Star Bound №3: Pluto Doesn’t Live Here Anymore
The Long Strange History of Our Search for Planet Next
We all have prejudices.
The one I regret most is my inclination to believe that what I learned yesterday — not literally yesterday, but fifty years or so ago, in grade school — is still true today. Because I’m human, I have a deep-seated urge to hang on to the things that make me me: ancient Aerosmith lyrics, memories of my first bicycle, a swim meet ribbon I won so long ago that the red has faded to pink. In this spirit I’ve kept a collection of outdated concepts in the back of my head like a pile of grass-stained sneakers. Unfortunately, this has made me something of a scientific idiot, since all kinds of things I learned in school were incomplete, misleading, or completely wrong. It turns out we weren’t just a few years away from landing on Mars. Nuclear power hasn’t solved all our energy problems. And don’t get me started on flying cars.
But I’m trying to catch up. Take Pluto, for example. I grew up knowing Pluto was the ninth planet. It was small and a little odd, like my friend Richie Weingarten, and it seemed fitting that Pluto was out there in right field, at the bottom of the batting order, where it couldn’t hurt anyone. After all these years, I’m still reluctant to let anyone claim it’s not the ninth planet. And yet schools these days are teaching that Pluto is no longer a member of the club. In fact, schools have been teaching this for well over a decade. Pluto has been demoted. And just as I think I’m ready to get my head around this factoid, someone comes along and says, Wait, there IS a Planet 9!
It’s just not the orb you think it is.
So what’s the story? Is there a new Planet 9? What happened to the old Planet 9? And in an era when scientists are using space-based telescopes to study planets orbiting stars in distant constellations, when are we going to be able to say for sure that we know how many planets are in our own solar system? The answer is more complicated than you might think — and trust me, I hate that as much as you do. But really, when it comes to the cosmos, isn’t the answer always more complicated than we might think? The more we look, after all, the more we see. And science aside, talk about the possibility of a new planet lurking in trans-Neptunian space is just the latest way of expressing a deep-seated human fascination with the unseen — the hidden, or “occult” — that may never really die.
The Basics
Counting the planets in our solar system is a matter of both technology and terminology. The technology part is easy to understand. As our ability to see farther out into space has increased, so too has our knowledge of what’s out there: Neptune’s moons, Uranus’s rings, comets crashing into Jupiter. Terminology is a little tougher. There are millions of objects circling the sun, from the largest — Jupiter, with 120 times more surface area than Earth — to any number of jagged, bowling ball-sized asteroids and fragments thereof. The definition of “planet,” and what fits within it, is still being debated today. And even as we debate, the search for far-flung residents of our solar neighborhood continues.
The ancient Greeks studied the stars as if they were paint-by-numbers portraits created by the gods, elliptical renderings of heroes and monsters caught mid-motion in the crowded sky. Amidst this fixed and glittering panorama they counted seven planets, or “wanderers”: the sun and the moon, Mercury, Venus, Mars, Jupiter, and Saturn. In the third century B.C., the brilliant Aristarchus of Samos theorized that the earth revolves around the sun. He was, as they say, ahead of his time. Way ahead. The easier-to-envision Earth-centered cosmology of Plato, Aristotle, and Ptolemy prevailed instead. The notion that the sun revolved around the earth became dogma, and guided the Western mind for almost two millennia. It wasn’t until the 16th Century A.D. that this started to change. Copernicus and Galileo, scientists so great they only needed one name, like Brazilian soccer players, became convinced, like Aristarchus, though for different reasons, that Earth was the wanderer, rather than the sun. Further, the moon wasn’t a planet either, but rather a satellite of Earth.
This shift in our conception of the heavens didn’t come easily. When Galileo became a vocal proponent of Copernicus’s ideas over the old Ptolemaic model, he was put on trial and forced to renounce his “heresy” against the laws of God and Nature as understood by the Roman Catholic Church. Trial for heresy was a serious matter in those days. Galileo’s contemporary Giordano Bruno, also an advocate for Copernican ideas, was found guilty of various unorthodoxies in 1600 and burned at the stake. Galileo avoided this fate by recanting. Even so, he spent the rest of his life under house arrest, a mind — and a mouth — too dangerous to be left at large.
And yet the shift occurred. Thanks to the first relatively primitive telescopes, modestly sized instruments with curved, or refracting, lenses, the facts were plain to see. And they were astonishing. As the historian Michael Hoskins writes:
Galileo saw with his telescope wonders vouchsafed to no one before him: stars that had remained hidden from sight since the Creation, four moons that orbited the planet Jupiter, strange appendages to Saturn that would be recognized as rings only half a century later…mountains on the Moon not very different from those on Earth, even spots on the supposedly perfect Sun.
The transit of Venus across the sun was first observed via telescope in 1639. It helped support the notion that the sun was relatively static and the planets, including Earth, were in motion. We know now that nothing is static and that the entire universe, including the sun, is moving, but ascertaining the relative relationship between the planets and the sun was a hugely important step in our understanding of our place in the cosmos.
The new discoveries were important. But it wasn’t just what was discovered that resonated. It was the fact that suddenly it seemed there was so much about the universe still to learn, and the corollary that perhaps humanity wasn’t as smart as it had previously assumed. William Herschel discovered Uranus in 1781, as what we in America call the Revolutionary War dragged on. German by birth but enthusiastically British by inclination, Herschel was a gifted man whose list of accomplishments, like those of his contemporaries Ben Franklin and Georges Buffon, is difficult to comprehend. Before he became an astronomer, Herschel was a musician and composer, capable of writing pieces for oboe, clarinet, violin, and organ. Though he wasn’t a biologist, he was the first man to demonstrate that coral is not a plant but rather an invertebrate animal. He seems to have taken up astronomy as a hobby.
Whatever the impetus, he soon became obsessed with stargazing and the mechanisms needed to do it. He ground his own mirrors, sometimes for as many as sixteen hours a day, and constructed his own telescope — a Newtonian reflector model with a 6.2-inch aperture and a seven-foot focal length. Working with his sister, Caroline, who became a celebrated skywatcher herself, he set about cataloguing binary stars. He wasn’t trying to find a planet. No one suspected there were any out there to find. In fact, Herschel figured at first that he’d identified a comet. But his find was too big, and too slow, to be a comet, and the scientific community eventually realized that Herschel was the first person in recorded history to identify an entirely new “wanderer” — though it turned out that his discovery actually had been seen before, and mistaken for a star.
As well as providing an easy pun for generations of jokesters, the name of Herschel’s planet presents a bit of a puzzle. Everyone knows who Jupiter and Venus were. Uranus is more obscure. Uranus (properly pronounced UR-an-us) is the only planet to bear the original Greek appellation for a god; all of the others, except for Earth, bear the Roman names of the Greco-Roman deities. Though he originally wanted to name his discovery after King George III — the same monarch who struts and preens so memorably through the musical “Hamilton,” spitting out nastygrams to us colonial upstarts — Herschel eventually chose to honor instead Ouranos, the sky god.
Uranus, as we now spell the name, was the father of Saturn and thus the grandfather of Jupiter in classical mythology. These three planets therefore have a tidy sort of mythological lineage, with son, father, and grandfather appearing in order of their distance from Earth. Other facts: Uranus has 27 fairly small moons, which are named for characters in the works of Shakespeare and Alexander Pope, with the five main satellites being Miranda, Ariel, Umbriel, Titania, and Oberon. The Uranian satellite system is the least massive among those of the giant planets. In fact, the combined mass of its five largest satellites would be less than half that of Neptune’s largest moon, Triton. And Uranus has rings — faint rings, to be sure, but rings nonetheless, made mostly of water ice.
Like Sam Phillips on the day Elvis walked into Sun Records to cut a demo, William Herschel was a man who found himself presented with the answer to a question he hadn’t even thought to ask. Reaction to the discovery of Uranus was rapid and overwhelming, and the prospect of finding another huge but previously unknown celestial wanderer — some heavenly sphere so massive as to seem godlike in its transit through the cosmos — has motivated astronomers, scientists, theorists, and outright quacks ever since. Soon a small army of astronomers, professional and amateur, was watching the skies. In 1801, an Italian named Guiseppe Piazzi spotted a large sphere orbiting the sun in the vast region of space between Mars and Jupiter. Astronomers had long wondered why there was such a gap. According to a numerical sequence which came to be known as Bode’s Law, and which was apparently more an encapsulation of coincidence than a law, the planets could be slotted into particular, predictably greater distances from the sun. Bode’s Law indicated that there ought to have been a planet between Mars and Jupiter. This area was therefore a logical place to look. In fact, given its shape and size, Piazzi’s discovery was widely considered to be a planet. He named the sphere Ceres, for the Roman goddess of agriculture and fertility. The year after Piazzi’s find, a German astronomer named Heinrich Olbers discovered another large object between Mars and Jupiter. He named his discovery Pallas, for Pallas Athena, the Greek goddess of wisdom and warfare. Wisdom and warfare might seem like two very different specialties to us, but they were evidently viewed differently by the Greeks. Why an Italian would name a celestial body after the goddess of fertility and a German would name a similar object after the goddess of war is unclear, but it didn’t bode well for the future of Europe.
Similar large objects Juno and Vesta were spotted later in the early 19th Century — and, like Ceres and Pallas, were considered to be planets, for lack of a better designation. For years in the early 19th Century, therefore, there were 11 planets in the commonly accepted solar system. By 1851, 15 more large orbital masses were found in the Mars-Jupiter corridor. By 1901, the number had climbed to over 500. There are now thought to be millions of these objects. Observations indicated that none of the original discoveries were actually as large as originally believed, and that they were all irregularly shaped, with the exception of Ceres, which is spheroidal. Eventually the name asteroid (or “star-like”) was proposed for such objects, and Ceres, Pallas, and their larger companions in what we now call the asteroid belt were demoted from planetary status. As we shall see, a similar demotion would take place a century and a half later.
Interestingly, while several constituents of the asteroid belt lost their status as planets, the belt itself became the basis for a hypothetical “fifth planet” (fifth from the sun, that is) which was eventually dubbed Phaeton. Phaeton, son of Helios, is a minor figure in Greek mythology, best known for a single story. Young Phaeton badly wanted to prove himself to the gods. He therefore tried to drive his father’s sun chariot — the horse-drawn cart that pulled the sun across the heavens. Unfortunately, he wasn’t up to the task. He lost control of the chariot’s divine steeds, which flew too low and scorched the earth. Angered by the destruction Phaeton was causing, Zeus killed the youth with a lightning bolt.
Phaeton’s tale, then, is a story of sudden destruction in the sky. This is apt, because some scientists have theorized that the asteroid belt was formed by the destruction of a large planet — Phaeton — that formerly occupied the space between Mars and Jupiter, but which was smashed to pieces many millions of years ago by a collision with a comet or asteroid. Actually, there have been a number of causes advanced for the demise of Phaeton, and there are scientists who still think the “sudden impact” scenario is possible, though not likely. Rather, most astronomers now suppose not that there was a violent disruption of an existing planet in the Mars-Jupiter corridor, but rather that the right conditions never formed for accretion of the various asteroids into a single mass in the first place. In other words, Phaeton isn’t a former planet, but rather one never existed at all, like an IKEA box full of parts for a desk that no one ever bothered to assemble. (Not that I would know about anything like that.)
While the search for planets grew more competitive, astronomers were in the meantime busy finding other marvels in the heavens as well. As telescopes grew larger, and the lenses and mirrors inside them more precise, the solar system began to reveal itself. In 1610 Galileo had used a nine-inch refractor telescope he’d made himself to spot Io, Europa, Ganymede, and Calisto, four of Jupiter’s many moons. By 1677 Edmond Halley was cataloging the stars of the southern hemisphere from a perch on the island of St. Helena using, among other devices, a refracting telescope with a focal length (in crude terms, a “barrel,” or “tube”) of some twenty-four feet. William Herschel built the famous “Great Forty-Foot Telescope” with royal backing in the late 1780s, and used it to find two moons of Saturn and individual stars within various nebulae. (Construction of the instrument gave rise to one of the great science quotes. When King George III prepared to duck into the long, tunnel-like shaft of the telescope to inspect the device, he turned to a bishop of the Anglican church who accompanied him. “Come, my Lord Bishop,” he reportedly said. “I will show you the way to heaven.”) A fifty-four foot-long reflector telescope with a six-foot aperture, the so-called “Leviathan of Parsonstown,” was constructed in the center of Ireland in 1845 and reigned as the world’s largest such instrument for many years. Using a 56-foot long, 26-inch-aperture refracting telescope at the U.S. Naval Observatory, American Asaph Hall first identified Deimos and Phobos, two moons of Mars, in 1877. The largest-ever refractor telescope, with a 40-inch main lens housed in a 60-foot tube, was installed in the University of Chicago’s Yerkes Observatory in 1897. Gradually, though, reflector telescopes, of a general type first proposed by Sir Isaac Newton, became standard for serious observation. These instruments use mirrors rather than lenses to gather and focus light. They can function with larger aperture sizes and shorter tubes, and are not subject to chromatic aberration problems that plague refractors. The Kcck Observatory in Hawaii has two reflector telescopes with 400-inch main apertures.
Despite the interest aroused by the discovery of each new moon of Jupiter or faraway comet by one of these giant telescopes, the stargazer’s holy grail, the great white whale, was a new planet. And it wasn’t always the size of your harpoon that counted in this quest. In 1846, for example, a French astronomer named Urbain LeVerrier solved a puzzle that had stumped sky watchers for a quarter-century. Uranus’s observed orbit didn’t seem to correspond with mathematical predictions. It was slightly off. The planet moved too quickly at some points, too slowly at others. Contemporary astronomers theorized that some large cosmic object, invisible from earth, must be exerting a gravitational pull on Uranus.
Given what was known about the mass of Uranus and the way its orbit wobbled, LeVerrier set out to calculate where this mysterious disruptive object must be. Remarkably, he was proven right on the very first night that observations based on his mathematics were made, when a German astronomer named Johann Gottfried Galle located the faint object we know as Neptune, for the mythological god of the oceans. Neptune was the name that stuck, anyway. LeVerrier at one point suggested naming the planet after himself, a move that was supported in France, particularly in the LeVerrier household, but not so much elsewhere. So, Neptune: third largest of the planets, a cerulean blue giant whose surface is scoured by supersonic winds of up to 1200 miles per hour, as if populated by a race of tornados with a methamphetamine problem. The planet is circled by fourteen moons, all named for nymphs of the sea. Tilted in its orbit at almost the same angle as the earth and Mars, it has similar “seasons” — except that Neptune’s seasons last something like forty years. Winter is coming indeed.
Attempts to use mathematics as a conceptual divining rod for previously undetected celestial bodies continued. In the 19th Century some astronomers, including the redoubtable Urbain LeVerrier himself, grappled with the fact that Newtonian physics seemed unable to explain the fact that Mercury’s perihelion — its closest approach to the sun — advanced in its orbit by a small amount each year. LeVerrier theorized that a hitherto unknown planet, or possibly a number of asteroids, existed between the sun and Mercury, and that the presence of this mass exerted enough gravity to cause Mercury’s misbehavior. Indeed, despite the obvious difficulties involved in spotting anything against the glare of the sun, there were purported sightings of the hypothetical planet, which scientists of the day named Vulcan, after the Roman god of the forge. By 1876, indeed, The New York Times declared the matter settled; Vulcan occupied an orbit between Mercury and the sun. And yet, despite the conclusions of journalists, astronomers continued to question the reported sightings. They proved difficult to verify, or turned out to be something other than a planet. Worse, most reputable observers were unable to find any trace of the mysterious little planet at all, and scientists eventually became convinced that Mercury’s behavior was a riddle with no immediately available solution. Vulcan was the dark matter of its day.
As detailed in Thomas Levenson’s The Hunt for Vulcan…And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe, the Mercury problem vexed none other than a young patent clerk-turned-academic named Albert Einstein. After a series of fits and starts in his development of the theory of general relativity, the theory that radically recast our understanding of what gravity is and how it works, Einstein was able in 1915 to formulate a mathematical explanation for Mercury’s perihelion advance that disposed of the need for a mystery mass. Objects in space, he argued, don’t travel on flat planes like the balls on a pool table. Rather, matter “curves” the space-time around it. The sun’s large mass creates a commensurately large curve, or gravity well, into which Mercury travels in its orbit, and the small changes in Mercury’s orbit reflect the influence of this distortion. Einstein’s theoretical work was confirmed by observations in 1919. It was a knock-out blow, a mathematical uppercut from the slipper-clad bantamweight, and Vulcan eventually faded from popular consciousness and astronomical imagining, a planetary chimera that was never really there. Just as LeVerrier had found a planet with his pen, Einstein disposed of one the same way.
Interestingly, though, there is a planet “Vulcan” orbiting another sun. That sun is 40-Eridani A, approximately 16 light years from earth. Some years ago, Star Trek creator Gene Roddenberry identified 40-Eridani A as the anchor of the solar system in which his fictional Mr. Spock’s Vulcan race resides. In 2018, scientists located a planet orbiting 40-Eridani A, and believe it might actually be habitable. Inevitably, some are now calling the newly found exoplanet “Vulcan.” In Andy Weir’s recent opus Project Hail Mary, a five-armed spider-like creature named “Rocky” hails from Vulcan’s solar system.
Planet X
While our solar system’s Vulcan didn’t survive past the early years of the 20th Century, a more durable mystery planet did. This one was located in another realm altogether — not between the sun and Mercury but rather far beyond Neptune. Starting around 1906, Percival Lowell, a wealthy New Englander, studied apparent eccentricities in the orbit of Neptune and Uranus and concluded that another large object, a so-called “Planet X,” must be roaming the skies beyond the eighth planet. Lowell had a prominent forehead, a boomerang-shaped moustache, and thinning hair that rose in tufts on either side of his scalp, like a pair of genteel imp’s horns. He was a big thinker with lots of money, a sort of Elon Musk of his day, minus the rockets. His musings attracted significant media attention. Lowell’s most popular theory involved the notion that Mars had “canals,” constructed by an ancient race of sentient beings in an attempt to bring water from the planet’s polar ice caps to other portions of the globe. This notion was unfounded, as it turned out, but it exerted enormous influence on imaginative Americans, particularly American writers. Ray Bradbury’s The Martian Chronicles, which was more or less required reading for sci-fi fans during the middle years of the 20th Century, deals extensively with the idea of canals on the red planet.
Lowell’s Planet X theory was similarly seductive. The Science History of the Universe, published in 1909, cites the work of astronomer W. H. Pickering, who calculated that the mystery planet, which he called “Planet Zero,” would be 52 times as far distant from the sun as the earth, and that its mass would be roughly twice that of Earth. Pickering’s reputation was brilliant but not unblemished. He earned plaudits, for example, for “discovery” of a moon of Saturn that turned out not to exist after all. He is also said to have discounted the possibility that airplanes could ever be used to drop bombs, and claimed at one point to have spotted vegetation on the moon. Despite predictive missteps by some important supporters of the Planet X theory, the possibility of a mystery planet took root in the scientific imagination. A Nebraska newspaper reported in 1919, under the headline “New Planet Causing Trouble,” that “a new planet is believed to be responsible for irregularities in the motion of the planet Neptune,” and added that astronomers were hoping to photograph the sphere later that year. Some astronomers indeed predicted discovery of not one but two planets out beyond Neptune. Indian scientist Venkatesh P. Ketakar, for example, apparently proposed naming the mystery planets Brahma and Vishnu.
Percival Lowell didn’t live long enough to locate his celestial lurker. But he did contribute to the next great step in mankind’s solar system census by building, in 1915, a remarkable observatory in what was then called the Arizona Territory (since Arizona wasn’t yet a state). Situated just outside of Flagstaff, near the San Francisco Peaks, Lowell Observatory was among the first to be situated in a remote spot, far from city lights, in order to maximize nighttime viewing capabilities. On the way to becoming what Time Magazine has called one of the World’s 100 Most Significant Places, the Lowell Observatory offered a job to an earnest young Kansas farm boy named Clyde Tombaugh, who applied for a position on the strength of a number of astronomical drawings he had made. It was Tombaugh who, just a few months later, found a dim spherical object orbiting the sun at a distance of three billion miles.
Tombaugh’s discovery was quite different from both Herschel’s accidental spotting of Uranus in the 18th Century and LeVerrier’s elegant deduction of the location of Neptune in the 19th. This was a uniquely American achievement. Tombaugh was only 24 at the time. He hadn’t even been to college. Unlike both Herschel and LeVerrier, upper middle-class educated men, Tombaugh’s family had little money to spare. As a kid, Clyde made his own telescopes from salvaged farm equipment. He was a straight arrow, bespectacled and bashful, a quiet man with a little bird’s nest of curls atop an otherwise close-shaven skull. He had grit, much of it beneath his fingernails. Along with other small-town visionaries like Eli Whitney, Philo Farnsworth, and the Wright Brothers, he seemed to represent something important about the American character — a conviction that hard work could take a man just as far as higher education. “Young Astronomer Tilled Soil By Day, Gazed Half of Night at Gleaming Stars,” the Chattanooga Daily Times wrote approvingly in March of 1930. And while LeVerrier relied on calculations to become the man who discovered a planet with the point of his pen, as one contemporary put it, Tombaugh’s search was exhaustingly empirical. He reasoned that any star-like object that moved couldn’t really be a star. Thus, the young Kansan spent long hours poring over photographic negatives of the night sky, comparing shots of tiny portions of the heavens. Thanks to dogged detective work with a device called a “blink comparator,” which allowed for rapid comparisons of the same section of sky taken at different times, Tombaugh noticed a tiny object that moved from one exposure to the next. The discovery of Planet X was announced on March 13, 1930.
For an American public reeling from the first blows of what would come to be called the Great Depression, the discovery was a welcome sensation. One journalist called it the “glory of American science.” Congratulations poured in to Flagstaff, and the public wondered almost immediately what to call the new ninth planet. The head of the observatory nominated Kronos, father of the six main Greek gods. Others suggested Percival, for Percival Lowell, or Constance, for his wife. Lowell’s nephew thought Atlas might be appropriate. One newspaper recommended Minerva as a tribute to both scientific wisdom and what it optimistically called the “feminist age.” None of these stuck. Rather, as a result of an international naming contest won by an 11-year-old English schoolgirl, the planet eventually became known as Pluto. One author explains that the name didn’t suggest itself immediately to Americans because “Pluto Water” was the brand name of a popular laxative beverage. Nevertheless, it caught on. Like the Roman god of the underworld, the planet lived in the cold and dark, hidden from sight. The name had the additional advantage that its first two letters, P and L, were the initials of Percival Lowell, the man who had made the discovery possible. No one seems to have thought of naming the solar system’s newest addition “Tombaugh.”
In 1930 the little sphere settled into the lineup of our solar system’s planets and seemed to make perfect sense. We had the inner four planets, Mercury, Venus, Earth, and Mars, terrestrial spheres made of metal and rock. Then came the four giants: Jupiter, Saturn, Uranus, and Neptune, largely composed of helium and hydrogen gases. Pluto was like the period at the end of a sentence. Regardless of how difficult it was to remember Uranus and Neptune, Pluto was easy to recall: last in space, last in time, first in the hearts of the nation’s third graders. Clyde Tombaugh went on to discover a host of asteroids during a long and productive career, but Pluto was the find he’ll forever be associated with. Despite Pluto’s distance from Earth, we know at least a few things about it, thanks in large part to the 2015 flyby of the planet by NASA’s New Horizons space probe. New Horizons showed us that Pluto has a diameter of 1,473 miles, which is less than one-fifth the diameter of Earth and only about two-thirds as wide as our moon. It’s a small planet, not even as large as Earth’s moon. Pluto, meanwhile, has five satellites, all named for denizens of dark dreams: Charon (the largest, with a diameter just over half that of Pluto); Styx; Nix (Greek goddess of the night); Kerberos; and Hydra. The planet seems to consist primarily of ice and rock. Pluto and Charon revolve around each other, like ice skaters in a tandem spin, with a center of gravity just outside of the planet’s surface.
Discovery of Pluto didn’t end the search for wandering orbs. Astronomers eventually concluded that Pluto wasn’t nearly massive enough to cause perceived irregularities in the orbits of both Uranus and Neptune; it wasn’t really, in other words, Percival Lowell’s postulated Planet X. Because the wobbles persisted, and Pluto couldn’t have caused them, scientists theorized that there must be objects out beyond even this ultimate planet. Just as Lowell had at the turn of the century, they came to believe that a larger, hitherto unknown planet occupied space beyond the little god of the underworld. News reports in 1950 indicated that the Soviets had found such a sphere, located in the constellation Virgo, that “moves almost perpendicular to its ecliptic.” In 1960, astronomers in the Soviet Union again announced, on almost the same date as the discovery of Pluto thirty years earlier, that they had discovered a tenth planet — though they were unable or unwilling to share any facts about the purported find.
By the 1970s, a new “Planet X,” presumably wearing a luchador mask and dark sateen cape, was widely expected to exist. Appropriately, given that a new planet would have become the tenth known planet in the solar system, “X” can stand for both an unknown variable in mathematics and, in some circumstances, for the Roman numeral 10. In 1975, the Soviets were at it yet again. By this time, though, even the journalists were querulous. Canada’s Edmonton Journal carried the headline “Soviets Find New Planets?” The single-paragraph story stated that “Soviet astronomers said they have found evidence of one, and possibly two, undiscovered planets circling the sun beyond Pluto.” Once more, though, evidence for the find was lacking.
Blame it on the Black Star
Speculation about mystery planets hasn’t always been based on science. Plenty of people will always suspect that what we know isn’t all there is to the story. Anyone who’s lived through the presidency of Donald J. Trump and the COVID-19 pandemic can testify that conspiracy theories are alive and well. So too with the stars. Astrology, astronomy’s QAnon cousin, is one way to explain why we act like we do. There are entire libraries of books purporting to detail how the interaction of heavenly objects produces some elaborate and possibly inadvertent alchemy that governs our existence down here on Earth. Widely, decisively, and repreatedly debunked as pseudo-science, astrology nevertheless continues to influence huge numbers of people all around the world. Former First Lady Nancy Reagan was an ardent follower, and many U.S. newspapers run brief and wholly generic daily horoscopes for readers as a sort of psychic placebo. Even the most hardened naysayers can tell you what sign they are. As science fiction author Arthur C. Clarke once put it, “I don’t believe in astrology. I’m a Sagittarius, and we’re skeptical.”
There’s a wrinkle in the human psyche that compels speculation about what we can’t see. Certainly when we look up at the night sky we feel the same urges for fortune and glory that drove the conquistadores to cross the western sea and wander the New World. It’s cool to find a planet. It’s a ticket to fame. But I suspect that beneath or alongside this motivation is a deeper instinct, one born from our experience of day and night, sight and shadow. Our ancestors were threatened by predators that hunted nocturnally. Just because we couldn’t see them didn’t mean they weren’t out there. So to this day we look for the fin breaking the water, listen for the rustle of bushes in the shadows. And what do those gleaming points of light in the cosmos look like if not the tapeta of feral eyes flashing back at us from just outside the dancing circle of our campfire?
So maybe there’s a deeper impulse at work in our fear of the skies. It’s the one that senses the monster in the loch, the aliens in the cornfield, the 5G transmitters in our vaccines. Let’s not kid ourselves. We’re all susceptible to such thinking. In fact, paranoia may be an evolutionary adaptation. In a world full of teeth, it’s sometimes helpful to suspect there’s a killer behind every bluff. It’s as if we walk around with a dollar cinema in our heads, showing horror movies at all hours. We enjoy the fear. And from here it’s just a short step to wondering what else we can’t see, what reality lies beyond ours. We all occasionally wonder if the irrational isn’t somehow the key to understanding the algorithm of modern happenstance. But most of us eventually suppress these impulses. They may have helped to keep our ancestors alive, but they don’t work so well in a cockpit or a courtroom. I suppose the people who follow these roadways to the irrational are my spiritual opposites. While I dearly want things to be what they seem to be, and stay that way, others are always going to question not only common knowledge but also common sense. They’ve found planets too, though mostly in their imaginations.
Counter-Earth, for example. Counter-Earth is a hypothetical planet that exists on the far side of the sun, similar if not identical to Earth but orbiting in such a way, and in such a position, that it’s always just out of our sight. The idea goes back at least as far as the theories of the Pythagorean philosopher Philolaus, who postulated a doppelganger for Earth called Antichthon in the 5th Century B.C. Readers of the Tarnsman of Gor science fiction novels may recall that protagonist Tarl Cabot, a lusty but mostly virtuous rider of dragon-like tarns, lives on a counter-earth called Gor. The Marvel Comics Universe has an elaborate set of counter-earths that feature in the struggles of, among others, Adam Warlock and Dr. Doom. While the notion of a counter-earth has long since ceased to be a subject of serious scientific speculation, the could-be planet has proven to be a surprisingly long-lived vehicle for metaphysical science fiction. Indeed, counter-earths figure in dozens of stories and films, including the 1969 movie Journey to the Far Side of the Sun and the more recent, and considerably better, Another Earth (2011).
Counter-Earth is neither good nor bad; for sci-fi writers, it’s a template, a sort of pseudo-science carnival mirror that allows us to experiment with speculative versions of ourselves. Not so for other phantom planets. For years a theory has percolated on the internet that a giant black planet called Nibiru circles us in outer space just beyond our ability to detect it. Nibiru, it’s whispered, will someday wreak havoc with humanity when it collides with our little blue marble. The story seems to have started with the theories of a man named Zecharia Sitchin. In a 1976 book titled The Twelfth Planet, Sitchin claimed ancient Sumerian texts contained evidence that a race of extraterrestrials called the Anunnaki visited Earth from Nibiru many centuries ago. Nibiru, he said, was the name of a twelfth planet known to the Sumerians. The Sumerians counted the sun and moon as planets, which meant that their twelfth planet was really our tenth planet — Nibiru, in other words, was Planet X.
Sitchin died in 2010, but others took up the cause. Some were lone wolves. A 1992 article in The Tampa Tribune describes a lecture given at a local library to an audience of 25 by an individual who laid out his theory that UFOs were scouts from the planet Nibiru. Other priests in the cult had larger followings. A self-described psychic named Nancy Lieder worried her followers when she began writing in the 1990s about Nibiru’s impending collision with Earth, which she predicted would occur in 2003. When that didn’t happen, Nibiru was pressed into service as the instrument of destruction in other doomsday scenarios. Indeed, in 2012, NASA felt compelled to issue a denial that Nibiru was not an agent for the end of the world as allegedly spoken of in the ancient Mayan calendar. More recently, a “Christian numerologist” named David Meade attracted media attention with his interpretation of Biblical passages as support for the theory that Nibiru would slam into Earth in late 2017.
So far, the Nibiru theorists have been consistently wrong. It hasn’t stopped them. They say NASA is hiding evidence of Nibiru’s existence, though the agency denies it (as it would, of course). Astrologists, pseudo-scientists, and religious cranks have also fabricated prophecies regarding other purportedly mysterious but threatening celestial objects like Barnard’s Star, the Comet Elenin, and the so-called Nemesis star. For those so inclined, every discovery of a new object in the heavens is simply another opportunity to link old superstitions to new science.
As numerous Hollywood movies, including the recent Gerard Butler disaster flick Greenland, have illustrated, there is indeed a small but persistent danger that a large asteroid or comet will someday perforate Earth and cause considerable trouble for the planet’s population. It’s happened before. Maybe the most alarming example is the theorized collision of Earth and a hypothetical rogue planet called Theia, after the Greek titaness who was the mother of Selene, the moon. Some scientists have attempted to explain the unusually large size of both our moon and the earth’s mantle, its rocky core, as the result of Theia’s slamming into our planet some 4.5 billion years ago, when the Earth was young. If they’re right, there would be no Earth as we know it without the impact of something resembling “Nibiru” long ago. So score half a plausibility point for the conspiracy theorists.
But rogue planets are only one — and by far the least likely — potential earth impactor. Asteroids have repeatedly smashed into our little blue marble. Sometimes these collisions have been benign. Indeed, at least one asteroid is widely venerated: the so-called Black Stone of the Kaaba, said to have been sent by God as a message to Adam and Eve. The stone has been broken into fragments, several of which are incorporated in the shrine in the Saudi Arabian city of Mecca, a site so prized by the faithful that non-believers are not allowed even to approach it. Another asteroid that landed near present-day Ensisheim, France in November of 1492 was hailed as a sign of celestial favor for the family that would go on to found the Hapsburg Dynasty.
More often, though, asteroid intrusions have caused more concern than congratulation. Occasionally the results have been catastrophic. A giant meteor crashed into what we now know as the Yucatan peninsula of Mexico some 66 million years ago, arriving on Earth with such force that the resultant explosion and fallout may have led to the extermination of something like 75% of all animals on the planet, including the giant scaley ones with the big teeth. Some scientists speculate that the extinction of the dinosaurs allowed for the evolution of mammals, including human beings, meaning that the Chicxulub meteor impact may have helped to create civilization as we know it. Other collisions have been less massive but still destructive. In 1908, for example, a meteorite smashed into the Tunguska area of Siberia with such force that the meteor’s bolite was essentially vaporized. The resultant explosion flattened 800,000 trees. But most impacts have merely been alarming. In Sylacauga, Alabama, a meteor crashed through the roof of Ms. Ann Hodges in November of 1954, leaving her with a large bruise and a not-entirely-welcome claim to fame after she became the first person in history known to have been struck by a meteorite fragment. Her fame was nearly eclipsed on October 3, 2021, when a 2.8 pound meteorite slammed into the home of a woman in British Columbia, coming to rest on her bed, just inches from where she was sleeping.
Scientists have calculated that there is a one in 2700 chance that the asteroid Bennu will impact Earth sometime before the end of the century, in a collision that would release something like two million times the energy of the ammonium nitrate blast that flattened portions of Beirut in 2020. In fact meteors and random hunks of space junk enter Earth’s atmosphere on a more or less constant basis, flaring spectacularly as they burn, though we only see them when it’s dark; some 500 meteorites are estimated to actually hit Earth every year.
Comets — Halley’s and Kohoutek, Hale-Bopp, West, and Neowise, to name a few — are another threat. They periodically streak through the solar system, their passage accorded strange hieratic significance by the millennialists among us. While Halley’s isn’t the brightest comet to have visited our solar neighborhood, it’s undoubtedly the most famous. In 1066, William the Conqueror took the appearance of what we now call Halley’s Comet as a harbinger of the great victory he would soon win in Britain. Before the famous comet’s appearance in 1910, French astronomer and novelist Camille Flammarion predicted that its fiery tail would wreak havoc on earthly life, like the chariot of a latter-day Phaeton. Some Earthlings acted on their anxieties. According to one source, a group of religious fanatics in Oklahoma called the Sacred Followers attempted to sacrifice a virgin to the approaching “comet god.” A Connecticut man was arrested in downtown Manhattan after standing on a corner and praying for 24 hours straight in an attempt to protect city residents from the ravages of the coming comet. Several newspapers carried the story of another individual named Mathews who, upon viewing Halley’s Comet in the sky in May of 1910, grew so agitated that he leaped off a United Fruit Company steamer ship into the sea and was never seen again.
This sort of existential disorientation was demonstrated again more recently. In 1997, 39 spiritual seekers in the Heaven’s Gate cult, based in a suburb of San Diego, California, apparently came to believe that the approaching Hale-Bopp comet was accompanied by an alien spacecraft, concealed by the comet’s gaseous tail. The cultists killed themselves with a cocktail of phenobarbital, vodka, and apple sauce in hopes of having their souls sucked skyward by the alien visitors and thereupon entering into a level of existence “above human.”
In 2017 a new category of solar system wanderer was created when astronomer Robert Weryk of the University of Hawaii spotted what came to be called ‘Oumuamua, Hawaiian for “first messenger from afar,” a cigar-shaped object roughly a quarter-mile wide by over two miles long that appears to have sailed into our solar neighborhood from far far away, though no one knows exactly where. ‘Oumuamua thus became the first recorded “interstellar object,” neither planet, comet, nor asteroid, and the subject of considerable astronomical attention. Not only is it oddly shaped. It also behaves strangely. First, it doesn’t orbit the sun. Second, it tumbles end over end. And finally, it appears to have accelerated at some point after it entered our solar system, a change in velocity sometimes seen in comets, which can be propelled by heated gases, but not in a seemingly inert object like this whale-like wanderer. Some scientists, most notably the Harvard astronomer Avi Loeb, have theorized that the rogue rock might be an artifact of an alien civilization, or even a type of spacecraft. While most observers reject the notion of intelligent design behind the object, many have nevertheless advocated for a mission to catch up with and study ‘Oumuamua before it leaves the solar system, destined to travel for some incalculable period deep into space.
While it’s doubtful that prophecy will have much to do with future asteroid strikes, doomsayers continue to dream. And in this realm where history and hysteria teach the same lesson, NASA isn’t taking the matter lightly. Astronomers at the University of Hawaii’s Pan-STARRS (for Panoramic Survey Telescope and Rapid Response System) project keep a constant watch on the heavens for signs of incoming stellar objects, be they asteroids, comets, rogue planets, or interstellar objects like ‘Omuamua. Any sighting is reported to the Minor Planets Center, and, if the object is deemed to be a threat, to NASA’s Planetary Defense Coordination Office, established in 2016 to marshal what (so far) few resources Earth has to ward of an impending impact. As discussed above, collision with a vagrant space object is a slight but appreciable risk. The Double Asteroid Redirection Test, or “DART,” Mission, a joint project of NASA and the European Space Agency, will attempt to change the course of a modestly-sized asteroid by crashing a spacecraft into it. The idea is to test whether we can alter the direction of an object on its way to hit Earth. Even a slight nudge administered far enough away from our planet would be enough, over the course of millions of miles, to cause a major change of direction in a potential impactor. The mission is scheduled to launch sometime in the next year. The People’s Republic of China has announced that it is studying plans for a similar project. The Chinese space agency’s version will, as currently envisioned, employ something like 23 of its Long March 5 rockets to ram the asteroid Bennu and nudge it off its current course, which, as discussed above, will bring Bennu uncomfortably close to Earth by the end of the century.
The New Necklace
It’s easy to ignore the Nibiru fanatics, to stick them in the same dark corner as Flat Earthers and QAnon quacks. But some scientists too suspect that we haven’t yet found the most distant objects in the solar system. The “10th Planet” theory didn’t really pan out. The U.S. Naval Observatory’s Robert S. Harrington made a determined but unsuccessful attempt to find the mystery sphere in the 1980s. It wasn’t until late in the decade, when researchers recalculated Neptune’s mass, that the hunt ended. Further studies of the perceived irregularities in the orbits of the two planets — in particular the work of Dr. Myles Standish, Jr. of NASA’s Jet Propulsion Laboratory — indicated that previous calculations had been faulty and that their trajectories weren’t being impacted by some other large planet after all. By mid-1993, American newspapers were reporting that “New Analysis Questions Existence of Elusive ‘Planet X.’”
And yet, even as one proposed theory related to the existence of an object beyond Pluto was faltering, a new one was beginning to take shape. Planet X or no, there are indeed other objects out there in the far reaches of our solar system.
In fact, there are lots of them.
Proof came in 1992. Astronomers David Jewitt and Jane Luu had been searching the skies for years for evidence of objects beyond Pluto. They were initially frustrated by technological limits; the equipment they were using simply wasn’t powerful enough to allow them to find what they were looking for. They solved this problem by switching to the University of Hawaii’s 2.2- meter telescope, a reflector instrument with a focal length of some 75 feet, situated on the summit of Mauna Kea. After five years of work, the pair announced on August 30, 1992 the discovery of 15760 Albion, a large object orbiting the sun at a distance of some four billion miles. “We knew,” Jewett later wrote, “that we had found a solar system object far beyond Neptune and more distant than any seen before, that it was about 250 km in diameter, and that there were thousands of similar objects awaiting discovery.” (Emphasis added.)
Six months later, Jewitt and Luu discovered a second object, now known by the unprepossessing title of (181708) 1993 FW. These finds set off a sort of astronomical gold rush. In the twenty years after 15760 Albion was spotted, over a thousand asteroids were found in the same vicinity, evidence of a vast population of orbital masses besides Pluto and 15760 Albion. The pace of discovery quickened thereafter. By 2018, over 2000 objects had been discovered.
This celestial necklace of dim jewels was named the Kuiper Belt, in honor of longtime University of Chicago astronomer Gerald Kuiper. It’s a doughnut-shaped ring of frozen objects extending just beyond the orbit of Neptune from about 30 to 55 Astronomical Units, with each unit representing the average distance between the sun and the earth, or 93 million miles. The Kuiper Belt is similar to the asteroid belt between Mars and Jupiter, but it’s far larger — something like twenty times as wide, and a hundred times as massive. Like the asteroid belt, it consists mainly of relatively small objects, many of which are believed to be jagged remnants from when our solar system formed. But while asteroids in the Mars-Jupiter belt are composed primarily of rock and metal, most Kuiper Belt objects are composed largely of frozen substances such as methane, ammonia, and water — in other words, ice.
As mind-blowing as the Kuiper Belt discovery was, it is still not exactly the “end” of our solar system. The Kuiper Belt is distinct from the theoretical Oort cloud, which is a thousand times more distant and mostly spherical, a haze around our solar system something like the cloud that followed Pigpen around in the old Peanuts comic strip. In fact, the Oort Cloud, named after the Dutch theoretician Jan Oort, hasn’t really been “discovered” yet — only imagined, in part as a way to account for the origin of so-called long-period comets. Objects in the Kuiper belt, together with any potential Oort Cloud objects, are collectively referred to as trans-Neptunian objects (TNOs).
Along with the realization that the planetary “gas giants,” Jupiter and Saturn, contain huge internal oceans, suggesting that they may be capable of supporting life, discovery of the Kuiper Belt is one of planetary astronomy’s truly astonishing developments of the past fifty years — not just the finding of a few objects, but rather the discovery of a whole sea of them. And as with any sea, some suspect this one might be harboring a whale.
Big Changes for Little Pluto
Discovery of the Kuiper Belt led to big changes for Pluto. Michael Brown, the self-proclaimed “Man Who Killed Pluto,” is an outspoken and social media-savvy Caltech astronomy professor. Shortly after Jewitt and Luu announced their discovery of 15760 Albion, he began using Palomar Observatory’s 48-inch Samuel Oschin telescope, outfitted with a large digital camera, to scour the Kuiper Belt for evidence of planets. Beginning in 2002, he and his team found several large objects, each of which was initially judged to be larger than it actually was — and potentially larger than Pluto — due to considerations related to the objects’ albedos, or “shininess.”
By now the search for the Next Big Thing had become more competitive — indeed, almost cutthroat — than it was in William Herschel’s day. During the course of his work, Brown was scooped by a Spanish astronomer in announcing the discovery of a large Kuiper Belt object that was eventually named Haumea, for the Hawaiian goddess of fertility. Brown and his team had been studying the object for approximately six months before its “discovery” was announced by José Luis Ortiz Moreno and colleagues from the Sierra Nevada Observatory in Spain. Brown initially voiced support for Ortiz’s team’s being given credit for finding Haumea. However, as Brown relates in his book, How I Killed Pluto and Why It Had It Coming, information that came to light later suggested that Ortiz used a combination of archival information and published hints about Brown’s find to locate the new object and quickly announce it as if he’d found it himself. The controversy was major news in scientific circles, and raw feelings have never quite healed. Brown now takes credit for the discovery.
But Haumea was just a warm-up. Brown and his team next discovered an even larger Kuiper Belt object that they eventually named Eris, after the Greek goddess of discord. Like other newfound Kuiper Belt objects, Eris was thought to be larger than Pluto. (In fact, it was eventually shown to be slightly smaller, though more massive.) It even has a small moon. In the summer of 2005, Brown announced the discovery of Eris as the solar system’s “10th Planet.” He did this even though, according to his book, he didn’t really believe that it was the tenth planet. Rather, he went along with the characterization partly because his publicist argued, rightly, that announcement of the discovery of the “10th Planet” would get more attention from the media than discovery of a “new large trans-Neptunian object.” Brown is a ruddy, blond-haired man with wide-set eyes and an impish, gap-toothed grin. He looks like the kind of man who would know how to operate audio-visual equipment. Brown revels in controversy, and was prepared to argue that given the apparent abundance of Pluto-sized (more or less) objects in the Kuiper Belt, neither Eris nor Pluto should be considered “planets” at all. Just as had happened a hundred and fifty years earlier to the largest of the asteroids in the space between Mars and Jupiter, the largest object beyond Neptune — Pluto, the “King of the Kuiper Belt” — was suddenly at risk of losing its planetary status.
It’s worth noting that even as some astronomers were finding scores of objects in the Kuiper Belt, here in our solar system, others were observing “exoplanets,” the name for planets orbiting stars other than the sun. While the first exoplanet was discovered in 1992, a fruitful year for astronomy, the number of such objects is now up to something like 4,000 — and increasing fast. So Pluto faced pressure from both within and without our solar neighborhood. The more we could see of the heavens, the less special the little orb seemed to be. Simply put, there are lots of planets out there. Enshrining our solar system’s planetary population at nine for some nostalgic, patriotic, or even mnemonic reason started, by the first years of the 21st Century, to seem irrational. Indeed, some scientists had already taken to calling Pluto and similarly-sized and situated objects “plutinos,” suggesting that Pluto and its like belonged in some new class of objects altogether.
The matter eventually came to the attention of the world’s most important association of stargazers, the International Astronomical Union. The IAU has a membership of some 13,000 astronomers and astrophysicists, and is responsible for, among other things, assigning official names to celestial objects. Given the apparent existence of numerous “Pluto-like” objects in the Kuiper Belt, including objects of roughly the same size, some sort of reassessment of Pluto’s status was expected when the IAU met in the summer of 2006. The question was what. Brown has written that he had no idea what the IAU would do. To him, though, the range of options looked something like this: (1) make Eris the 10th planet in our solar system; (2) declare that neither Eris and Plato were planets, but rather were asteroids, leaving eight planets (the outcome Brown thought would make the most sense to people “who really understood the solar system”); (3) leave Pluto as the ninth planet, for not-very-scientific reasons, and ignore Eris; or (4) declare Pluto, Eris, and all other spherical Kuiper Belt objects to be planets, essentially opening the door to a whole host of new cosmic gods.
What actually happened didn’t fit neatly into any of these categories. One IAU proposal involved characterizing Pluto’s moon Charon as a planet. Another proposal, which was actually voted on and rejected, would have designated the “Big 8” planets, i.e., all except Pluto, as “classical” planets, as if they were Led Zeppelin songs on an FM radio station. In the end, the IAU chose to characterize Pluto not as a planet, full stop, but as a “dwarf planet,” a status it currently occupies with Eris and three other celestial bodies in our solar system. The IAU’s final Resolution 5A created a three-tiered system for categorizing celestial bodies orbiting the Sun, as follows:
The IAU … resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and © has cleared the neighborhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, © has not cleared the neighborhood around its orbit, and (d) is not a satellite.
(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies.”
It was the “clearing the neighborhood” requirement that got Pluto. By virtue of its place in the Kuiper Belt, the diminutive body travels with an unruly complement of icy asteroids. It simply isn’t massive enough to attract and consolidate the asteroids or to sling them off into another path altogether, which is what the IAU thinks “clearing the neighborhood” (not exactly a revered astronomical term) means.
So, according to the IAU, we have eight planets in our solar system (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune); five dwarf planets (Ceres, of the Mars-Jupiter asteroid belth, plus Pluto, Eris, Haumea, and Makemake of the Kuiper Belt); and countless “small solar system bodies” (everything else). There are also lots of small solar system bodies that will probably be “upgraded” in the future to dwarf planetary status. Indeed, one astronomy website lists 13 such prospective dwarf planets — dwarf planets in waiting, as it were.
In Search of a New Planet 9
The big problem with the IAU scheme for many people is the creation of a category for Pluto and other dwarf planets that specifically states that they are not “planets,” period. For a number of planetary astronomers, this definition is nonsense, an unnecessary and misguided attempt to impose order on an unruly and still-evolving field of study. But assuming we’re okay with the IAU’s strictures, we know that our list of planets is complete, right? It’s just different from what it was when Joe DiMaggio hit safely in 56 straight games and Apollo 11 touched down on the lunar surface and Michael Jackson did his first moon walk without even leaving the earth.
Not quite.
In January 2016, a decade after Pluto was re-categorized by the IAU, Michael Brown returned to the spotlight. This time, he and a Caltech colleague named Konstantin Batygin proposed the existence of what they call “Planet 9,” a major planet between the size of Earth and Neptune, situated far beyond Pluto. According to the pair’s initial announcement, the planet has a mass 10 times that of Earth, as a highly eccentric orbit, and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun. Working with data originally obtained by astronomers Scott Sheppard and Chad Trujillo, Brown and Batygin based their theory on a study of six Trans-Neptunian Objects, or TNOs, each smaller than Pluto, in extremely elongated and tilted orbits around the sun. The astronomers postulate that the orbits of these “extreme” TNOs are bunched together because Planet 9’s gravity nudged them there over billions of years. Several more extreme TNOs — nineteen, by one recent count — discovered since then seem to “cluster” as well.
So the search is on. And while Planet 9 — variously known to others as Planet X, Giant Planet Five, or Planet Next — has yet to be seen, the Hubble Space Telescope has found evidence to suggest its existence is at least possible, in the form of observations of an exoplanet in the vicinity of the constellation Crux, some 336 light years away. This odd planet, designated HD 106906 b, also orbits its star — a binary star, in this case — at a distance comparable to Planet 9’s projected orbit, and at an angle well off the horizontal plane typical of our solar system’s known planets; indeed, one astronomer calls HD 106906 b’s orbit “eccentric and highly misaligned.” First identified in 2013, HD 106906 b is actually much larger and farther away from its central star than Brown and Batygin’s supposed Planet 9 is from ours. But if HD 106906 can exist, the argument goes, so too can Planet 9.
A question may present itself to your mind now as it did to mine. Why can’t we use the Hubble Space Telescope to locate Brown and Batygin’s hypothetical sphere? The constellation Crux is 336 light years away. (A light year is the distance light travels in a year, which is around 6 trillion miles — about the same number of miles my dad always claimed to have put on his ’74 Volvo.) Astronomers at the University of Texas, meanwhile, are studying a relatively young exoplanet that is some 370 light years from Earth. Using Hubble, they have captured images of the planet, PDS 70 b, at two wavelengths: 656 nanometers (red optical, corresponding to the H-alpha emission line of the hydrogen atom) and 336 nanometers (in the ultraviolet range). Hubble, with its 8-foot primary mirror, has angular resolutions of around 50 milli-arcseconds at these wavelengths. This resolution is more than enough to separate the planet and its host star, which are 175 milli-arcseconds away from each other. Given the mind-boggling reach of the now 31- year-old space telescope, it would seem an easy matter to spot a similarly sized planet here in our own solar system.
Not according to Dr. Yifang Zhou, one of the UT researchers on the project. “First,” he explains, “because our solar system is much older than PDS 70 b and Planet 9 is so far away from our sun, Planet 9 is much colder and does not emit much energy. The reflected light from Planet 9 is also weak due to its long distance from the sun. As a result, although Planet 9 is much closer than PDS 70 b, it is significantly fainter.” Second, says Zhou, we still don’t know precisely where to look for the hypothetical Planet 9. The area in which Planet 9 may be located is so vast that it would probably take hundreds of thousands of Hubble images to cover. However, Dr. Zhou allows, if we did know the position of Planet 9 and pointed HST at it, Hubble would have the sensitivity to detect it.
Naturally, a claim as important as the one made by Brown and Batygin has attracted pushback from other scientists. The two were not surprised. As reported in Air & Space Magazine, as recently as December 2015, observers at the Atacama Large Millimeter Array in Chile detected what they thought could be a “super earth” beyond the Kuiper Belt. In that case, the astronomers posted their preliminary findings online in order to solicit opinions — and were quickly shot down. “Every time somebody sees something unusual, everyone always thinks ‘planet’ and it never pans out,” Batygin has said. “It is probably the most failure-rich idea that exists in astronomy today.”
Accordingly, one team of astronomers has argued that the orbits of the handful of distant lumps of rock are not bunched together by the gravity of Planet 9, as Brown and Batygin suggest, but only seem clustered because that’s where telescopes happened to be looking. There’s a sort of selection, or observational, bias at work. Other scientists have proposed that what Brown and Batygin believe to be a Planet 9 affecting the orbits of other TNOs could actually be a small black hole, or several such holes. “Primordial” black holes are predicted to have formed within the first few fractions of a second after the Big Bang, the result of unusually high concentrations of matter essentially folding in on themselves. If they exist in our solar system, a primordial black hole or holes would be located hundreds of times farther from the sun than Earth. But because black holes are incredibly efficient and powerful matter compactors, a black hole equivalent in mass to around five Earth-sized planets — and thus capable of exerting a strong gravitational influence — would be only about the size of a baseball. Think about that for a minute. An object that could fit in Max Scherzer’s hand — in theory, of course — might be massive enough to influence the orbital paths of hundreds of Kuiper Belt asteroids.
Brown is sensitive to the fact that Planet 9 has gone undetected five years after his announcement that it exists. He and Batygin recently released new calculations of the size and orbit of their hypothetical planet, and provided a sort of “treasure map” indicating what they believe to be the sectors of the sky in which astronomers should look for the orb. “I believe it’s within a year or two from being found,” Brown stated. He went on to concede that “I’ve made that statement every year for the past five years. I am super-optimistic.”
As of this writing, the jury is out on the existence of Planet 9. As NASA puts it, the current status is as follows:
To date, astronomers only have circumstantial evidence for Planet Nine. They’ve found a cluster of small celestial bodies beyond Neptune that move in unusual orbits compared with the rest of the solar system. This configuration, some astronomers say, suggests these objects were shepherded together by the gravitational pull of a huge, unseen planet. An alternative theory is that there is not one giant perturbing planet, but instead the imbalance is due to the combined gravitational influence of multiple, much smaller objects.
“Another theory” the agency points out, “is that Planet Nine does not exist at all and the clustering of smaller bodies may be just a statistical anomaly.”
So there’s that.
Pluto Strikes Back
Despite some unpleasant developments in the past thirty years, Pluto lovers aren’t backing down. Some are determined to see the little orb retain its position as the ninth planet they grew up with. It’s not hard to understand why this movement exists. In his book The Pluto Files, with its obvious pun on the term “Plutophiles,” Neil deGrasse Tyson surveys the occasionally irrational affection Americans have for Pluto, and its expression in jokes, songs, and other elements of popular culture. One driver of the phenomenon is national pride. Pluto was discovered by a Kansas farm boy, after all, in an act that was celebrated in its day as one of the pinnacles of American science. Part of it is nostalgia — a desire to keep things the way they were when we recited the names of our beloved nine planets, shortly after we said the Pledge of Allegiance and not long before recess. And maybe another portion of our resistance relates to our love of lists: Top Tens and Halls of Fame, Desert Island Discs and Hot One Hundreds. Who is this kid Eris anyway? What’s her batting average? How many hits has she had? Why does she deserve a place in The Nine?
Whatever the reason, a surprising number of people refuse to honor the IAU’s demotion of Pluto to the status of “dwarf planet.” They castigate Mike Brown for declaring that he “killed” the little planet, as if he were a sort of celestial Robert Ford, the man who assassinated the outlaw Jesse James. This is an irritating and nonsensical boast, they say. Unlike Einstein with Vulcan, Brown has never proved a planet didn’t exist; he simply found other celestial objects that look and act a lot like it. And the Pluto revanchists point out that by claiming to be looking for “Planet Nine,” Brown is essentially arguing from facts not in evidence — namely, that Pluto is not a planet. Their preferred term: Planet Next. It’s a surprisingly vicious spat. Pluto lovers wear t-shirts that say PLUTO: NEVER FORGET and TEAM PLUTO. They buy the “Nine Planet Ring” as advertised in Smithsonian. They belong to Facebook groups like The Society of Unapologetic Pluto Huggers and presumably frequent the website for the Society For the Preservation of Pluto as a Planet (http://www.plutoisaplanet.org/). This group advises that:
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet. We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects.
Planetary scientist Alan Stern, a leader of the NASA New Horizons mission that explored Pluto in 2015, is perhaps the foremost proponent of the idea that Pluto — and numerous other solar system objects, for that matter — should be classified as planets, pure and simple. In February 2021, at a talk given to Flagstaff, Arizona’s annual “I Love Pluto” Festival, Stern proposed a geophysical definition that’s completely agnostic to the total number of planets in the Solar System. Under this definition, a celestial object is a planet if:
1. It has enough mass (and therefore gravity) to be round [which would include Ceres].
2. It has insufficient mass to undergo nuclear fusion in its interior [which would make it a star].
This way, Stern says, Pluto easily qualifies as a planet — as do all “dwarf planets.” An influential group of planetary scientists has proposed a similar definition, one that includes dwarf planets and “moon planets,” and states that its members count “at least 110 known planets in our Solar System,” a number “that continues to grow as astronomers discover more planets in the Kuiper Belt.” Thus, they say, the search for an elusive Planet 9 should really be called something like The Search for Planet 111. Others make potent geophysical arguments aimed at reviving Pluto’s status in particular. Pluto is 70 percent rock and geologically layered into core, mantle, and crust — like a planet (like Earth, in fact). Pluto has an atmosphere — like a planet. Pluto has moons — like a planet. Pluto has ice mountains — like…well, it’s unclear what other celestial bodies might have ice mountains. But you get the idea. The argument about whether Pluto is a planet — and indeed over what constitutes a “planet” in the first place — continues.
What’s apparent is that even people who maintain that Pluto is a planet, not a “dwarf planet,” believe that the number of planets is greater than nine. Thus, the old paradigm, the one I grew up with, is irrevocably dead. It’s not even possible to say that Pluto is the ninth discovered planet, since Ceres, which everyone agrees meets most of the same criteria Pluto does, was discovered some 130 years earlier. The best we Pluto lovers can say is that we like the old system, the old nomenclature, the 20th Century country club where Pluto, like a harmless cousin, was allowed to occasionally take a tee time and hang out at the bar with the big boys. As comforting as that thought might be, it’s not science. Science isn’t comfortable — unless, of course, you’re a nine-year-old third grader, patiently memorizing your list of dwarf planets for next week’s test as if that cozy catalogue is the most natural thing in the world, appropriate and — why not? — apparently permanent.
But nothing is permanent. And as much as I resist the “demotion” of Pluto from the ranks of the Big Nine, what’s even worse is the realization that even this demotion is uncertain, that the ground shifting under my feet may actually be over my head. Pluto is a planet by some standards, but not by others — and in fact the desire to categorize it says as much about the person making the categories as it does about Pluto or Ceres or Neptune. I like having clear, consistent labels to organize my understanding — heck, even to organize my lack of understanding. But space isn’t the place to go for consistency. The future isn’t interested in stasis. The older I get, the more I wonder if the only constant we’re likely to find, for as long as we’re looking, is that the universe is going to surprise us. With black holes. With dwarf galaxies, spiral galaxies, and lensed galaxies. With exoplanets and pulsars and dark matter and doubtless additional riddles and mysteries that are difficult to fit in a file.
That’s not necessarily going to be easy. But maybe that’s all, finally, that a curious species needs — the opportunity not simply to number and group and memorize, but to understand the cosmos for what it truly is, whether it makes us happy or not. As the great Richard Feynman once stated, “I’d rather have questions that can’t be answered than answers that can’t be questioned.” The universe is almost incomprehensibly vast. If we ever think we understand it, we can relax and get back to memorization, recess, and the Pledge of Allegiance. Unless, of course, there’s reason to believe there’s something else out there, some new elusive Planet X or ephemeral Vulcan, the dark god Nibiru or a hypothetical Planet 9.
Given human nature — our greed and curiosity, our intellectual courage and existential terror — it’s likely there always will be.
The End
