Before the 20th century, some philosophers considered the possibility of the existence of intelligent aliens, from outside the Earth. We can cite Lucretius (De Rerum Natura, book II, 1st century BC), Nicholas of Cusa (15th century), and Giordano Bruno (16th century). The idea was happily adopted by science fiction writers, such as Lucian of Samosata (Vera Historia, 2nd century) and Cyrano de Bergerac (Comic History of the States and Empires of the Moon, 1656), of whom I spoke in another post.
During the 19th century, public attention focused on possible intelligent inhabitants of other bodies in the solar system, especially the moon and Mars. In 1835, The Sun newspaper published in New York six false reports declaring that flying men had been discovered on the moon. It is said that nine out of ten Americans believed it. In fact, The Sun had published a science fiction novel as though if it were real news, making reference to existing people, such as the astronomer Sir John Herschel. Near the end of the century, The War of the Worlds by H.G. Wells (1898) raised the possible existence of Martians, at the time of the scientific controversy over the canals of Mars, which was not finally solved until 1965.