The plurality of worlds?

The discovery of planets orbiting other stars and the widespread occurrence of dusty circumstellar disks, some with gaps in which planets may be lurking, has raised once again and in a dramatic fashion, the ancient question posed amongst others by Albertus Magnus in the 13th century:

“Since one of the most wondrous and noble questions in Nature is whether there is one world or many, a question that the human mind desires to understand, it seems desirable for us to enquire about it” [1].

These questions have been discussed under many headings for the past 24 centuries since Democritus and Epicurus in Greece favored a multitude of worlds in contrast to Plato and Aristotle who considered the Earth to be unique [2]. The concept of a multitude of habitable worlds has appeared historically under several headings such as “the plurality of worlds” [3], “the principle of plenitude (abundance)” [4] and “the principle of mediocrity” which states that our neighbourhood is more or less typical of the rest of the universe [5]. The whole question of the existence of Earth-like planets is inextricably intertwined with the debate over the existence of extraterrestrial life (which is usually assumed overtly or covertly to be intelligent). On this topic the biologists, familiar with the random course of evolution, have mostly been sceptical [6] while the physicists have been less so [7].