Page Content (OCR)
The Drake Equation, Take Two Some have argued that the Drake equation is incomplete, that it needs additional factors. These proposals lengthen the list of requirements for the emergence of communicating civilizations. Ward and Brownlee proposed a modified Drake equation in which the key biological factor is the percentage of a lifetime of a planet that is marked by the presence of complex metazoans (multicelled life). They added other factors: the fraction of planets with a large moon (to stabilize their rotation), the fraction of solar systems with Jupiter-sized planets (to protect smaller planets from large impactors), and the fraction of planets with a low number of mass extinction events.’ Papagiannis and Mauldin argued separately that the equation neglects the factor of time; it assumes that all factors are relatively unchanged over the billions of years for life to develop into civilizations. Yet, some disruptive processes in the Galaxy occur on times scales of 100 million years, which may cause us to overestimate the lifetimes of civilizations. Only a very small fraction of planets where life originates might be able o offer the long-term stability required for the slow evolution of life to high intelligence. The duration of the zone of habitability may be as impor- ant as its distance from the star.’ Ideally, the equation would address cultural evolution beyond the state known to us—not just the emergence of postbiological societies but other actors as well. This is extremely difficult to do when we have no agreed heory of cultural evolution. It is easiest to be pessimistic about the probability of alien technological civilizations when our sample for a factor is limited to one, such as the origin of life. On the other hand, our experience with the astronomical actors implicitly favors the believers. We first knew of many stars; although that did not prove that extrasolar planets existed, it did imply that they might be numerous. Now we know hat such worlds are abundant. That does not prove that Earth-like planets exist, but it does make them seem more likely. If we find large numbers of 111