" (1) the number of stars in the Milky Way that survive sufficiently long for intelligent life to evolve on planets around them; (2) the average number of planets around each of these stars; (3) the fraction of these planets with conditions suitable for life; (4) the probability that life actually arises on these suitable planets; and (5) the chance that life on such a planet evolves to produce an intelligent civilization, by which astronomers typically mean a form of life capable of communicating with ourselves. "

― Neil deGrasse Tyson , Origins: Fourteen Billion Years of Cosmic Evolution