How I killed Pluto and why it had it coming.

by Mike Brown.

Prologue.

PLUTO DIES.

As an astronomer, I have long had a professional aversion to waking up before dawn, preferring instead to see sunrise not as an early-morning treat, but as the signal that the end of a long night of work has come and it is finally time for overdue sleep. But in the predawn of August 25, 2006, I awoke early and was up sneaking out the door, trying not to wake my wife, Diane, or our one-year-old daughter, Lilah. I wasn't quite quiet enough. As I was closing the front door behind me, Diane called out, "Good luck, sweetie!"

I made the short drive downhill through the dark empty streets of Pasadena to the Caltech campus, where I found myself at 4:30 a.m., freshly showered, partially awake, and uncharacteristically nicely dressed, unlocking my office building to let in news crews that had been waiting outside. All of the local news affiliates were there, as well as representatives of most of the national networks. Outside, a j.a.panese-speaking crew was pointing a TV camera up at the sky, the beams of the flood lamps disappearing into s.p.a.ce.

Today was the last day of the International Astronomical Union meeting in Prague, and the final item on the agenda at the end of two weeks' worth of discussion was a vote on what to do with Pluto. Everyone's favorite ice ball was in imminent danger of being cast out of the pantheon of planets by the vote of astronomers a.s.sembled half a world away, and whatever happened would be big news around the globe.

I like planets, but I didn't care enough about Pluto to get up at 4:30 a.m. But this Pluto vote mattered enough for me to drag myself out of bed that morning. For me that vote had nothing to do with the ninth planet; it was all about the tenth. And I cared a lot about that tenth planet, because eighteen months earlier, I had discovered it, a ball of ice and rock slightly larger than Pluto circling the sun every 580 years. I had been scanning the skies night after night looking for such a thing for most of a decade, and then, one morning, there it finally was.

At the time of the Pluto vote, my discovery was still officially called only by its license plate number of 2003 UB313, but to many it was known by the tongue-in-cheek nickname of Xena, and to even more it was known simply as the tenth planet. Or maybe, after today, not the tenth planet. Xena had precipitated the past year of intensive arguments about Pluto, but it was clear that Xena would share whatever fate was dealt to Pluto. If Pluto was to be a planet, then so too Xena. If Pluto was to be kicked out, Xena would get the same boot. It was worth waking up early to find out the answer.

The previous two weeks in Prague had been perhaps the most contentious gathering in modern astronomical history. Usually the International Astronomical Union meeting is nothing but a once-every-three-years chance for astronomers to advertise their latest discovery or newest idea while spending some time in a nice international destination, having dinners with old friends and catching up on their celestial gossip. On the final day of each meeting, in a session attended by almost no one, resolutions are pa.s.sed, usually all but unanimously, on such pressing topics as the precise definition, to the millisecond, of Barycentric Dynamical Time (I have no idea what this actually even means).

This year was different. The usually placid astronomers had spent their time in Prague arguing and bickering day and night about Pluto and about planets. While several of the typically unintelligible resolutions were indeed to be voted on this last day, the final two resolutions would be all about Pluto. The usually spa.r.s.ely attended final session was likely to be full of surly astronomers itching for a fight.

While the astronomers were gathering for their vote in Prague, the news crews and I were arriving in the early morning on the Caltech campus in Pasadena, California, so that we could watch the excitement via a webcast. My job was to provide commentary and a.n.a.lysis for the press and moral support and scientific coverage for the astronomers who were-rightly, I thought-trying to take the bold move of ridding the solar system of the baggage of planet Pluto. I found the webcast, projected it on the large screen, and we all sat back to watch.

Three mostly esoteric and tedious hours later, it was all over. On the final vote, the air was filled with yellow cards with which the astronomers in Prague were voting "no" to Pluto's planethood. There was no need to count; the vote was not even close. After hours of detailed explanation and a.n.a.lysis and discussion of the subtleties of all the different possible outcomes, I could finally just say: "Pluto is dead."

The cameras whirred; correspondents talked into their microphones, and on a screen on the other side of the room, I could see myself on some local television station repeating, like an echo, "Pluto is dead."

Before anyone else could ask a question, I quickly picked up the phone and called Diane, who was now at work. I had made a similar phone call eighteen months earlier, only minutes after I had discovered Xena. Back then, the moment she picked up the phone I said, "I found a planet!"

Back then, her voice had risen. "Really?"

Yeah! Really!

This time, instead, the moment she picked up the phone, I said, "Pluto is no longer a planet!"

Her voice dropped. "Really?"

Yeah! Really! I was still excited about the vote and had not quite grasped her mood.

She paused for a long time. "And Xena?" she finally asked quietly.

But Diane already knew the answer. Xena had indeed gotten the same boot as Pluto, and Diane was already mourning the little planet that we had gotten to know so well.

In the days that followed, I would hear from many people who were sad about Pluto. And I understood. Pluto was part of their mental landscape, the one they had constructed to organize their thinking about the solar system and their own place within it. Pluto seemed like the edge of existence. Ripping Pluto out of that landscape caused what felt like an inconceivably empty hole.

That first morning, Diane was having the same reaction, but for Xena instead of Pluto. For her, Xena was more than just that thing that used to be called "the tenth planet." She had listened to me enough over the previous eighteen months that she had gotten to know all about the onetime tenth planet. She knew about its tiny moon, its incredibly shiny surface, and its atmosphere frozen in a thin layer all around the globe. Diane and I had discussed the excitement of the search, what to name the tenth planet, and how many more like it might be out there. Xena had become as much a part of our own mental landscapes as Pluto might have been for anyone else. And Xena would be forever linked in our minds to our daughter, Lilah, who was only three weeks old when Xena was announced to the world. All of those memories of the first months of our Lilah's life-the lack of sleep, the dazed confusion, the questions about what life would be like after this sudden change-were tied up with all of our memories of what became tenth-planet mania-the rush to learn more, the push to discover others, the questions about what life would be like after this sudden change. And now, just a little after Lilah's first birthday, Xena was gone.

I had to tell Diane: The astronomers did the right thing.

Xena is not really gone, of course. It is now actually the largest of the dwarf planets, which it rightfully deserves to be.

Lilah will probably not learn about Xena in school, but someday, we'll tell her that when she was three weeks old the world first heard about the tenth planet, and we'll pull out our little box of Xena news clippings and talk about that year when Lilah and the tenth planet were both burning themselves into our lives as things that we could never again imagine the universe without.

Chapter One.

WHAT IS A PLANET?.

One December night in 1999, a friend and I were sitting on a mountaintop east of San Diego inside a thirteen-story-tall dome. Only a few lights illuminated the uncluttered floor of the cavernous interior, but above you could vaguely see the bottom half of the ma.s.sive Hale Telescope at Palomar Observatory. The Hale Telescope was, for almost fifty years, the largest telescope in the world, but from where we sat, with the weak yellow incandescent lighting being swallowed in the darkness above, you would never have guessed where you were. You might have thought you were deep in the interior of a pristine Hoover Dam, with cables and wire and pipes for directing the flow of water around. You might have believed that the steel structures around you were part of the far underground support and control of a spotlessly clean century-old subway system. Only when the entire building gently rumbled and a tiny sliver of the starry sky appeared far over your head and the telescope began to move soundlessly and swiftly to point to some new distant object in the universe, only then would you be able to make out the shadowy outline of the truss all the way to the top of the dome and realize that you were but a dot at the base of a giant machine whose only purpose was to gather the light from a single spot beyond the sky and focus it to a tiny point just over your head.

Usually when I am working at the telescope I sit in the warm, well-lit control room, looking at computer screens showing instrument readouts, staring at digital pictures just pulled from the sky, and pondering meteorological readings and forecasts for southern California. Sometimes, though, I like to step out into the cold, dark dome and stand at the very base of the telescope and look up at the sky through the tiny open sliver high overhead and see-with my own eyes-exactly what the giant machine is looking at. This December night, however, as I was sitting with my friend inside the dark dome, there was no sky to see. The dome was fastened closed, and the telescope was idle because the entire mountain was covered in cold, dripping fog.

I tend to get quite glum on nights when I'm at a telescope with the dome closed and the precious night is slipping past. An astronomer gets to use one of these biggest telescopes only a handful of nights per year. If the night is cloudy or rainy or snowy, too bad. Your night on the telescope is simply lost, and you get to try again next year. It's hard not to think about lost time and lost discoveries as the second hand very slowly crawls through the night and your dome stays closed. Sabine-my friend-tried to cheer me up by asking about life and work, but it didn't help. I instead told her about how my father had died that spring, and how I felt unable to really focus on my work. She finally asked me if there was anything anything that I was excited about these days. I paused for a few minutes. I momentarily forgot about the freezing fog and the closed dome and the ticking clock. "I think there's another planet past Pluto," I told her. that I was excited about these days. I paused for a few minutes. I momentarily forgot about the freezing fog and the closed dome and the ticking clock. "I think there's another planet past Pluto," I told her.

Another planet? Such a suggestion would have generally been scoffed at by most astronomers in the last days of the twentieth century. While it is true that for much of the last century astronomers had diligently searched for a mythical "Planet X" beyond Pluto, by about 1990 they had more or less convinced themselves that all that searching in the past had been in vain; Planet X simply did not exist. Astronomers were certain that they had a pretty good inventory of what the solar system contained, of all of the planets and their moons, and of most of the comets and asteroids that circled the sun. There were certainly small asteroids still to be discovered, and occasionally a bright comet that had never been seen before would come screaming in from the far depths of s.p.a.ce, but certainly nothing major was left out there to find. Serious discussions by serious astronomers of another planet beyond Pluto were as likely as serious discussions by serious geologists on the location of the lost continent of Atlantis. What kind of an astronomer would sit underneath one of the biggest telescopes in the world and declare, "I think there's another planet past Pluto"?

Almost a decade earlier, in the late summer of 1992, I was in the long middle years of my graduate studies at Berkeley (the place where I was taught that Planet X certainly did not exist and that we already knew pretty much everything we needed to know about what there was in the solar system). I didn't think much about Planet X those days. I was midway through a Ph.D. dissertation about the planet Jupiter and its volcanic moon Io. When you're midway through a Ph.D. dissertation, your mind acquires narrow blinders, so I didn't think much about anything anything other than Io and how its volcanoes spewed material into s.p.a.ce and affected Jupiter's powerful magnetic field. I had so few thoughts to spare for most of the quotidian universe that I had fallen into a pattern of every day eating the same lunch at the same coffee shop right next to the Berkeley campus and having dinner at the same burrito stand a block away. At night I would ride my bicycle back down toward the San Francis...o...b..y to the marina where I lived on a tiny sailboat. The next morning I would start all over again. Less time thinking about what and where to eat and sleep meant more time thinking about Io and volcanoes and Jupiter and how they all fit together. other than Io and how its volcanoes spewed material into s.p.a.ce and affected Jupiter's powerful magnetic field. I had so few thoughts to spare for most of the quotidian universe that I had fallen into a pattern of every day eating the same lunch at the same coffee shop right next to the Berkeley campus and having dinner at the same burrito stand a block away. At night I would ride my bicycle back down toward the San Francis...o...b..y to the marina where I lived on a tiny sailboat. The next morning I would start all over again. Less time thinking about what and where to eat and sleep meant more time thinking about Io and volcanoes and Jupiter and how they all fit together.

But, occasionally, even obsessive Ph.D. students need a break.

One afternoon, as on many times previous, after spending too much time staring at data on my computer screen and reading technical papers in dense journals and writing down thoughts and ideas in my black bound notebooks, I opened the door of my little graduate student office on the roof of the astronomy building, stepped into the enclosed rooftop courtyard, and climbed the metal stairs that went to the very top of the roof to an open balcony. As I stared at the San Francis...o...b..y laid out in front of me, trying to pull my head back down to the earth by watching the boats blowing across the water, Jane Luu, a friend and researcher in the astronomy department who had an office across the rooftop courtyard, clunked up the metal stairs and looked out across the water in the same direction I was staring. Softly and conspiratorially she said, "n.o.body knows it yet, but we just found the Kuiper belt."

I could tell that she knew she was onto something big, could sense her excitement, and I was flattered that here she was telling me this astounding information that no one else knew.

"Wow," I said. "What's the Kuiper belt?"

It's funny today to think that I had no idea what she was talking about. Today if you sat next to me on an airplane and asked about the Kuiper belt, I might talk for hours about the region of s.p.a.ce beyond Neptune where vast numbers of small icy objects circle the sun in cold storage and about how, occasionally, one of them comes plummeting into the inner part of the solar system to light up the skies like a comet. I might talk about the very edge of the solar system, where millions of little icy bodies never quite got gathered up into one big planet but instead stayed strewn in the disk surrounding the solar system. And I might tell you a little history, about how in the early 1990s no one had seen such a thing as this Kuiper belt, but a small group of astronomers who had predicted its existence had named the region the Kuiper belt after Dutch American astronomer Gerard Kuiper, who had speculated about its existence decades earlier. And finally, if you were still listening and the plane had not yet landed, I would tell you how this Kuiper belt was finally seen, for the first time, in the late summer of 1992, and how I first learned about it on the roof of the Berkeley astronomy building a day before it appeared on the front page of The New York Times The New York Times.

But when Jane told me she had just found the Kuiper belt, I didn't know any of this. Jane explained. She had not found this vast collection of bodies beyond Neptune, exactly, but simply a single small icy body circling the sun well beyond the orbit of Pluto. The body was tiny-much, much smaller than Pluto-and as far as anyone knew for sure, it might have circled the sun all alone at the edge of the solar system. But still, exciting, right?

Cute, I thought. But it's just one tiny object, and it's farther away than Pluto. How could that matter?

So I nodded and listened and, like any diligent Ph.D. student midway through a dissertation, eventually walked back down the stairs, stepped into my office, and reentered the world of Jupiter and Io and volcanoes, where I actually resided.

I was wrong, of course. Even though the object discovered was only a lonely, relatively tiny ball of ice orbiting beyond Pluto, it showed that astronomers had been wrong: They didn't actually know everything; there were things still to be found at the edge of our own solar system. Some astronomers were reluctant to consider this new possibility seriously, and they dismissed the discovery as nothing more than a fluke that presaged absolutely nothing. But soon, as more and more astronomers became excited about the possibility of discovery and started searching the regions beyond Pluto, more and more of these small bodies began to be found.

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 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. 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 ma.s.sive 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 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. 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 a.s.sertions 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, s.p.a.cecraft had already visited Mars and Venus and Mercury, so some of the pictures were quite detailed. (I didn't know about these s.p.a.cecraft 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 Ura.n.u.s 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 pa.s.s inside the orbit of Neptune, but the poster showed only this inner bit of Pluto's...o...b..t. 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 would 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 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. 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 Ura.n.u.s 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 pa.s.sionately 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 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 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. 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 a.s.sumed 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 s.p.a.cecraft, 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 pa.s.sionately 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 planets, 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 was determined by the positions of the planets at the moment of my birth. 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. n.o.body 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 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 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. 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 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. 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.

In a pre-electric-light, pre-urban world, people must have been much more intimately connected with the sky and the planets. Mercury and Venus, which are close to the sun and thus only show up low in the early-evening or early-morning sky, are these days frequently mistaken for airplanes; even I sometimes mistake them. But before we became used to the idea of artificial lights in the sky, the recurring appearance of the evening or morning star would have been an obvious and spectacular event that would have been hard to miss. Mars, distinctly red in the sky, even to the naked eye, always stands out. It is no wonder that some of the earliest recorded scientific records of any sort are of the positions of the planets. Everyone would have known what a planet was back then. Planets mattered. And it is no wonder that all of our basic units of time are based on the sky: A year traced the time it took for the sun to go all the way around the sky to reappear at the same location again, while a month ("moon"-th) is about the amount of time it takes for the moon to circle the earth. The seven days of the week are even named after the seven original planets. Sunday, Mo[o]nday, and Satur[n]day are the most obvious, while Tuesday through Friday are more than a bit obscure. Tiw was an ancient Germanic G.o.d of war, as Mars was to the Romans, so Tuesday is actually Mars's day. Wednesday is Woden's day. Woden was the carrier of the dead-a Germanic grim reaper-fulfilling one of Mercury's less well known jobs. Thor was the Norse king of the G.o.ds, like Jupiter, and Friday is the day of Venus in the guise of the Norse Frigga, the G.o.ddess of married love.

Though planets were so deeply embedded into many aspects of everyday life, there is no recording of the public reaction to the first and most significant shock to the word planet planet. In the sixteenth century the idea began to spread that the sun, rather than the earth, was at the center of the universe and that the earth and the planets revolved around it. Suddenly, the wanderers were in disarray. Instead of the sun and the moon and the other planets revolving around the earth, five of them (the planets) went around one of them (the sun), while the seventh (the moon) went around the earth. The earth, like five of the wanderers, also went around the sun. Copernicus wrote down what is perhaps the most startling proposition of all time: "The motions which seem to us proper to the Sun do not arise from it, but from the Earth and our orb, with which we revolve around the sun like any other planet." We revolve around the sun like any other planet! The sun doesn't move; the earth does. The earth under our feet is like any other planet in the sky. The earth is is a planet. What seems so obvious and ingrained in us today must have been profoundly disorienting. I've tried to put myself in the frame of mind of the time and tried to understand how shocking it would have been, but I've never come close. It is as hard for me to image an Earth-centered universe as it would have been for them to imagine anything else. Everybody thought they knew what a planet was, and suddenly, one appeared beneath their feet. a planet. What seems so obvious and ingrained in us today must have been profoundly disorienting. I've tried to put myself in the frame of mind of the time and tried to understand how shocking it would have been, but I've never come close. It is as hard for me to image an Earth-centered universe as it would have been for them to imagine anything else. Everybody thought they knew what a planet was, and suddenly, one appeared beneath their feet.

And what of the moon? At least Earth was special in that, of all the planets, it alone had another body going around it. But when Galileo first pointed his crude telescope at the sky in 1609, he discovered that Jupiter, too, had objects going around it (now called the Galilean satellites). Any reasonable pair of binoculars will show you the same thing. Find Jupiter, point the binoculars up (lean against a wall to steady your shaky hands), and you'll see the disk of Jupiter and maybe even some bands of wispy clouds on the disk. Perhaps you'll also see four tiny white dots strung in a line all to one side of Jupiter. The next night look again, and one of the tiny dots might be missing-hidden behind Jupiter-and one might have moved to the other side. The next night they will change again. The little moons are wandering around the wanderer. One of them even has volcanoes. I could tell you a lot about those volcanoes.

Even with Galileo's primitive telescope, he could tell that there were stars in the sky that were too faint for the eye to see. Did he or anyone else think about whether or not there were planets in the sky that were too faint for the eye to see? No one appears to have written about the possibility. Perhaps no one even thought about it. Though the planets had been rearranged and now were secondary to the sun, and the earth had been demoted from the center of the universe to the same status as the other planets, perhaps the possibility of additional planets circling the sun so faintly that we wouldn't know about them was simply beyond comprehension. Why would such invisible things have been put there in the first place?

It took almost two more centuries to stumble upon the answer. In 1781 the British astronomer William Herschel was charting faint stars that could be seen only through his new advanced telescope. He came to one star that looked bigger than the stars around it, which was strange, since all of the stars look simply like points of light and none appears bigger than another. When he looked again the next night, the unusual star had moved. He had found a new wanderer. But since it couldn't be a planet (obviously, since all of the planets were known, right?), what was it? Herschel a.s.sumed it was a comet near the earth. Within only a few months, however, he realized that the new object was in a circular orbit well beyond Saturn, where nothing else had ever been seen before. It was no comet, it was a planet. Herschel measured the size of the greenish disk he had found and realized that this new body had to be big-not quite as big as Jupiter or Saturn, but much bigger than any of the other planets in the solar system. The word planet planet quite naturally expanded to include this new body distantly circling the sun: the seventh planet had been found. Jupiter, the largest planet, was named after the king of the G.o.ds. Saturn, originally the most distant known planet, was named after the father of Jupiter. The new wanderer, even more distant than Saturn and unrecognized throughout history until the moment Herschel distinguished it from the stars around it, was-eventually, after sixty years of debate-named Ura.n.u.s, for the most ancient of all the G.o.ds. The element uranium, discovered only seven years later, was named in honor of the new planet. quite naturally expanded to include this new body distantly circling the sun: the seventh planet had been found. Jupiter, the largest planet, was named after the king of the G.o.ds. Saturn, originally the most distant known planet, was named after the father of Jupiter. The new wanderer, even more distant than Saturn and unrecognized throughout history until the moment Herschel distinguished it from the stars around it, was-eventually, after sixty years of debate-named Ura.n.u.s, for the most ancient of all the G.o.ds. The element uranium, discovered only seven years later, was named in honor of the new planet.

Everybody had known there were only six planets until the moment the seventh was found, but once the prejudice against the idea of new planets was overcome, the idea that there could be other unseen planets was infectious, and as the techniques to build telescopes became more and more available, people began to systematically search the skies for new wanderers. Success came more quickly than expected. On the first day of 1801, Italian astronomer Giuseppe Piazzi-who, like Herschel, had been busy studying stars, not wanderers-discovered the new planet Ceres, the eighth planet, orbiting between Mars and Jupiter.

The eighth planet? Ceres? Most people today have never heard of "planet Ceres," but there was little question at the time that Ceres was indeed a planet. Within a few years it could be found in all astronomy textbooks, alongside Ura.n.u.s and the others. In keeping with tradition, the element cerium, discovered two years later, was named for the new planet. Most people today have never heard of the element cerium, either, but it is used in the walls of most self-cleaning ovens.

Planet Ceres's problems began just a year after its discovery, when German astronomer Heinrich Olbers, investigating the new planet with his telescope, accidentally stumbled upon yet another unknown object wandering through the sky: the ninth planet, Pallas! Again, there was little question that Pallas was the ninth planet, and the element palladium was named for it in 1803.

Ceres and Pallas, though considered full-fledged planets, had a few puzzling properties. While all of the other planets were well s.p.a.ced in their orbits around the sun, Ceres and Pallas were, in the cosmic scheme of things, right on top of each other between Mars and Jupiter. They were different from the other planets in other ways, too. The recently discovered Ura.n.u.s was too faint to be seen without a telescope simply because it was so far past Saturn. With the aid of a telescope, though, the green outline of the disk of Ura.n.u.s was apparent. But Ceres and Pallas were closer to us than Jupiter, closer than Saturn. They could not be seen without the aid of a telescope not because they were far away, but simply because they were so small compared to all of the other planets. They were so small, in fact, that even with the best telescopes of the day they just looked like little points of light. Herschel, the discoverer of Ura.n.u.s-wanting to preserve the uniqueness of his own discovery, I suspect-coined the term asteroid asteroid ("aster" is Greek for "star," as in ("aster" is Greek for "star," as in astronomy astronomy, while "oid" means "like") to describe these new objects. To Herschel, Ceres and Pallas weren't like real planets with their visible disks; they appeared "starlike" instead.

Astronomers quickly found two more planets in this same region between Mars and Jupiter-the tenth planet, Juno, in 1804 and the eleventh planet, Vesta, in 1807-and then, for almost forty years, nothing new came along. This was too many new planets for some people, chemists in particular. There are no elements named after Juno or Vesta. But still, forty years was long enough for the eleven-planet solar system to firmly emplace itself into the teachings of the day. In a secondary school text from 1837, the chapter between "The fourth planet, Mars" and "The ninth planet, Jupiter" is simply called "The fifth, sixth, seventh, and eighth planets." The schoolkids who had learned about the eleven planets were probably unhappy with what was about to come.

I have never seen these fifth, sixth, seventh, or eighth planets, even though they are just as easy to see in my binoculars as the satellites of Jupiter, and I look at the satellites of Jupiter in my binoculars all the time. In fact, I love a solar system tour with good binoculars. The rings of Saturn pop out, as does the redness of Mars, and sometimes even the little crescent-moon-shaped sliver of Venus that proved to Galileo that Venus...o...b..ts the sun. I can explore the craters and mountains and shadows on the moon for hours. I've carefully tracked down the position of Ura.n.u.s and stared at it several nights in a row just to experience how Herschel might have felt about his discovery. But I've never even thought to look for any of these objects that were the most exciting astronomical discoveries of the early nineteenth century.

The reason I've never looked for these four individuals, I think, is that just as the four new small planets were becoming accepted as part of our understanding of the universe, a deluge of new objects started to be discovered. By 1851, fifteen more of the new asteroid planets were found, as well as one more large planet-Neptune. Neptune was even deemed large and important enough to name an element, neptunium, in its honor, but almost no one can recall the names of the other fifteen. It was a confusing time. What counted? What didn't? On the wall in my office at Caltech I have a collection of maps of the solar system dating from about 1850 to 1900. Each map labels the solar system differently. A page from a sky atlas drawn in 1857 clearly shows Ceres, Pallas, Juno, and Vesta as "small planets," while dozens of other asteroids are generally shown in the "zone of asteroids" between Mars and Jupiter. A German map from a year earlier lists all of the known Asteroiden Asteroiden by date of discovery, with no references to their being planets at all. Even by 1896, the solar system map from the Rand McNally Atlas explicitly states that the solar system contains only the sun, planets, and comets-asteroids are not mentioned at all-and that planets are either primary (what we would call planets today) or secondary (what we would call moons). In the margins of my Rand McNally map are drawings of how big the sun would look from the planets. At the top of the margin, the sun, seen from Mercury, is huge. At the bottom, the view from Neptune shows a tiny, distant disk. In the middle are the views from Ceres, Pallas, Juno, and Vesta, still tenuously holding on to their claims to be planets. The sun looks exactly the same from each of these four since they are all the same distance away. by date of discovery, with no references to their being planets at all. Even by 1896, the solar system map from the Rand McNally Atlas explicitly states that the solar system contains only the sun, planets, and comets-asteroids are not mentioned at all-and that planets are either primary (what we would call planets today) or secondary (what we would call moons). In the margins of my Rand McNally map are drawings of how big the sun would look from the planets. At the top of the margin, the sun, seen from Mercury, is huge. At the bottom, the view from Neptune shows a tiny, distant disk. In the middle are the views from Ceres, Pallas, Juno, and Vesta, still tenuously holding on to their claims to be planets. The sun looks exactly the same from each of these four since they are all the same distance away.

By the turn of the century, though, somehow all of the confusion about what was and wasn't a planet had settled. I cannot find anything written or drawn in this period that doesn't separate the asteroids from the planets. What was their offense that they were cast down from the pantheon? In the end, their major sin seems to have been that there were too many of them all in the same place. The big planets go around the sun in orbits far from one another with no overlap, but the hundreds of known asteroids had crossing and overlapping orbits and were all one big jumble. How many is too many? When there were only four and the solar system appeared stable at eleven planets-which it did for forty years-no one (except the chemists, who couldn't discover elements fast enough) seems to have complained. But the prospect of a never-ending parade of smaller and smaller planets all in essentially the same orbit around the sun was too much. There was no official vote or p.r.o.nouncement, but by the early 1900s it became conventionally agreed that the solar system had only eight planets. Planet Ceres, which had held on for a century, along with all of its smaller neighbors, was demoted, with no outcry from the citizens of planet Earth.

By recognizing that Ceres and the swarm of other new bodies were fundamentally different from planets and should be cla.s.sified differently, astronomers had-perhaps inadvertently, but certainly profoundly-changed the scientific meaning of the word planet planet. The word no longer simply meant anything that moved around the sun and wandered around the sky. Asteroids wandered, but they wandered in a swarm; they were the schools of minnows swimming among the pod of whales. Planets were the whales of the solar system.

As a kid I knew asteroids, too. On my poster on the wall they looked like tiny pebbles strewn in a vast band between Mars and Jupiter. They were the things-the meteors-that sometimes. .h.i.t the moon and made those giant craters. I had seen shooting stars, which I knew were tiny fragments of these asteroids burning up in the earth's atmosphere. Maybe I didn't know their individual names or anything specific about them, and perhaps as individuals they were indistinguishable. But from what I knew by the time of my 1970s childhood, the difference between a planet and an asteroid was as obvious as the difference between a boulder and a handful of sand.