“Still, statistically the probability that there are other thinking beings out there is good. Nobody knows how many stars there are in the Milky Way – estimates range from 100 billion or so to perhaps 400 billion – and the Milky Way is just one of 140 billion or so other galaxies, many of them even larger than ours. In the 1960s, a professor at Cornell named Frank Drake, excited by such whopping numbers, worked out a famous equation designed to calculate the chances of advanced life in the cosmos based on a series of diminishing probabilities.
"Under Drake’s equation you divide the number of stars in a selected portion of the universe by the number of stars that are likely to have planetary systems; divide that by the number of planetary systems that could theoretically support life; divide that by the number on which life, having arisen, advances to a state of intelligence; and so on. At each such division, the number shrinks colossally – yet even with the most conservative inputs the number of advanced civilizations in the Milky Way always works out to be somewhere in the millions.