Drake equation (F. Drake; 1961)
A technique of estimating the number of intelligent, technological species (that means, able to communicate along with other species) in existence in our Galaxy.
N = R fp ne fl fi ft L.
N refers to the number of species defined above at any given moment in our Galaxy. The parameters it is calculated from are as follows:
R is the rate of star formation in our Galaxy (in stars per year);
Fp is referring to the fraction of stars which have planets;
neis refer to the number of habitable planets per system with planets;
flis refer to the fraction of habitable planets on which life arises;
fi is refer to the fraction of these planets on which life develops intelligence;
ft refer to the fraction of these planets where the intelligence develops in a technological civilization able of communication; and
L refers to the mean lifetime of such a technological civilization.
Of these quantities, just the first R is known to anything like any reliability; this is on the order of 10 stars per year. The others, most particularly the fractions, are almost completely pure speculation at this point. Calculations made through respectable astronomers differ by something such as ten orders of magnitude in the ultimate estimation of the number of species out there.