How I Killed Pluto and Why It Had It Coming - Mike Brown (2010)


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.

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 god 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 gods, like Jupiter, and Friday is the day of Venus in the guise of the Norse Frigga, the goddess 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. 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 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 assumed 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 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 gods. 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 Uranus, for the most ancient of all the gods. 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 Uranus 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 spaced 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 Uranus 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 Uranus 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 Uranus—wanting to preserve the uniqueness of his own discovery, I suspect—coined the term asteroid (“aster” is Greek for “star,” as in 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 orbits the sun. I can explore the craters and mountains and shadows on the moon for hours. I’ve carefully tracked down the position of Uranus 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 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 pronouncement, 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 classified differently, astronomers had—perhaps inadvertently, but certainly profoundly—changed the scientific meaning of the word 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 hit 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.

After the uncertainty and confusion about planets had been settled for a few decades and textbooks were clear that there were eight and only eight planets, the ninth was finally discovered. Clyde Tombaugh found Pluto by taking repeated pictures of the sky and comparing them to see if anything had changed. On February 18, 1930, he found a faint object that moved from one night to the next: a new wanderer! Unlike the myriad asteroids (hundreds were known by then), Pluto was not between Mars and Jupiter; it was well beyond Neptune, where a real ninth planet should be. Still, it was a bit odd. It was found to go around the sun in an elongated, rather than circular, orbit, and while all of the planets orbit the sun in a flat disk, Pluto was found to be tilted by almost twenty degrees away from the rest. Pluto also looked different. It was so small that you couldn’t tell it was a planet at all. In fact, it appeared starlike. Some astronomers didn’t want to call Pluto a planet. Shouldn’t it just be called an asteroid instead? By then, though, the word asteroid had lost its literal meaning of “starlike” and instead referred specifically to that belt of objects between Mars and Jupiter. Should it be called a comet? Comets can have elongated and tilted orbits like Pluto’s, but none had ever been seen so far away, and the word comet (from coma, Latin for “hair”) specifically refers to the fuzzy appearance of comets in the sky. Pluto was not fuzzy; it looked like a star, albeit one that moved. Though it looked and behaved like no other planet known, there was no other way to classify it, so it became accepted as the ninth planet, had the element plutonium named for it, and remained unchallenged for almost seventy years as the tiny lonely oddball at the edge of the solar system, the planet with the ice spires, the planet with the orbit so extreme that it couldn’t even fit on my poster on the wall, the incongruous period at the end of the solar system.

What I didn’t immediately grasp when Jane Luu joined me on the roof overlooking the San Francisco Bay at the Berkeley astronomy department in 1992 was that the discovery of the Kuiper belt gave Pluto a context. It took me and most other astronomers a few more years to realize that Pluto is neither lonely nor an oddball, but rather part of this vast new population called the Kuiper belt. Just as the explosion of asteroid discoveries 150 years earlier had forced astronomers to reconsider the status of Ceres, Pallas, Juno, and Vesta and change them from full-fledged planets to simply the largest of the collection of asteroids, the new discovery of the Kuiper belt would certainly force astronomers to reconsider the status of Pluto. It was becoming more and more clear that if the asteroids were the schools of minnows swimming among the pod of whales, then Pluto and the Kuiper belt objects were simply a previously overlooked collection of sardines swimming in a faraway sea. If Ceres was to be thought of as just the largest of the vast collection of asteroids and thus not a planet, why should Pluto not suffer the same fate? What, after all, was a planet?