"The probability that intelligent life which eventually attempts interstellar communication will evolve in a star system is usually expressed by the :p = f_pn_ef_lf_if_cwhere f_p is the probability that a given star system will have planets, n_e is the number of habitable planets in a solar system, f_l is the probability that life evolves on a habitable planet, f_i is the probability that intelligent life evolves on a planet with life, and f_c is the probability that an intelligent species will attempt interstellar communication within 5 billion years after the formation of the planet..."