equation is Ward and Brownlee's riposte to the Drake equation. It calculates , the number of Earth-like planets in the Milky Way having complex life forms, as: where: N* is the number of stars in the Milky Way. This number is not well-estimated, because the Milky Way's mass is not well estimated, with little information about the number of small stars. N* is at least 100 billion, and may be as high as 500 billion, if there are many low visibility stars. is the average number of planets in a star's habitable zone. This zone is fairly narrow, being constrained by the requirement that the average planetary temperature be consistent with water remaining liquid throughout the time required for complex life to evolve. Thus, =1 is a likely upper bound. We assume . The Rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, which are all fractions, is no greater than 10−10 and could plausibly be as small as 10−12. In the latter case, could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of , because the numerical values of quite a few of the factors below can only be conjectured. They cannot be estimated simply because we have but one data point: the Earth, a rocky planet orbiting a G2 star in a quiet suburb of a large barred spiral galaxy, and the home of the only intelligent species we know; namely, ourselves. is the fraction of stars in the galactic habitable zone (Ward, Brownlee, and Gonzalez estimate this factor as 0.1). is the fraction of stars in the Milky Way with planets. is the fraction of planets that are rocky ("metallic") rather than gaseous. is the fraction of habitable planets where microbial life arises. Ward and Brownlee believe this fraction is unlikely to be small. is the fraction of planets where complex life evolves. For 80% of the time since microbial life first appeared on the Earth, there was only bacterial life. Hence Ward and Brownlee argue that this fraction may be small. is the fraction of the total lifespan of a planet during which complex life is present. Complex life cannot endure indefinitely, because the energy put out by the sort of star that allows complex life to emerge gradually rises, and the central star eventually