habitable; Fcivil is the fraction of inhabited planets where life has led to a technological civilization capable of communicating, < Tcivil > is the average lifetime of a technological civilization, and T* is the lifetime of stars.
The first two parameters are relatively well-known. If we take stars close to the Sun's spectral type, N* is about 109. Fpl, according to the first results in the search for exoplanets, may be about 0.1 to 0.2; this parameter should be appreciably improved in the next decade. The lifetime of stars is known. It is a function of their spectral type. In contrast, the other parameters remain utterly unknown at the present time.
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