Aliens and the Enlightenment

For thousands of years, it was widely accepted that humans occupied a special, central position in the universe. This confidence began to wane when Nicolaus Copernicus proposed that the Earth revolves around the Sun, introducing an exciting yet unsettling question: could life exist on other planets?

For French astronomer Jérôme Lalande, the answer was obvious. He argued that just as one wouldn’t assume some sheep in a distant flock have stones inside them instead of internal organs like humans, it stands to reason that planets resembling Earth are also inhabited. However, even among those who accepted this line of reasoning, uncertainties grew as more stars were discovered with the aid of increasingly powerful telescopes. Could other planetary systems exist, and might they support life as well?

Similar to Lalande, many writers of the time relied on analogy—a persuasive rhetorical tool—to make their case. Fueled by scientific discoveries, religious beliefs, and philosophical ideas, debates about the “plurality of worlds”—as it was known then—thrived throughout the 18th century.

Cosmic conversations

In 1686, animated debates spread rapidly when Bernard Le Bovier de Fontenelle, a young French poet, published Conversations on the Plurality of Worlds. This work presented a series of nocturnal dialogues between an articulate philosopher and a beautiful yet somewhat naive marquise as they strolled under the stars.

Although Copernicus had revolutionized the understanding of the cosmos nearly 150 years earlier, Fontenelle still found it necessary to underscore the merits of adopting a heliocentric model. At that time, Isaac Newton had yet to introduce his theory of gravity, so Fontenelle celebrated his fellow countryman René Descartes.

Descartes envisioned a mechanical universe teeming with swirling particles that perpetually collided, forming immense vortices of cosmic matter. Each vortex revolved around a central sun, suggesting that our solar system was just one of countless others extending across the universe.

Although René Descartes remained silent on the topic of life beyond Earth, Bernard Le Bovier de Fontenelle employed a similar analogical approach to that of Jérôme Lalande. He argued that invisible planets orbiting distant suns must also harbor life. Fontenelle’s bestseller was translated into English by three different individuals, including the playwright Aphra Behn. Annoyed by Fontenelle’s patronizing portrayal of the marquise, Behn discreetly modified the text by replacing references to “men” with “Men and Women.” She was also unsettled by the absence of God in the narrative. The marquise’s knowledgeable companion likened the cosmos to a vast theater operated from behind the scenes by an unseen puppeteer. For English readers, Behn added a bold preface criticizing Fontenelle for attributing everything to nature without mentioning God, noting that “one would almost take him to be a Pagan.”

Much like René Descartes, Isaac Newton wrote very little on the subject of extraterrestrial life. However, his followers interpreted this silence as tacit approval, and the belief in inhabited planets soon became a core tenet of Newtonian philosophy. Scottish mathematician John Keill, a leading advocate for Newton, borrowed Fontenelle’s theatrical metaphor to instruct his Oxford students, asserting that “all the Worlds or System of Worlds, are as so many Theatres.”

In contrast to France, where secular ideas were more prominent, God played a central role in British cosmology. Keill proclaimed that “we are to consider the whole Universe as a glorious Palace for an infinitely Great and everywhere present GOD.” Theologians and clergymen turned to the Bible for support, interpreting passages like “In my Father’s House are many Mansions, I go to prepare a Place for you” to suggest that “there are Worlds besides this whereon we inhabit; and it may justly be concluded, that they are inhabited by Beings who are far superior to us in Goodness.”

The concept of extraterrestrial life rapidly transitioned from a debated hypothesis to a foundational aspect of Enlightenment thought. Astronomers often quoted the line “An undevout Astronomer is mad” as if it were scripture, even though it originated from Night Thoughts (1742–45), a lengthy poem by Edward Young aimed at converting a skeptic through an imagined celestial journey. Depicting a vast universe filled with worlds of devoted worshippers, Young encouraged readers to marvel at God’s grandeur. Similarly, Alexander Pope reinforced the idea of multiple inhabited worlds in his Essay on Man (1733–34):

He, who through vast immensity can pierce,
See worlds on worlds compose one universe,
Observe how system into system runs,
What other planets circle other suns,
What varied being peoples every star,
May tell why Heaven has made us as we are.

Circular reasoning became widespread. John Adams, who would later become the president of the United States, argued that if the planets had slightly different initial velocities, “the Inhabitants would be tormented yea destroyed and the Planets left barren and uninhabitable Wastes by Extreme Vicissitudes of Heat and cold.” Therefore, he concluded, “an intelligent and benevolent mind” must have determined their actual velocities. Robert Chambers’ Cyclopaedia employed similar logic, suggesting that because the Moon resembles Earth, it must have rivers whose sole purpose is to support life akin to our own

Analogy and conjecture

Earlier Earth-centered cosmologies held that the universe was specifically designed for human beings. However, Newtonian thinkers challenged this view, arguing that it was merely “the old vulgar Opinion” to believe everything was made for man. Some went even further, proposing that God’s main purpose in creating countless stars was to provide more realms where intelligent life could exist to worship Him, in whatever forms that might take.

Using analogy, classicist Richard Bentley sought to reconcile Earth’s newfound insignificance with human superiority. He posited that just as Earth was created for human existence and contemplation, other planets might serve similar purposes for their own inhabitants endowed with life and understanding. Other theologians urged humans to adopt a more humble perspective. Henry Baker penned his poem The Universe (1727) expressly to curb human pride, asserting that our planet is so negligible compared to the vast universe that for us to consider ourselves the lords of all creation is as absurd as an ant claiming the entire Earth was made solely for it.

Another approach involved postulating an Aristotelian Chain of Being that ascended toward God. Relying on “analogy and conjecture,” Member of Parliament Soame Jenyns described subtle gradations leading from stones and plants up through birds and animals to “the brutal Hottentot,” then ever upwards to reach Earth’s pinnacle—a “Bacon or a Newton”—and eventually to “the inhabitants of other planets, to angels, and archangels.” Benjamin Franklin similarly suggested that the Infinite has created many beings or gods far superior to humans, who can better comprehend His perfections than we can.

The problem of Christ

By the late 18th century, even the most advanced telescopes had failed to provide conclusive evidence of life elsewhere. Debates persisted, but now some used science to challenge Christian theology. Thomas Paine bluntly questioned why, if multiple worlds exist, Christ would “quit the care of all the rest and come to die in our world, because they say one man and one woman had eaten an apple?” Literary figure Horace Walpole articulated his own conflict: “Fontenelle’s Dialogues on the Plurality of Worlds first rendered me an infidel. Christianity and a plurality of worlds are, in my opinion, irreconcilable.”

One solution was to propose that although God created many worlds, only ours was tainted by original sin. In Messiah, Friedrich Klopstock portrays Christ’s life on a corrupt Earth surrounded by countless worlds inhabited by innocent beings. Another argument suggested that humans should feel grateful for being singled out. Scottish moralist James Beattie declared that our fall and redemption might serve as an example to others, and that the divine grace shown in our redemption could elevate their adoration and gratitude to even greater heights.

In 1801, Edward Nares, a country rector and former Oxford Fellow, concluded that “those who have thought most soberly, and reasoned most coolly, have in few points been perfectly agreed. I am not so sensible of being able to decide any point in dispute.” The debates over the plurality of worlds offered ample opportunities to make definitive statements without the burden of providing solid evidence: after all, it was impossible to be proven wrong.