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Guthke may have been too dismissive when he wrote that the Drake equa- tion is made up entirely of unknowns.' Since that equation was written, there have been significant scientific findings affecting some of its factors. We now have circumstantial evidence lending credibility to a plurality of inhabited worlds: Extrasolar planets exist in large numbers, strengthening the arguments of believers. Research showing that life on Earth is tougher and more adaptable that we had believed also may help their case. Deniers have presented more detailed arguments casting doubt on the probability of Earth-like environments elsewhere. Some research results imply that astrophysical disasters may be more frequent than we once thought, shortening the lifetimes of intelligent species and the civilizations they may create. Each factor deserves a book of its own. Here we can only touch on fe din an fen HL net Meee etd naan ane Aan a6 Stars Fermi’s classical question “Where is everybody?” may be answered with “where the late F stars and the early G stars are.” —Space visionary Krafft Ehricke, 1975° Astronomers have developed fairly reliable estimates of the rate of star formation in our galaxy, although the rate for the universe as a whole may have been decreasing for the past 5 to 8 billion years.* The question now is how many of these stars could sustain conditions suitable for the evolu- tion of life on their planets. Conventional wisdom long assumed that only relatively recent third- generation stars like our Sun could have planets where life, intelligence, and technology might evolve, because only those planets would have the necessary heavier elements (known to astronomers as “metals”) in 58 Probabilities: The Astronomical Factors