It is the sunset of the 1600s. Milton has just pioneered the use of the word space to connote outer space. Kepler has just pioneered science fiction by imagining space travel, but going only as far as the Moon. Gravity is a brand new concept and the notion of a galaxy is still more than two centuries away . The universe is as big as our Solar System, which has six planets orbiting a sun we have only just conceded, after burning the seers at the stake, does not revolve around us. Against this backdrop, having set the Scientific Revolution into motion with his landmark contributions to optics, mechanics, and astronomy, the Dutch polymath Christiaan Huygens has just finished his most daring work: Cosmotheoros: or, Conjectures Concerning the Inhabitants of the Planets — our world’s first treatise speculating on the existence of life on other worlds not from a theological but from a scientific standpoint.…