The Drake Equation

N = N_* ´ f_s ´ f_p ´ n_w ´ f_l ´ f_i ´ f_c ´ f_L

N = number of communicating intelligent civilizations in our galaxy

N_* : number of stars in our galaxy

f_s : fraction of stars in our galaxy suitably long-lived for life to develop

f_p : fraction of suitable stars that have planetary systems

n_w: number of planets in a given system with the possibility of liquid water

f_l : fraction of habitable planets where life arises

f_i : fraction of inhabited planets where intelligence arises

f_c : fraction of planets inhabited by intelligent beings on which a communicative technical civilization develops

f_L : lifetime of a communicating technical civilization, expressed as a fraction of a planet’s existence