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One of the greatest debates in the long history of astronomy has been that of exceptionalism versus mediocrity—and one of the great satisfactions of modern times has been watching the arguments for mediocrity emerge triumphant. Far more than just a high-minded clash of abstract ideas, this debate has shaped the way we humans evaluate our place in the universe. It has defined, in important ways, how we measure the very value of our existence.

In the scientific context, exceptional means something very different than it does in the everyday language of, say, football commentary or restaurant reviews. To be exceptional is to be unique and solitary. To be mediocre is to be one of many, to be a part of a community. If Earth is exceptional, then we might be profoundly alone. There might not be any other intelligent beings like ourselves in the universe. Perhaps no other habitable planets like ours. Perhaps no other planets at all, beyond the neighboring worlds of our own solar system.

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MEDIOCRITY #3: In the heliocentric system of Nicolaus Copernicus, Earth is just third in a set of planets circling the sun. But there is comfort in being part of a family. Mikołaj Kopernik

If Earth is mediocre, the logic runs the other way. We might live in a galaxy teeming with planets, many of them potentially habitable, some of them actually harboring life. In the mediocre case, we bipedal little humans might not be the only sentient creatures peering out into the depths of space, wondering if anyone else is peering back.

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Today, the broadest version of exceptionalism has been thoroughly disproven, as astronomers have discovered 4,150 confirmed exoplanets, a tally that increases almost daily. The roster of alien worlds includes a remarkable variety of forms, many of which have no equivalent in our solar system. And that is just a limited sampling from the stars in our local corner of the galaxy.

We do not yet have the technology needed to find a close analog of Earth orbiting a close analog of the sun, so we still know little about how common or rare such worlds may be. The question of alien life is still wide open. What we do know is that the Milky Way is home to a tremendous number of other planets. In that sense, at least, we are certainly not exceptional, and Earth is certainly not alone.

The notion of cosmic mediocrity is so old that it predates modern observatories. It predates the 17th-century invention of the telescope. It predates even what could recognizably be called “science” in the modern sense, tracing its origins at least back to the Greek philosopher Anaxagoras of Clazomenae, writing and teaching in Athens in the 5th century B.C.

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Anaxagoras proposed that the cosmos is ruled by an all-pervasive intellect that he called nous, which functioned as a set of universal laws—a philosophical ancestor of Isaac Newton’s theory of universal gravitation. Under the action of nous, the elements of nature were set into circular motion, separating into different components. The sun, a ball of incendiary metal, was cast off into the sky by this process. So, too, were the stars and planets. Although what survives of Anaxagoras’s writing is fragmentary and mostly secondhand, it seems that he imagined the stars to be fiery lumps much like the sun, just drastically more distant. In one especially intriguing passage, he further hints at the existence of other lands similar to Earth and expansively argues “that there are a sun and a moon and other heavenly bodies for them, just as with us.”

The question of alien life is still wide open. What we do know is that the Milky Way is home to a tremendous number of other planets.

Many of these ideas reappeared in even more modern-looking style in the philosophy of Aristarchus of Samos. During the 3rd century B.C., Aristarchus advanced the first known heliocentric model of the solar system, evicting the Earth from its long-assumed central position and completely reworking the order of the cosmos. There is no surviving description of this iconoclastic model in Aristarchus’s own words. Fortunately, his contemporary Archimedes provided a succinct summary:

His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun in the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.

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That final idea, though somewhat obscure in its phrasing, is pregnant with significance. Aristarchus is saying that the stars are so far away that we cannot see their parallax: They appear stationary even as the Earth moves in a great circle around the sun. The implications are twofold. First, he imagined a cosmos vastly larger than the one implied by the geocentric system. Second, he reiterated and expanded on Anaxagoras’s deduction that the stars might be other suns, this time explicitly spelling out the kinds of grand distances necessary for the stars to nevertheless appear as fixed, cold dots in our sky.

The budding possibility of a multitude of worlds fully blossomed in the philosophy of the Greek atomists, most notably Epicurus. They envisioned not just other stars but other entire kosmoi (cosmic systems) beyond the one we know, each following the inexorable rules of the atoms it contains. Writing at about the same time as Aristarchus, Epicurus declared that “there is an infinite number of worlds, some like this world, others unlike it. For the atoms being infinite in number … are borne ever farther in their course.” His atoms were mathematical and ethical constructs, quite unlike the physically described quantum units of today’s physics, and yet in the way Epicurus reached toward a boundless universe he sounds shockingly prescient.

MANY WORLDS: Recent studies indicate that there may be a trillion planets in our galaxy—and then a trillion other galaxies in the observable universe.NASA, ESA, ESO / M. Kornmesser

That pinnacle of glorious Epicurean mediocrity, alas, was followed by a lengthy retreat back into a constricted, Earth-centered cosmology. Aristotle retorted that “there cannot be more worlds than one,” and his great authority carried the day. Around 150 A.D., Claudius Ptolemy shrank even further from the kosmoi when he merged Aristotelian physics with state-of-the-art observations of stars and planets into a unified, Earth-centered model. The Ptolemaic system consisted of a set of nested celestial spheres, dispensing with exotic speculations about infinite space and other suns. By Ptolemy’s reckoning, the outermost crystalline sphere containing the fixed stars was about 20,000 times the radius of the Earth, making his entire cosmos just 160,000 miles wide in modern terms.

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What the Ptolemaic system lacked in grandeur, it made up in practicality. It predicted the motions of the planets and stars with admirable precision using a combination of mathematically appealing circular motions. Ptolemy’s astronomical writings, later translated by medieval Islamic scholars as the Almagest (literally “the greatest”), reigned supreme for more than a millennium. His authority was cemented when prominent theologians like Thomas Aquinas merged the Ptolemaic system with the Roman Catholic worldview during the Middle Ages. The outermost sphere of the cosmos equated with heaven; the Aristotelian “prime mover” that set the spheres in motion became one and the same with the Christian God.

The same attributes that make exceptionalism appear impoverished from a scientific perspective made it precious from a theological point of view: only one Earth, one heaven, one God. But the fire of human imagination is not so easily snuffed. Some medieval Islamic astronomers continued to speculate about the existence of other worlds. Catholic scholars, too, pushed against the boundaries. Around 1450—a full century before the mystical speculations of Giordano Bruno—the German philosopher and astronomer Nicolas of Cuna wrote about the notion of infinite space, in contradiction to Ptolemaic concepts. Nicolas framed his ideas within a Catholic framework, exploring infinity as a natural corollary to the limitless glory of God, but his philosophy kept alive the possibility of a physically unbounded universe as well.

Then along came Nicolas Copernicus, and mediocrity began a full-on comeback.

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From outward appearances, Copernicus was an unlikely figure to knock the solar system askew and to set astronomy on its modern path to a multitude of planets. He worked as a canon in Warmia, a small, semi-autonomous Catholic state in what is now Poland, tending to various local political and economic disputes. He was a modest, well-liked figure, not particularly known for his controversial opinions. Professionally, his most notable achievements were probably in economics and monetary theory. There was a spark within that set him apart, however: the bold, revisionist astronomical ideas brewing in his head.

Sometime before 1514, while he was still in his 30s, Copernicus wrote a summary of his new model of the solar system. Influenced by the arguments of Aristarchus, as well as by his own strong sense of the mathematical ugliness of the Ptolemaic system, Copernicus returned the sun to the center and set the Earth in motion about it. He circulated his short document, called the Commentariolus, among his friends, with the intention of expanding its arguments into a fully developed work of heliocentric cosmology. That magnum opus, De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) famously was not published until 1543, when he was on his deathbed. Copernicus was unconscious when a finished copy was thrust into his limp hands, and died that same day.

The first U.S. and Soviet space probes almost uniformly made the solar system seem shockingly hostile to life.

The publication delay was not, as popular accounts often claim, a simple matter of Copernicus’s fear of the Catholic church. He was more afraid of the Church’s intellectual partners, the Aristotelian philosophers, whom he worried (not unreasonably) might be brutal to this upstart living far from the intellectual heart of Europe. He also needed to perform detailed mathematical analysis and to collect astronomical observations in support of a theory that he was developing only in his spare time. Only in retrospect do those fears look absurd. It turns out that the time was ripe for a critical reexamination of entrenched classical Greek thinking. In the decades after its publication, De Revolutionibus was extensively read and discussed across Europe. The influential Danish astronomer Tycho Brahe even described Copernicus as “a second Ptolemy.”

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Two disciples of Copernicus were especially pivotal in establishing Copernican mediocrity—the notion that Earth does not sit in a privileged position, but rather is representative of the richness of the universe as a whole. In 1575, Thomas Digges, a leading astronomer in 16th-century England, published the first English translation of De Revolutionibus. He added commentary to clarify that the Copernican system was a physically realistic model of the solar system (not just a computational trick), and he overtly broached the idea that a sun-centered universe could be infinite in extent. To drive home this last point, Digges created a drawing showing, for the first time in history, how the stars might be scattered through endless space outside our solar system.

A few years later, the German astronomer Michael Maestlin adopted Copernicus’s system as superior to Ptolemy’s, and spread heliocentric thinking broadly from his prominent position as a teacher at the University of Tübingen. Most notable among his students was a clever young fellow named Johannes Kepler, who starting in 1609 figured out that planets go around the sun in elliptical paths. This discovery thoroughly and finally smashed Ptolemy’s claustrophobic crystalline spheres. The universe was now wide open to all possibilities, and to endless worlds.

From there, the concepts of Copernican mediocrity spread with astonishing rapidity—historically speaking. By the middle of the 17th century, heliocentrism was widely accepted across the Western world. By the 18th century, many leading intellectuals embraced not only the idea of other worlds, but even other inhabited worlds. Cyrano de Bergerac’s Comical History of the States and Empires of the Moon, published in 1657, introduced the reader to imaginary inhabitants of the moon. Jonathan Swift’s Gulliver’s Travels (1726) and Voltaire’s Micromegas (1752), whose central character is from a planet orbiting the star Sirius, casually assume a multiplicity of inhabited worlds as a backdrop to their social satire. Mediocrity was in vogue.

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On the scientific side, William Herschel (perhaps the most famous astronomer of the late 18th and early 19th century) was a firm proponent of the idea that life is common on other planets. Right around the time that he discovered the planet Uranus, Herschel shared what he believed to be telescopic evidence of intelligent life on the moon. He later argued that all worlds might be inhabited; improbable as it sounds, he even suggested that there is life on the sun, huddled beneath the luminous clouds covering its surface.

RED AND DEAD: The Mariner 4 probe flew past Mars in 1965, revealing a landscape that looked cratered and lifeless. NASA-JPL

Although many other researchers were not so enthusiastic, each generation found its champion of life beyond Earth. American astronomer Percival Lowell was especially effective at promoting such ideas well into the 20th century with his popular (if increasingly eccentric) writings about an imperiled advanced civilization on Mars. Science-fiction writers like Ray Bradbury, Arthur C. Clarke, Isaac Asimov, and Robert A. Heinlein further popularized many-worlds mediocrity with their compelling visions of alien beings on far-off worlds.

That optimism suffered a major setback with the advent of the actual Space Age. Data from the first U.S. and Soviet space probes almost uniformly made the solar system seem shockingly hostile to life. NASA’s Mariner 2 flew past Venus in 1962 and found that the planet is not steamy jungle at all; rather, it is an Earth-size sterilizing oven, with a crushing atmosphere and surface temperatures hovering around 800 degrees Fahrenheit. Two years later, Mariner 4 flew past Mars and beamed back images of a barren, cratered landscape to crestfallen planetary scientists. In 1976, NASA sent the twin Viking landers to Mars to do a Hail-Mary search for life right there on the surface. The $1 billion effort, equivalent to $5 billion today, yielded no conclusive signs of anything alive.

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In the decade after Viking, the astrobiologist Carl Sagan energetically raised the possibility that many habitable worlds could exist undetected around other stars, but attempts to find such “extrasolar planets” proved a bust again and again. For a few decades, it seemed possible that Earth was a genuine outlier. The march of mediocrity resumed only in 1995, with the first unambiguous detection of a planet around another sunlike star. It was a weirdo—bigger than Jupiter, hotter than Mercury, clearly unsuitable for life—but the discovery provided the scientific confidence needed to gain approval for the big-budget Kepler space telescope and its successors, including the new TESS (Transiting Exoplanet Survey Satellite).

Those missions indicate that there could be more than a trillion planets scattered through our galaxy, including many billions of them similar to Earth in size and temperature. Sara Seager of MIT, the deputy director of the TESS mission, has publicly set a lifetime goal of finding 500 planets similar to Earth. “If we’re lucky, maybe 100 of them will show biosignatures,” she says, referring to the data readings that would indicate the presence of life.

With a sample that size, scientists could compare the different types of worlds that support life, different styles of metabolism, and different stages of evolution. They could navigate to whole new levels of mediocrity, exploring Earth’s place within an entire pantheon of inhabited worlds. But identifying even a single other living world would deliver an unprecedented connection between humanity and the rest of the universe.

There is no way to know when such a discovery will happen. There’s no way to be certain it will happen at all. But today, more than ever before, the prospect of cosmic mediocrity spreads wide open and inviting before us.

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Corey S. Powell is an Earth-based science writer and editor, and co-host of the Science Rules! podcast. He is a frequent presence on Twitter: @coreyspowell

Adapted excerpt from The Lost Planets: Peter van de Kamp and the Vanishing Exoplanets around Barnard’s Starby John Wenz, foreword by Corey S. Powell. Copyright © 2019 Massachusetts Institute of Technology.

Lead image: NASA’s retro-style poster celebrates the seven Earth-size planets recently found around the nearby red dwarf star TRAPPIST-1. Credit: NASA-JPL / Caltech

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