The recent discovery of planets around neighbouring stars has given new topicality for a question that human beings have been asking ever since the origins of conscious thought. Are we alone in the universe? We find discussions in ancient text, notably those by the Greeks Democritus and Epicurus, then by the Roman poet Lucretius, about the question of the multiplicity of inhabited worlds. The idea of extraterrestrial life was reopened by Giordano Bruno in the 16th century. A victim of the Inquisition, he paid for his speculations with his life. Galileo, and then numerous astronomers and philosophers (Kepler, Kant, Huygens, Fontenelle, etc.) all picked up in their own time this doctrine which was held to be heretical by the Catholic church.