The Drake Equation

(see Schilling 1998, Sky and Telescope, Dec. issue, p. 36)

N = N_* x f_s x N_p x f_e x f_l x f_i x L/L_MW

• N = number of communicating civilizations in our galaxy now
• N_* = number of stars in the galaxy
• f_s = fraction of sunlike stars
• N_p = average number of planets per star
• f_e = fraction of planets suitable for life
• f_l = fraction of those planets where life actually develops
• f_i = fraction of planets with life where intelligent civilizations arise
• L/L_MW = Lifetime of civilization with ability and desire to communicate compared to the lifetime of the Milky Way galaxy

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