During the 17th century, as knowledge of the Universe and its contents increased, so did speculation about life on other planets. One such source, as Hugh Aldersey-Williams explores, was Dutch astronomer, mathematician, and inventor Christiaan Huygens, whose earlier work on probability paved the way for his very modern evaluation of what alien life might look like.

When things look bleak in this world, it is perhaps natural to turn one's mind to conditions on other worlds. This is what the Dutch astronomer Christiaan Huygens did in the late 1680s. He had been ejected from his influential post as a government scientist of Louis XIV in Paris and found himself isolated back home in the provincial town of The Hague, frequently ill with depression and fevers, and missing the companionship of his brother Constantijn, who was away serving as secretary to the Dutch King William III in England.

It was then that Huygens began to write Cosmotheoros, a book-length speculation on the possibility of life on other planets, and the first such work to be based on recent scientific knowledge rather than philosophical conjecture or religious argument. Fearful of censure by “those whose Ignorance or Zeal is too great”,1 Huygens instructed his brother to publish the work only after his death, which he did in 1698. Originally written in Latin, Cosmotheoros was quickly translated into Dutch and other languages. A lively English translation appeared that same year under the audacious title, The Celestial Worlds Discover’d.