Today, we have rovers busily studying the surface of Mars, but 100 years ago, it wasn't uncommon for people to believe that intelligent Martians occupied the Red Planet, and 100 years before that, visions of Moon dwellers danced in many people's heads. Where did these ideas of nearby intelligent extraterrestrials come from? And why did we stop believing in them?
Top image by JD Hancock.
Lunar inhabitants turn up surprisingly early in the canon of Western literature. In the seventh century BC, Thales of Miletus described the moon as a spherical body, much like the Earth, providing people who dreamed of non-human intelligent life a perfect platform for their imaginings. Some followers of the philosopher and mathematician Pythagoras, among them Philolaos, claimed that the moon was populated by animals and plants more beautiful than those on Earth. (The animals, notably, were 15 times more powerful and for some reason produced no excrement.) In Lucretius' first century BC poem De rerum natura, the Roman philosopher states that, given the expansive nature of the universe, it is likely that life is not unique to Earth, but must exist elsewhere in the cosmos.
Famously, Lucianus of Samosata's satire True History appears on the scene around 177 AD and features a war between the denizens of the sun and the inhabitants of the moon. Of course, contrary to the title, Lucianus didn't actually believe in the existence of these particular aliens; he was just spoofing over-the-top travel tales and creating an ancient forerunner to science fiction in the process.
But as Giancarlo Genta notes in Lonely Minds in the Universe, other planets weren't necessarily a prerequisite for believing in alien life in ancient Europe. Philosophers of the Epicurean school believed in the existence of a plurality of worlds, which was to say a multitude of universes. Our reality, the theory went, isn't a special snowflake. There are bound to be other realms of existence, with their own forms of life.
E.T. and the Renaissance
During medieval times, folks like Thomas Aquinas and Dante Alighieri shaped the popular European view of the universe, but the Renaissance brought a new crop of thinkers who looked back to the Greek and Roman writers of old—and employed the developing science of astronomy. On the eve of the Renaissance, a pair of 15th century theologians, Nicholas of Cusa and William Vorilong, contemplated a multiple worlds view of the universe. Nicholas admitted that any speculation of life on other worlds was groundless, but still considered the possibility of intelligent beings living on the sun and the moon. Vorilong argued that there were reasons for believing God could create another inhabited world, but that left the problem of the inhabitants' redemption through Christ. Vorilong, for his part, argued that Christ's suffering and death could redeem an infinite number of worlds, "but it would not be fitting for Him to go unto another world that he must die again."
The research of Nicolaus Copernicus would challenge the geocentric vision of the universe, placing our planet and nearby celestial bodies around the sun instead. This would inflame the imagination of a 16th century Dominican friar, Giordano Bruno. Bruno's cosmology wasn't based on his own research; instead he used the writings of other philosophers and scientists to form his vision of the world. Remarkably, Bruno asserted that the stars in the night sky were suns just like out own, and that planets must revolve around them as planets in our solar system revolve around the sun. God, Bruno believed, had created innumerable solar systems with innumerable planets, including other life forms. Also, more dangerously, he asserted that God had no particular attachment to any portion of the universe; He was as present on Earth as he was in Heaven and in any other corner of the cosmos.
Bruno was eventually arrested for blasphemy, heresy, and immoral conduct and was subsequently burned at the stake in 1600. Because some of the charges did involve his then-peculiar cosmology, Bruno is often held up as a man who was executed because he believed in little green men. Most historians who study belief in the extraterrestrial, however, note that Bruno's belief in aliens was a minor charge compared to his rejection of the divinity of Christ and the accusations that he practiced diabolic magics—those latter charges are probably what got the man executed.
As the astronomical picture of our solar system became clearer, some of the scientists who peered through telescopes and read the reports of their fellows likewise found themselves wondering about the state of life on other planets. After reading Galileo's 1610 pamphlet Siderius nuncius, Johannes Kepler speculated on the possibility of life not on one of our immediate neighbors, but on Jupiter, writing:
Those four little moons exist for Jupiter, not for us. Each planet in turn, together with its occupants, is served by its own satellites. From this line of reasoning we deduce with the highest degree of probability that Jupiter is inhabited.
However, in his book Somnium, which imagines a student of Tycho Brahe transported to the moon, Kepler states that no planet is as well suited to life as the Earth. Galileo, for his part, was skeptical about the idea of plant, animal, or human-like life on other celestial bodies.
The Age of Enlightenment Opens with Aliens
Bernarnd le Bouvier de Fontenelle's 1686 book, Conversation on the Plurality of Worlds, proved an influential treatise on the notion of life on other planets. Fontenelle was a believer in explaining scientific concepts so that they could be understood by a lay audience, and Conversation on the Plurality of Worlds is seen as one of the early works of the Age of Enlightenment. Through it, he introduced a larger public to the Copernican model of the universe and the possibility of life on other world. Although the Conversation was ostensibly a scientific text (and earned the author an election to the Académie française) Fontenelle's descriptions of these possible aliens are enormously fanciful; he describes the inhabitants of Mercury and Venus as scorched by the sun (naturally, the Venusians are also "amorous"), while the denizens of Jupiter are "Flegmatik" and the moons function as colonies for the main planet. Thanks to Fontanelle's witty, conversant tone (it's a fun read, even for modern audiences), the book proved a runaway hit. It was endlessly reprinted and by 1800 was already available in English by 1688; by 1800, it was also available in Danish, Dutch, German, Greek, Italian, Polish, Russian, Spanish, and Swedish.
It's no wonder that so many educated people in the 18th century would state unequivocally that the various planets were inhabited. Consider the 1749 edition of Poor Richard's Almanack, in which Benjamin Franklin writes, "It is the opinion of all the modern philosophers and mathematicians that the planets are habitable worlds." Franklin may not have been completely right on that score, but as the Renaissance gave way to the modern era, belief in Martians, selenites, and other alien neighbors was quite common. In 1725, reverend and natural philosopher William Derham published a popular book attempting to reconcile modern astronomy with theology, including an assertion that the sun, planets, and comets were all inhabited. Immanuel Kant's 1755 Universal Natural History and Theory of Heaven included lengthy speculations on inhabitants on other solar worlds, although he forbade his publisher to include those speculative passages in the 1791 edition of his book, feeling that his youthful imagination had run too wild. In 1768, Scottish astronomer James Ferguson wrote An Easy Introduction to Astronomy for Young Gentlemen and Ladies, a dialogue that includes one character saying, "I cannot imagine the inhabitants of our earth to be better than those of other planets." Among Americans and Europeans of a certain class and education, belief in life on our neighboring bodies was the norm.