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.

Ancient Aliens

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.


The Search for Alien Life

It may well be that the promise of discovering life on other planets was what spurred Sir William Herschel to abandon a career in music and turn his attentions instead to telescopes. Inspired by Ferguson's astronomical writings (and, in all likelihood, Ferguson's lectures), Herschel became a tireless searcher of the skies, building hundreds of telescopes in hopes of getting a better view of our solar system. In 1781, he "discovered" Uranus (he wasn't the first person to spot it, but his observation led to it being correctly identified as a planet), and was also the first to observe Uranus' moons Titania and Oberon, and Saturn's moons Mimas and Enceladus. He discovered the existence of infrared radiation. But he never made one discovery he'd hoped for throughout his career: the discovery of alien life. Herschel believed that that the moon, planets, and even the sun were all inhabited, and his close observations of the moon in particular were performed in hopes of discovering life there. He sometimes would write that he had observed cities or vegetation on the moon, but in 1783, he confessed that, though he had dedicated years of his life to searching the moon, he had found no optical evidence that it was inhabited.


It's worth mentioning that Herschel's fellow astronomers didn't universally share his conviction that the moon was inhabited. When he submitted a paper on his lunar observations to the Royal Society, it included the line, "the knowledge of the construction of the Moon leads us insensibly to several consequences…such as the great probability, not to say almost absolute certainty, of her being inhabited." By this time, astronomers were already aware that the moon likely lacked an atmosphere, and when Nevil Maskelyne, the Astronomer Royal, inquired about the line, Herschel prefaced part of his argument for life on Mars with the demand that Maskelyne not call him a "Lunatic." But Herschel was still not alone in his lunarian beliefs. Johann Hieronymus Schröter, for example, would publish his Selenotopographische Fragmente in 1791 and 1802, arguing that a lunar atmosphere was possible (and in fact, he claimed to have observed a lunar twilight in 1792—though the twilight later proved an optical illusion) and that selenites might be perfectly situated for life on the moon.

But even as life on the moon was looking less and less likely, there were still scientists hoping to find nearby intelligent life. If they couldn't spy it with a telescope, why not send a message to our fellow sentients? Joseph Johann von Littrow, director of the Vienna Observatory, and famed mathematician Carl Friedrich Gauss have both been linked to early 19th century plots to create messages that would be viewed from space, although it's not clear that either man actually participated in these planned schemes. According to the Littrow story, Littrow suggested digging enormous ditches in the Sahara in various geometric shapes and then filling them with kerosene and setting them aflame. In the Gauss story, Gauss proposed building a heliotrope made up of 100 16-square-foot mirrors and then use it to send light to the moon, alerting the lunar residents to our presence.


This is the world into which the Great Moon Hoax was born.

Another element in the debate about life on other planets would literally fall from the sky. In 1859, chemist Friedrich Wöhler studied a meteorite and found that it contained a carbonaceous substance; the presence of organic material in the meteorite, he said, provided a "powerful argument" for life beyond Earth. (Ironically, a few decades earlier, Wöhler had proved that organic compounds could be synthesized in a lab.) A meteorite that landed in Orgueil, France, in 1864 was found to contain certain compounds associated with the decay of ancient life. The possibility of alien life was looking better—so good, in fact, that Sir William Thomson, then president of the British Association, proclaimed the organization's confident belief in many worlds containing life during his 1871 presidential address.


And, in the eyes of some astronomers, Mars would soon look like a likely candidate for life.

The Great Martian Canal Debate

So here's a story that I remember hearing time and again as a kid: In 1877, Giovanni Virginio Schiaparelli saw some squiggly lines on the surface of Mars and described them as canali, the Italian word for both "channel" and "canal." This caused an uproar among folks who assumed the canali referred to man-made canals. In my childish brain, I assumed the whole thing was cleared up later, but the truth is more complicated than that.


The truth is that astronomers began pointing their telescopes at Mars, hoping to see the same features that Schiaparelli had reported, but came up canali-less. Not only did these astronomers not believe that Martians had built the canali; they didn't believe the features existed at all. (Their appearance was actually caused by an optical illusion.) By 1882, the sole champions of Schiaparelli's canals were the French astronomer Camille Flammarion (in his pre-doomsaying days) and Schiaparelli himself, who by then actually began to suggest that the canali were, in fact, canals rather than channels. These claims for Martian life were apparently bolstered by bright flashes seen on the planet in 1888. Most astronomers figured there was a non-alien explanation for the flashes (there was; it was the reflection of sunlight on the mountaintops), but Martians began popping up in works of popular science. The journal Science published an article suggesting that the Martian seas were actually great swaths of vegetation, and Flammarion published La planète Mars et ses conditions d'habitabilité. Flammarion supposed that Mars housed a superior civilization, arguing that Mars was an older planet and thus life had had more time to progress.


Into this dubious fray stepped Percival Lowell, scion of an old Boston family. After reading Flammarion's book, Lowell became convinced that life must exist on Mars, and decided to devote himself to astronomy. He rushed out to Flagstaff to build an observatory (the Lowell Observatory) in hopes of catching the Mars opposition in 1894. When the opposition arrived, Lowell put eye to telescope in his unfinished observatory and mapped out 184 canals.


Lowell fueled a Martian mania among the general public, who largely believed his reports of the canals and wondered who might have built them. When astronomy journals no longer accepted his work, he would publish his findings in newspapers and magazines, where they seemed none-too-fantastical alongside tales of uncontacted peoples and expeditions to the South Pole. Among professional astronomers, Lowell picked up many detractors, notably the directors of the Lick and Harvard Observatories and even the President of Harvard himself, Charles Eliot. But, while some astronomers of the era would maintain their belief in a Martian civilization and others would declare Mars utterly uninhabitable, many were coming to the realization that they couldn't know for certain whether Mars was inhabited—at least not yet.

Tesla and Marconi Receive a Martian Message

We never did get around to digging those kerosene trenches in the Sahara. In 1909, William Pickering, a former associate of Lowell's, proposed erecting a giant mirrored surface (to the tune of $10 million) to use the sun's rays to signal to Mars. In a letter to the editor of the New York Times, Nikola Tesla outlined the flaws in Pickering's mirror plan, and revealed that he believed that the Martians had already made contact:

Personally I base my faith on the feeble planetary electrical disturbances which I discovered in the summer of 1899, and which, according to my investigations, could not have originated from the sun, the moon, or Venus. Further study since has satisfied me that they must have emanated from Mars. All doubt in this regard will be soon dispelled.


He had his own ideas for signaling the Martians, which naturally involved the use of his own wireless transmitters:

But there is one method of putting ourselves in touch with other planets. Though not easy of execution, it is simple in principle. A circuit properly designed and arranged is connected with one of its ends to an insulated terminal at some height and with the other to earth. Inductively linked with it is another circuit in which electrical oscillations of great intensity are set up by means now familiar to electricians. This combination of apparatus is known as my wireless transmitter.

By careful attunement of the circuits the expert can produce a vibration of extraordinary power, but when certain artifices, which I have not yet described, are resorted to the oscillation reaches transcending intensity. By this means, as told in my published technical records, I have passed a powerful current around the globe and attained activities of many millions of horse power. Assuming only a rate of 15,000,000, readily obtainable, it is 6,000 times more than that produced by the Pickering mirrors.


In 1919, Guglielmo Marconi, too, began receiving mysterious signals, and in 1921, the New York Times reported that Marconi believed the origin of those signals was certainly Martian. The United States Navy ended up participating in this hunt for the Red Planet dwellers; for three days in August 1924, every naval station in the Pacific ceased transmission so that the Martian signals could come in unimpeded. But there were no chatty Martians on the radio.

Looking Beyond Mars

Gradually, visions of real intelligent life on Mars was overshadowed by fictional depictions of Martians. And, over the course of the 20th century, our technology improved with better telescopes and more sophisticated methods for studying the planet. By the time we sent crafts to photograph and later explore Mars, we weren't dreaming of finding intelligent life so much as wondering if life was even possible there.


The universe may not be as teeming with life as Giordano Bruno imagined, but that hasn't stopped us from looking to distant and nearby celestial bodies in search of life. We've gone from schemes to flash light at Mars with giant mirrors to SETI listening for alien signals. We've abandoned the plurality of worlds and contemplated the significance of the Fermi Paradox. In the hunt for Martians and moon dwellers, we've expanded our understanding of our little corner of the universe and better prepared ourself to see if life could exist in our solar system and beyond. But the history of these early extraterrestrial hunters provides a valuable reminder that we must unravel the mysteries of the universe as they come, and not assume where we will find life and what form that life might take.

Further Reading:

Michael J. Crowe, The Extraterrestrial Life Debate, 1750-1900


David E. Fisher and Marshall Jon Fisher, Strangers in the Night: A Brief History of Life on Other Worlds

Giancarlo Gena, Lonely Minds in the Universe: The Search for Extraterrestrial Life

Christ Impey, The Living Cosmos: Our Search for Life in the Universe