LEFT: Bernard Vaillant, “Portrait of Christiaan Huygens,” 1686. RIGHT: Sizes of planets and sun, from the 1698 ‘Cosmotheoros’ of Christiaan Huygens.

PART IV: In the 1800s, the idea of a Plurality of Worlds was challenged by the fact that, just as stars are not all other suns, planets are not all other earths — and that even on those planets that might be other earths, inhabitants might not naturally spring out of the ground.