The term fp – indicating the fraction of suitable stars with planetary systems – is anachronistic to much of the time period under discussion. This chapter will survey a number of theories about planetary systems in Western thought prior to 1961 and will consider what values the term would have been assigned based on the natural philosophy or science of periods before 1961. In antiquity, two strands of thought, the Aristotelian and the atomistic, would have provided very different values for fp. The former proposed a very clear value: zero. There are no planetary systems other than our own. The value that the latter would have proposed, however, presents a more complex situation, as atomists believed there are infinite other inhabited worlds, though none could be seen from our own. The Aristotelian view held sway in the West until the Copernican Revolution sparked a dramatic shift in cosmological ideas. Ultimately, the notion that our solar system was merely one of many planetary systems won out. Such a view was promoted, alongside the idea of plentiful extraterrestrial life, by two popularizers from the late seventeenth century: Christiaan Huygens and Bernard le Bovier de Fontenelle. The view that planetary systems were plentiful persisted through the nineteenth century, and so fp must be understood to have a value of close to 1. Numerous authors posited life even in other places, such as comets and the Sun; planetary systems, then, were not the only abode for extraterrestrial life during this period of optimism about the plurality of worlds, suggesting that fp was an inadequate criterion for the location of life. In the second half of the nineteenth century, William Whewell posited that little was scientifically known about other planetary systems and made various arguments against their commonality. This was reinforced in the first half of the twentieth century with new theories on the origins of planetary systems, which suggested that fp would need to be assigned a much lower value.

The second term of the Drake Equation, fp, assumes that life outside the Earth will arise on planets. Only recently has observable evidence shown that other star systems have planets. In the long era prior to 1961, Western cosmological systems did not always assume the existence of other planetary systems. Only after the Copernican Revolution did Western natural philosophers and scientists understand that the stars in the sky potentially represented planetary systems. But even during that shorter period in which the modern cosmological worldview took form, a variety of theories suggested, in turn, that planets might not be the only source of life and then that planetary systems might not be so common. The historical survey of this chapter will highlight some of the theories in which applying a number to the term fp would be problematic.1