Mike Brown

by How I Killed Pluto;Why It Had It Coming

  Using the words “classical planets” is a compromise which allows more than one kind of planet in the universe. Yet advocates of the [eight-planet] model have refused this term. They will tell you why. Listen carefully. The word planet is being restricted to just one narrow point of view. Their restriction means that a dwarf planet is not a planet. It would be like saying a dwarf star is not a star. We can fix this. Will we have too many planets? Will we confuse the public? No. The distinction is as simple as an umbrella. Pluto is a planet, but it is in the dwarf planet category. So please pass 5B. The word planet must be shared.

  I almost felt bad enough to want to give in. I didn’t object to dwarf planets being considered planets, which was all he was talking about. But I did object to the other planets being termed “classical planets.”

  For the anti-Pluto side, a British astronomer stood up and spoke:

  The key issue is the definition of the concept planet. This is a very important decision to be taken by the IAU; Resolution 5A is very close to the definition that was agreed by consensus at the meeting on Tuesday. There it was made clear that 3 distinct categories were being defined. Planet, dwarf planet, and small solar system body. The amendment [5B] proposes to insert the word “classical” in front of the word planet. It is inconsistent with the 1st paragraph of Resolution 5A. And it transforms 3 distinct categories into 2, planets and the rest, and that too has been made clear. In answer to the question, how many planets in the solar system? Resolution 5A gives the clear answer: 8. Resolution 5B implies at least 11 and soon several dozen. Both Pluto and Ceres become planets, and probably several main belt asteroids and several Kuiper belt objects as well. Resolution 5B not only removes a fundamental dynamical distinction for a planet, it is confusing and internally inconsistent. In my view it should be rejected.

  Sadly, even though I was all in favor of rejecting the resolution, I found almost none of the arguments compelling. Who cared what the consensus was on Tuesday? The final vote was today! And really, if the concepts were significant, wasn’t it more important to make sure to get them right than to worry about the precise wording of the resolution? Besides, would it matter if there were eleven or more planets? It wasn’t the number that mattered, it was getting the concepts right. I realized that I wouldn’t have minded if there were “major planets” and “dwarf planets” instead of “classical planets” and “dwarf planets.” I guess my own version of pickiness was just as bad as anyone else’s.

  The vote was called. If the resolution passed, Pluto would be a planet again, and Xena would officially be part of the club. Chad, David, and I would be the only living discoverers of a planet in the world. At least for now. And still I didn’t want it to happen.

  “All in favor of the resolution?”

  Astronomers in favor of 5B—in favor of repromoting Pluto—held up their yellow cards. There were many. The counting took a few minutes.

  “Mister President, we report ninety-one votes in favor.”

  That didn’t seem like enough, but I couldn’t tell from the tiny webcast precisely how many astronomers were there in the auditorium.

  “All opposed to the resolution?”

  Astronomers opposed to 5B, who wanted to firmly cap the solar system at eight planets, held up their cards. A sea of yellow filled the auditorium, which immediately erupted in applause.

  “I think, Mister President, a further count is not honestly needed.”

  “Then it’s clear that Resolution 5B is not passed.”

  At that point it was final. And I said to the assembled press: “Pluto is dead.”

  The cameras whirred; correspondents talked into their microphones; 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.”

  Most of the remainder of the day was a blur of interviews, condolences, and congratulations. That afternoon I made my way to the studio of a radio station, where I was scheduled to be on a call-in program broadcast throughout Los Angeles. When I showed up at the studio, they told me that another astronomer would be calling in as a guest.

  Great, I thought. Another guest would help me to stay focused and coherent.

  When we went live on air, I suddenly realized that the astronomer was none other than the member of the once-secret planet-definition committee, live from Prague! It had been an even longer day for him than it had been for me.

  He seemed tired, and he definitely didn’t seem happy. He talked about how he thought the vote had done a disservice to astronomy. I said I thought astronomy had done a great service to the world.

  He said that he was sad that no one would ever again be able to discover a new solar system planet under the current definition.

  “You know,” I said, over the radio to him half a world away, “when you tell me that no one will ever discover a planet again, I just take that as a challenge.”

  Over the course of the radio show, we both answered questions from callers. It was becoming clear that the idea that Pluto was no longer a planet was not going to be an easy sell.

  Throughout the hour, the host collected suggestions for a new mnemonic for remembering the order of the planets. Some gave a slight modification of the previous standard—My Very Educated Mother Just Served Us Nine Pizzas—by turning “Nine Pizzas” into “Nachos” or into “Nothing,” which was a bit funnier. But the best mnemonic, and the one that I still tell people to use to this day, sent in by an anonymous listener, sums up the feelings that would envelop much of the world over the next days, weeks, and months:

  Mean Very Evil Men Just Shortened Up Nature.

  Chapter Thirteen

  DISCORD AND STRIFE

  Keeping Pluto dead has taken a lot of work.

  In the days, months, and years since the decision was made, I’ve been accosted on the street, cornered on airplanes, harangued by e-mail, with everyone wanting to know: Why did poor Pluto have to get the boot? What did Pluto ever do to you?

  It is at these moments that I am most happy that astronomers ignored my initial advice to simply keep Pluto and add Xena and forget about a scientific definition. I am thrilled that astronomers instead chose to put a scientific foundation behind what most people think they mean when they say the word planet. They don’t mean “everything the size of Pluto and larger,” and they certainly don’t mean “everything round.” Instead, when people say “planet,” they mean, I believe, “one of a small number of large important things in our solar system.”

  My job is just to explain the solar system as it actually is. People, I think, will then realize themselves that Pluto is not one of these large important things in our solar system.

  Here is what I say to people:

  Many astronomers, tired of the endless debates before and after the demotion of Pluto, will tell you that, in the end, none of this matters. Whether Pluto is a planet or not is simply a question of semantics. Definitions like this are unimportant, they will say. I, however, will tell you the opposite. The debate about whether or not Pluto is a planet is critical to our understanding of the solar system. It is not semantics. It is fundamental classification.

  Classification is one of the first processes in understanding something scientifically. Whenever scientists are confronted with a new set of phenomena, they will inevitably, even subconsciously, begin to classify. As more and more things are discovered, the classifications will then be modified or revised or even discarded to better fit what is being observed and what they are trying to understand. Classification is the way that we take the infinite variability of the natural world and break it down into smaller chunks that we can ultimately understand.

  So how should we classify the solar system? It’s hard, because we are sitting in the middle of it and have known planets our whole lives. But let’s try to do it from the perspective of someone who has never seen a planet before. Imagine that you are an alien who has lived your whole life on a spaceship traveling from a distant star to the sun. Yo
u don’t know that planets exist. You don’t even have a word for planet in your language. All you know is your spaceship and the stars you can see surrounding you. The sun—which originally looked like any other star—now gets brighter and brighter as your destination nears.

  As you start to stare at and wonder about the sun, you suddenly notice that—wait!—the sun is not alone! You see that there is something tiny right next to it. You’re excited beyond alien words. As your spaceship gets closer and you look even more carefully, you suddenly realize there are two tiny things next to the sun. No, three. No, four!

  You have just found the things that we call Jupiter, Saturn, Uranus, and Neptune: the giant planets. From your perspective, still quite far from the solar system, they look tiny and so close to the sun as to be barely distinguishable. You don’t have a word to describe them, so you make one up in your alien language: Itgsan.

  You keep looking for a fifth Itgsan out beyond that fourth one you found, because it seems logical that there should be more, but even as your spaceship gets closer and closer to the system, you don’t see anything out there. Trust me, I understand your disappointment.

  Finally, as you get close and the four Itgsan get brighter and appear more distinguishable from the sun, you realize you were looking in the wrong place all along. There are other things next to the sun, but they are inside the first Itgsan, not outside. There are four of them, but they’re much smaller than the first four things you found. So you come up with a new word. You call them Itrrarestles. You don’t know it, but you’ve just found Mercury, Venus, Earth, and Mars.

  For a very long time, as you keep getting closer, there is nothing new. Finally, when you’re almost on top of the solar system, you realize that between the small Itrrarestles and the large Itgsan there is a band of millions and millions of tiny things going around the sun. And looking even more carefully, you see that outside the large Itgsan there is another band with even more. You call them something that I can’t pronounce, but I call them the asteroid belt and the Kuiper belt.

  Nowhere in that alien brain of yours would it be likely to occur to you to take one or two or even a few hundred of the things sitting in the Kuiper belt or in the asteroid belt and put them in the same category as the big things, the Itgsan and the Itrrarestles. Instead, you would quite rationally declare that the solar system was best classified by four major categories. And you would, I think, be correct.

  The only thing wrong with our current classification of the solar system as a collection of eight planets and then a swarm of asteroids and a swarm of Kuiper belt objects is that it ignores the fundamental distinction between the terrestrial planets—Mercury, Venus, Earth, Mars—and the giant planets—Jupiter, Saturn, Uranus, Neptune. In the class on the formation of planetary systems that I teach at Caltech, I try to convince my students that, really, there are only four planets and that Mercury, Venus, Earth, and Mars shouldn’t count. But even students who worry about their grades aren’t willing to go that far. So, even though the aliens call them Itgsan and Itrrarestles, we’ll lump them together and just call them all Tsapeln.

  You can classify anything at all in many different possible ways. If you are studying birds, you might split them into land birds and seabirds; carnivorous birds and seedeaters; red birds, yellow birds, black birds, and brown birds. All of these distinctions can be important to you, depending on what it is you are studying about birds. If you are studying their mating habits, you might classify them in categories of monogamous and polygamous. If seasonal migration is your thing, you could classify them by those that stay put and those that fly south for the winter.

  Things in the solar system can equally well be categorized in many different ways. Things with atmospheres. Things with moons. Things with life. Things with liquids. Things that are big. Things that are small. Things that are bright enough to see in the sky. Things that are so far away that only the biggest telescopes will ever see them. All of these are perfectly valid categories, and they might be of utmost importance to you if you study one specialized type of thing about the solar system. As with birds, your favorite solar system classification will depend on your interests.

  Most people, though, don’t have specialized interests in the solar system. The only classification scheme they will ever know is the word planet. They will know what a planet is and how many planets there are and what their names are. Their entire mental picture of what the solar system is, of how our local bit of the universe is put together, will be carried in the understanding of that simple word. The definition of the word planet, then, had better carry with it the most profound description of the solar system possible in a single word.

  If you think of the solar system as a place consisting of eight planets—or, better, four terrestrial planets and four giant planets—and then a swarm of asteroids and a swarm of Kuiper belt objects, you have a profound description of the local universe around us. Understanding how such a solar system came to be is one of the major tasks of a wide range of modern astronomers. If, on the other hand, you think of the solar system as a place with large things that are round and smaller things that are not quite round, you have a relatively trivial description of the universe around us. There is nothing important to study here: We’ve known for hundreds of years that gravity pulls big things in space into the shape of a sphere.

  • • •

  Sometimes you don’t even have to go through such extensive arguments. If you catch a person early enough, before the idea that Pluto deserves to be a planet has sunk in, you can teach things correctly from the start. Take Lilah, for example. Everywhere I went in the months following the IAU decision, people wanted to know if I thought Pluto had been treated fairly. Did I think Pluto was a planet? After a few weeks, I taught Lilah to answer for me.

  “Lilah, is Pluto a planet?” I would ask, beginning our choreographed banter.

  She would frown and shake her head.

  “No no no no no no no.”

  As she got older the banter continued: “So what is Pluto, Lilah?”

  “He’s not a real dog. He’s a dwarf dog.”

  My friends would laugh, and then invariably go out and buy Lilah Pluto toys. She has stuffed dogs, of course, but also a collection of nine-planet memorabilia. Early on she learned to figure out which one of the nine little circles on whatever picture she had was Pluto and then promptly declare, “Pluto is a dwarf dog.” The continued laughs from that line were more reinforcement than I could possibly have given.

  Another friend was worried how Lilah would react when she got older and discovered that I was a planet killer. “What will Lilah think,” the friend said, “when she learns that Pluto is not a planet and that you are to blame?”

  “I know what’s going to happen,” I replied. “In second grade or third grade, when she learns about planets she’ll come home and say, ‘Daddy, today we learned about the eight planets,’ and I’ll say, ‘Lilah, did you know that when you were born we thought there were nine or even ten planets?’ She’ll look at me, shake her head, and say, ‘You know, adults are so stupid.’ ”

  • • •

  Now that Xena, too, was officially called a dwarf planet, it finally got a real name. The possibilities were wide open, but Chad, David, and I had decided that because—at least in our minds—Xena had been the tenth planet in good standing for an entire year, we wanted to give it a Greek or Roman name, like all of the other planets have. The problem was that there were very few left to go around. Back in the 1800s, when asteroids were first being discovered, they were, of course, called planets. And people wanted them to have Greek or Roman names, like the other planets. So they used up almost all of the major gods and goddesses and most of the minor ones, too. Every time we found a name we thought might be nice, we had to look it up in the databases of asteroid names to see if it had been used. Usually it had been. Finally, David wrote a quick computer program to correlate all asteroids with all names of Greek and Roman gods so we could see what
—if anything—was left.

  There wasn’t much, and what there was was hardly recognizable. Obscure demigods of long-forgotten activities. Minor protectors of long-gone professions. But one name grabbed my attention. I remembered this name from my high school mythology readings, and I couldn’t believe no one had used it before. Here was a major goddess with a fascinating backstory, overlooked in the solar system for two centuries. I quickly double-checked all of the asteroid databases. I double-checked that my mythological memory was correct. And then I sat down and wondered, for the first time since I had correctly predicted my sister’s pregnancy, whether or not there was some sort of cosmic force governing the stars and planets and even the dwarf planets after all. Maybe there was some sort of fate that had kept this name free until now, the perfect time for it to be unveiled. Maybe there was no free will in any of this. That idea is, of course, crazy, but it’s hard not to think crazy thoughts now and then.

  I quickly e-mailed Chad and David, and we all agreed: the largest dwarf planet, temporarily nicknamed Xena, cause of the largest astronomical showdown in generations and the killer of Pluto, would henceforth be called Eris, after the Greek goddess of discord and strife.

  I love the myth of Eris. As a perpetrator of discord and strife, she was not everyone’s favorite goddess to have around, so when the human Peleus and the sea nymph Thetis decided to wed, they didn’t invite her to the wedding. I understand their dilemma. Having gotten married myself, I know that there are always touchy issues involving the invite list. There are A lists and B lists and whole categories where you think, “Well, if I invite one person from this category, I should really invite everyone from this category,” and then the bar tab gets out of control. If you find yourself having a wedding and are trying to decide whether or not to invite the goddess of discord and strife, my only recommendation to you is that if you decide not to invite her, make sure that she is not the only goddess who is not invited, which was the mistake Peleus and Thetis made.