How I Killed Pluto and Why It Had It Coming

by Mike Brown

  By the end of 1999, on the foggy December night when Sabine and I were sitting underneath the Hale Telescope at Palomar Observatory and I was proclaiming that I thought there were new planets to be found, astronomers around the world had already discovered almost five hundred of these bodies in a vast disk beyond the orbit of Neptune in what looked very much indeed like the Kuiper belt. From being something that most astronomers had perhaps heard of once or twice, the Kuiper belt had become the hottest new field of study within the solar system.

  Of the five hundred bodies that were then known in the Kuiper belt in 1999, most were relatively small, maybe a few hundred miles across, but a few moderately large objects had also been found. The largest known at the time was somewhere around a third the size of Pluto. A third the size of Pluto! Pluto had always enjoyed a somewhat mythical status as a lonely oddball at the edge of the solar system, but it turned out that it had more company than astronomers had originally thought.

  Over the years since I had dismissed the entire Kuiper belt as not quite interesting enough to pull my mind away from Jupiter, I had actually been thinking a bit about Pluto and about those five hundred small icy bodies recently discovered in the distant solar system. By now it seemed to me inevitable that, whether anyone realized it or not, astronomers were on an unstoppable march that would eventually lead to a tenth planet. It seemed to me obvious that it was there, slowly circling the sun, just waiting for the moment when someone somewhere pointed a telescope at the right spot, noticed something that hadn’t been there earlier, and suddenly announced to an unsuspecting world that our solar system had more than nine planets.

  Sitting beneath the massive Hale Telescope that foggy night, ever the scientist, Sabine asked, “What evidence do you have?”

  I told her about all of the recent astronomical discoveries. But when pressed for evidence, I had to admit: I had none whatsoever. I had a hunch. Officially, scientists don’t work on hunches. We work on hypotheses and observations and plenty of evidence. Hunches don’t get you research funding, tenure at your university, or access to the world’s largest telescopes. But a hunch was all I had. No one had systematically looked across the sky for a new planet since the 1930s, when Pluto itself was found, and even though astronomers knew of almost five hundred bodies in the Kuiper belt, the searches had been, of necessity, piecemeal, and no one had yet mounted a careful search like the one that had uncovered Pluto. Now, seventy years after the discovery of Pluto, telescopes were bigger and better, computers made searches vastly more powerful, and astronomers simply knew more about what they were looking for. How could it be that if someone went and looked again for a new planet they wouldn’t find something that had been just beyond the reach of the telescopes in the 1930s? There had to be a tenth planet. The possibility that Pluto was a unique planetary oddball out at the edge of the solar system seemed absurd to me.

  “I don’t have any evidence,” I told Sabine. “I don’t have any proof. I don’t have anything other than this deep feeling that another planet past Pluto makes sense. And I’m willing to bet that there’s one there.”

  Scientists don’t bet much. We are supposed to deal in quantifiable levels of certainty and in statements that can be backed up with experiments and observations. Bets are simple assertions that you think you are right and that you believe what you are saying enough to risk something valuable if you’re wrong. There is nothing scientific about a bet at all; in fact, it is almost the opposite of science. In earlier years many scientists would have bet the farm against the big bang, evolution, and quantum mechanics, and the farm would be gone.

  But still, there’s something appealing about betting. I had no solid evidence to go on, but bits and pieces of different facts and discoveries had, somehow, shaken together in my mind to form a hunch. Though I couldn’t prove it to a scientist, I was all but certain that I was correct. I couldn’t prove it, but I could definitely bet on it.

  Sabine took the wager. The bet was that someone would find a new planet by December 31, 2004. The winner of the bet would receive five bottles of champagne, to be drunk in celebration of new planetary frontiers or in mourning for the sad limitations of our solar system.

  We sat for a few minutes staring up at the telescope, thinking about planets.

  “We’ve got one problem. We’ll never know if someone wins the bet,” I said.

  “What?” she asked. “How could we not know whether or not someone finds a planet? Surely the entire world will hear about it. It’ll be pretty obvious.”

  “Well, okay,” I said, “then I have one question for you: What is a planet?”

  I needed to know the answer, because I wanted to find one myself.

  • • •

  Like most everyone else, I’ve known what a planet is since I was four or five years old, which for me would have been about 1970. I knew the moon even earlier. I grew up in Huntsville, Alabama, a thoroughly dedicated rocket town. The father of everyone I knew—mine included—was some sort of engineer working to build the Apollo rockets to send men to the moon. For a while as a child, I thought that when you grew up you became a rocket engineer if you were a boy and you married a rocket engineer if you were a girl; few other options in the world appeared to exist. When Neil Armstrong stepped on the moon, I was pretty sure that that was exactly what I was going to be doing eventually, too. I drew picture books of rockets blasting off, of command capsules in orbit about the moon, of lunar modules landing next to giant lunar craters, and of parachutes deployed in the moments before splashdown.

  By second grade I had learned enough about the moon to know that those giant craters I had been drawing earlier had been formed by meteors slamming into the moon’s surface. I figured out that if I went to the backyard and soaked the deep red dirt with a hose, I could throw rocks from above and make the mud look just like the craters on the moon. I could even throw the rocks sideways into the mud and make oblong craters like some I had seen in lunar pictures.

  Though the moon was my favorite, I learned about planets, too. But planets were a little more abstract than the moon, since you couldn’t see them and no one had stepped on them or taken pictures from the surface. Still, by first grade I had a poster on my bedroom wall that showed the solar system with an artist’s conception of each of the planets. Though I didn’t realize it at the time, spacecraft had already visited Mars and Venus and Mercury, so some of the pictures were quite detailed. (I didn’t know about these spacecraft at the time because Huntsville was totally dedicated to the Apollo rocket program and the moon, as far as I could tell. The robotic exploration of the other planets was being run out of a small town I’d never heard of called Pasadena, on the other side of the continent.) On my poster, Mercury looked much like the moon, battered by meteors. Venus appeared only as a swirl of clouds. Mars had giant volcanoes and deep canyons. In the outer part of the solar system, things on my poster got fuzzier, since truly no one had ever seen them except through powerful telescopes; but Jupiter had its clouds and great red spot, Saturn had its rings, and Uranus and Neptune had their retinue of moons. Pluto, however, was the most exciting of them all, because it was so different from all of the other planets.

  Even as a first grader I could see that Pluto didn’t travel in perfect circles around the sun the way the other planets did. I could see on the poster that it came close enough to the sun to momentarily pass inside the orbit of Neptune, but the poster showed only this inner bit of Pluto’s orbit. The outer parts of the orbit were so far away that Pluto would have to travel off my poster, onto my wall, out my window, and midway across the front yard toward the street before it turned around and came back in toward the sun. Even stranger, Pluto didn’t orbit the sun in the same nice flat disk that all of the other planets did: It was tilted away from the others by almost twenty degrees. On the poster, all of the other planets were represented by paintings of a global view of the surface seen from high above, but Pluto—only special Pluto—had a painting of what the planet woul
d look like if you were standing on the surface looking back at the tiny dim Sun. The surface of the planet was covered in icy spires. These days I realize that the artists would have had no idea what Pluto looked like and probably felt the need to make the surface look like something interesting, but as a first grader I was thoroughly convinced that Pluto was covered in icy spires and that they would shatter at the slightest touch by a future Neil Armstrong. Clearly Pluto was different and mysterious and potentially very fragile. It would take another thirty-five years for me to learn just how fragile it really was.

  In third grade we finally learned about planets in school. Most people I know memorized their order by learning some variant of the mnemonic “My very excellent mother just served us nine pizzas” for “Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto,” but for some reason, in my school we learned one that I have never heard since: “Martha visits every Monday and just stays until noon. Period.” The “and” appears between Mars and Jupiter, just where the asteroids are, though I always suspected that that was just dumb luck. The “period” at the end, though, seemed fishy even in third grade. It didn’t seem so much as to make Pluto special, as the other odd characteristics did, as much as to make Pluto seem an afterthought or a late addition or just perhaps an undesirable misfit.

  Oddly, though, for a kid interested in planets, I had never been very interested in the actual night sky. Sure, I could name some of the more obvious constellations and sights—the Big Dipper, Orion, the North Star. I could point out the Milky Way galaxy, which was actually visible in the dark skies above Alabama, and I could even convince the other kids that it really was the Milky Way they were seeing and not just clouds in the sky as they always seemed to think. Once, I even saw a real comet through binoculars when my father dragged me out of bed one cold winter night in 1973 and drove us to the top of a dark mountain to see what was supposed to be the spectacular Comet Kohoutek but instead looked to me like a shaky little smudge and please could I go back to sleep now? But I was never one of those kids who built his own telescope by grinding mirrors from blanks or who memorized the locations of each of the nebulae hidden among the constellations or, even, who could tell you that the bright light above the just-set sun was, in fact, not an airplane but the planet Venus. I could passionately describe the rings of Saturn, the number of moons of Jupiter, the rocky plains of Mars, and, of course, the icy spires on Pluto, but the fact that these distant worlds were up in the sky above me was never really part of how I thought about them, much like when I think of Antarctica now I think of pictures and descriptions and maps, but I never really think about the fact that if I jumped in a boat, turned south, and started sailing, I would actually end up there.

  I did get a telescope for Christmas when I was in the third grade—the seemingly perfect gift for a kid like me—but I could never make it work. My brother was capable of constructing elaborate LEGO structures for any purpose and could make balsa wood airplanes that looked sleek and flew straight and were painted beautifully. I was lucky if my LEGO constructions stayed together and were made of more or less the same colors. My attempts at balsa airplanes usually ended in my deciding that, really, I had meant to make that model of an airplane wreck, and yes, it would be fun to burn the whole thing now. Trying to make the telescope work went little better. I needed to carefully align mirrors and keep the tripod steady and adjust eyepieces, and it never worked. I think I found a single star once—though in retrospect, knowing now what a star should have looked like in such a small telescope, it is entirely possible that I only looked at an out-of-focus streetlight with a shaky telescope.

  One night in the late fall when I was fifteen years old, I was awake late enough to find myself looking up at Orion—the one truly familiar part of my winter night sky—and I noticed that something didn’t look right. Orion is full of bright stars that make very clear patterns even for the casual sky glancer: three stars for the belt, a dagger beneath, and a quartet of bright stars outlining the rest of the body. They are among the brightest stars in their region of the sky and nearly impossible not to recognize. And yet somehow, overhead, a little to the left, there was a pair of stars every bit as bright as those of Orion that I didn’t recall ever having seen before. I was not a photographic-memory-star-pattern-recognizing kid and just assumed I had somehow overlooked them, much the way I would also overlook my allegedly lost shoes even when they were right in the middle of the floor in my room. As the months went on, however, the two stars did something extraordinary. They moved! You would have never noticed it in a single night or even in a single week. But over the months, they very slowly crawled closer together. As the winter wore on and moved into spring, the two then moved apart and then around each other in an elaborate dance high overhead, while the remainder of the stars remained fixed in their constellations. I found myself eager to go check on the stars night after night. In the winter, I would have to stay up late before they rose in the sky, but as spring came, the dancing stars were directly overhead as soon as the sun went down.

  I didn’t ask or talk to anyone about the moving stars; I just silently kept track. At some point that spring, though, I came across a single-paragraph article in the newspaper describing the once-every-twenty-years close conjunction of the two largest planets, Jupiter and Saturn, which looked like two bright stars wandering near Orion. They were planets! Today, I am surprised that I could possibly have been as shocked as I was. How could I not have known? What did I think those moving stars were? How at fifteen could I have seen something unknown in the sky and not immediately needed to know what it was?

  I guess no one had ever mentioned to me that you could actually see planets in the sky overhead. As soon as I realized that my two moving stars were Jupiter and Saturn, however, it became clear: Planets were not just an artist’s conception on my poster, nor even just images sent from distant spacecraft, but they were bright points of light that moved among the stars. Imagine how you might feel if you had been looking at pictures of the Grand Canyon all of your life and passionately studying the layered geology and tracing Powell’s trip down the canyon on the first raft expedition on a topo map, and then, suddenly, while out on what was supposed to be an ordinary afternoon stroll, you turned a corner and came unexpectedly to the canyon rim and almost fell in. At that point, how could you not want to explore every corner, every tributary, and learn everything that you could possibly learn about this wonder in your own backyard?

  I have been hooked on the real planets in the sky ever since. I’ve kept track of Jupiter and Saturn in their travels through the stars season after season. Each year they move a little farther east in the sky as they orbit around the sun. Saturn is so far away and moves so slowly that it takes a full thirty years to complete one orbit. Today, almost thirty years after I first noticed Saturn above, it has finally almost completed one of its transits all the way around the sky—one full Saturn year—and when I look outside at night I see that it is almost back in the same place where I first saw it when I was a teen wondering what those bright stars were that danced. With luck, I’ll get to watch Saturn trek all the way around the sky and end up in this spot once more in my lifetime, but probably not twice.

  Jupiter, closer to the sun, is comparatively fast; it takes only twelve years to go completely around the sky. When it gets to where it started, though, Saturn has moved on. It takes another eight years—twenty years in total—for Jupiter to finally catch up to Saturn once again so they come close together in a conjunction just like the one I noticed when I was fifteen. I’ve often wondered about the timing of this conjunction. If I had been born a few years earlier, I would have looked up at age fifteen, but Jupiter would not yet have caught up to Saturn’s position in the sky. I would have noticed only one bright planet moving a little below Orion instead of two. Would I have noticed their dance? Would I have become the person I am today, someone whose first instinct when walking outside at night is to always look up, check the stars, look for pl
anets, locate the moon? It’s impossible to know, but it’s always hard not to feel that in some ways, for me at least, perhaps the early astrologers were right: Perhaps my fate actually was determined by the positions of the planets at the moment of my birth.

  Whether or not the planets controlled my fate, one thing was clear: I knew what a planet was. As a child, I knew planets from my poster on the wall. As a teen, I knew them from watching them move across the sky. And later I knew them from years of writing a Ph.D. dissertation. Nobody was going to be able to change my mind about what a planet was. Right? So then, as my friend Sabine and I were sitting inside the Hale Telescope dome at Palomar Observatory on a cloudy, drizzly night finalizing our bet about whether or not someone would find a new planet, why was it that astronomers around the world suddenly could no longer agree on a definition of the word planet? How could it be that even I was unsure about what would and would not count?

  Chapter Two

  A MILLENNIUM OF PLANETS

  The end of the twentieth century was not actually the first time that the word planet had become confusing. The word has existed for thousands of years, and its meaning has been continually updated to reflect our continually shifting view of the cosmos. Over the millennia there have been a few major events leading to dramatic changes.

  The original ancient Greek meaning of the word planet was simply “wanderer,” or something that moved in the sky. When, as a teenager, I first noticed Jupiter and Saturn dancing among the stars, I was seeing the sky as it had been seen for millennia and noticing that there were things that were special, things that stuck out, things that moved in a different way. As the sky slowly revolves throughout the year, the stars stay in fixed patterns while the wanderers move separately and conspicuously through the constellations of the zodiac. The ancient Greeks and Romans knew seven wanderers in the sky: the five visible planets—Mercury, Venus, Mars, Jupiter, and Saturn, which are all easy to see if you know where and when to look—plus the moon and the sun, which both also move through the sky and were also considered planets in good standing.