 At the time, such speculation was of a rather rarefied sort, and was limited to astronomers like Christiaan Huygens who wrote a book, "Cosmotheoros" (1698) considering the possibility of life on other planets; or to philosophers like Campanella, who wrote in defense of Galileo.The concept of life on distant planets was not, however, much utilized in fiction.The most popular target of 17th century "science fiction" was the Moon ("visited" in fiction by Kepler, Godwin, Cyrano, and Defoe).