This equation expresses the number of technological civilizations that might exist among the stars.

R * FP * NE * FL * FI * FC * LN = N = number of advanced technical civilizations emitting detectable radio signals.

R = mean rate of star formation averaged over the lifetime of the galaxy.
FP = fraction of stars with planetary systems.
NE = mean number of planets per system with environments suitable for the origin of life.
FL = fraction of such planets on which life does develop.
FI = fraction of such planets on which intelligent life rises during the lifetime of the local sun.
FC = fraction of planets on which advanced technical civilizations rises.
L = lifetime of this technical civilization.

When explaining this equation, Carl Sagan once said the formula means that at least 1 million of the 100 billion stars in our Milky Way galaxy have planets which have developed advanced intelligent civilizations. Now, if we consider the astronomical fact that there are over 10 billion galaxies in the universe, each holding at least another million solar systems, one may start to think that we may not be the only life in the universe.

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