Drake Equation, The

   Named after American astronomer Frank Drake, this formula estimates the distribution of life in the universe. The formula N = R* • fp • ne • f1• fi • fc • L, according to some astronomers, suggests the number of civilizations in the Earth's galaxy is somewhere between 1,000 and 1,000,000. The letters in the formula sometimes change, according to the source, but the meaning and order of definitions always stays the same.

   N - The number of civilizations capable of communicating across interstellar space using radio emissions.

   R* - The mean rate of star formation.

   fp - The fraction of stars with interplanetary systems.

   ne - The mean number of planets in each planetary system that are suitable to support life.

   f1 - The fraction of planets where life can actually develop.

   fi - The fraction of life bearing planets on which intelligent life has evolved.

   fc - The fraction of planets with intelligent life that can give rise to a civilization capable of interstellar communication.

   L - The lifetime of a technical civilization.