How I Killed Pluto and Why It Had It Coming

by Mike Brown

  I probably should have known better than to ask him. I remembered that in college he had told me that he woke up every morning surprised that reality was still reality.

  Still, maybe there was something to it. Maybe words do just mean what we think they mean.

  Perhaps it is wrong for astronomers to attempt to redefine a word when people already know what it means when they say it. Perhaps the job of astronomers is, instead, to discover the definition of the word planet as people use it. After all, the word planet has been around much longer than, well, our understanding of planets.

  So what do people mean when they say the word planet? That spring, well before anyone knew that the world was about to be handed a tenth planet, I started asking everyone I saw. The answers were diverse and, more often than not, scientifically misguided: large rocky bodies in the solar system (well, no, there are gas giants), things with moons (not Mercury or Venus!), things that are big enough to see with your eye (Uranus, Neptune, and Pluto are out), things that pull the earth around in its orbit (that’s just the sun). But when I then asked people to name the planets, everyone had exactly the same answer, starting with Mercury and ending with Pluto. People who felt themselves quite up to date and informed would then explain that maybe Pluto shouldn’t be called a planet, but they certainly knew that it currently was one.

  So, again, I ask: What do people mean when they say the word planet? They mean a slew of unscientific clutter. And then they mean nine specific objects in the solar system.

  I always pressed people further: How would you know if something new was a planet? The answer was always the same: If it was as big as the other planets. Or, as I interpreted it, according to my unscientific springtime poll, everything the size of Pluto and larger that orbits around the sun is a planet.

  Isn’t that the real definition, then? Shouldn’t astronomers leave the word alone if it already has a meaning?

  I remained torn. If Pluto was a planet, why were the many things just a little smaller than Pluto not considered planets? It made no scientific sense at all. Why draw such an arbitrary line right around the size of Pluto? Isn’t the job of scientists to guide the public’s understanding of nature rather than acquiesce to unscientific views?

  In addition to everything else happening that spring, while Xena and Santa and Easterbunny were just being found and studied, and Lilah—still known as Petunia—was growing and beginning to kick inside Diane’s stomach, I was teaching introductory geology at Caltech for the first time. I’m not a geologist. I’ve never taken a single class in geology. If you gave me a handful of different types of rocks, chances are I could identify only a small number of them. I still get confused by the meanings of strike and dip.

  Luckily, most of my students didn’t realize this.

  I was pretty good at teaching that class, actually. The class was the equivalent of what is called a “rocks for jocks” class at many other universities, meaning that it is intended for people who won’t end up majoring in geology. Caltech, though, is not known for its jocks. All of the kids in the class who don’t major in geology are majoring instead in physics or biology or mathematics or engineering. I affectionately referred to the class as “earth science for eggheads.”

  But why was I teaching a class about which I knew nothing? One reason only: I had begged. As an astronomer who studies planets, I have ended up at Caltech not in an astronomy department but in a planetary science department. And the planetary science department is tacked on to the side of the geology department. The people I see walking around the halls and coming to my classes tend to be geologists. After having been at Caltech for almost a decade, I thought it might be time to actually learn some geology. And what better way to learn than by teaching it myself?

  I had intended to spend most of the winter preparing for the class; instead, I spent it working on the newly discovered Santa and Xena. As the first class came around in April, I was barely on track with the teaching. And then we discovered Easterbunny that week.

  Still, I stayed about two weeks ahead of the class, learning the material as I went along. Over the course of the term, I said only one thing that I now know to be blatantly wrong. (To anyone who took my Ge 1 class in 2005, I apologize. The mineral peridotite does not change into spinel as it is compressed by high pressure; its crystal structure collapses to one that is identical to that of spinel, but the chemical compositions of the two minerals are totally different.)

  Teaching earth science to eggheads has so far been the highlight of my teaching life. The earth is a spectacular laboratory that you can get to by simply walking out the back door. The eggheads and I took trips to the local arroyo to understand debris flows in the Los Angeles mountains; we walked one mile south of Caltech to our local thrust fault; we took a bus up the east side of the Sierra Nevada, stopping to see ancient volcanic flows, now-dry Ice Age lakes, and a 50-million-year-old mountain range now buried almost to its top in debris. All the while, I tried to pull the students out of the mind-set that is all too easy to get into in the middle of a hard first year at college: Give me the information; tell me what I need to know, what’s on the exam. In earth science for eggheads the message was instead: Look around you! What is happening here? Why?

  Because my head was so immersed in the geological world that spring, it is perhaps not surprising that I started looking to the earth sciences for examples of the ways in which scientists were confronted with words that had previous meanings. Geologists, in fact, have had a more difficult time than astronomers on this issue. While planets are up in the sky and don’t form part of most people’s everyday experiences, daily life is filled with geology. People see mountains, rivers, lakes, oceans. Or should they really be called hills, streams, ponds, and seas? When is something a mountain instead of a hill? A river instead of a stream? A lake or a pond? An ocean or a sea?

  Geologists have never attempted to define these things. The words simply mean what people think they mean when they say them.

  I grew up on a little rise in northern Alabama called Weatherly Mountain. As a child I assumed that the word mountain had some sort of meaning. When we took our first family trip west and encountered the Rocky Mountains rising six thousand feet from their base, I was stunned. Our three-hundred-foot-high mountain looked to be a molehill in comparison. But still, Weatherly Mountain will always be Weatherly Mountain.

  The best geological equivalent to the word planet is the word continent. What does the word continent mean? As far as I can tell, the definition is something like: big coherent chunk of land. How big? The only answer I could ever find was “big enough.” Australia is big enough. Greenland is not.

  I began to quiz people about continents as much as I quizzed them about planets. I heard all sorts of interesting theories about how the word continent was defined, including a few from people who knew a little geology. I was told, emphatically: A continent is any island on its own continental plate. Greenland doesn’t qualify because it is on the same plate as the rest of North America and thus is not separate. I pointed out that continents have been around much longer than the plate tectonic theories of the 1970s. And I pointed out that by the “scientific” definition, we should really count the south island of New Zealand as a separate continent.

  So how do we really define continents? Simply by tradition. The seven continents are the seven continents because that’s what people mean when they say the word continent.

  But even that is not entirely true. Apparently some people mean different things. As I quizzed more and more people, I learned that, for example, many Europeans do not consider Australia to be a continent. Argentinians consider North and South America a single continent (the Panama Canal is not enough of a break for them, I guess). And rational people in many places believe that Europe is considered a separate continent only because, well, that’s where the people who defined the continents in the first place all came from.

  Can it really be that the most important classification schem
e for our understanding of landforms has no scientific basis whatsoever? Shouldn’t geologists get to work defining their terms more carefully?

  And yet, when geologists talk about continental crust or continental shelves they know exactly what they are referring to. They never use the word continent by itself unless it is just to refer to one of those landmasses that we agree are called continents.

  For the public, having a handful of continents whose names everyone can remember (even when everyone doesn’t always agree) is an important way to organize our understanding of the world around us. It is too difficult to make sense of the hundreds of countries on the earth without an organizing principle. The continents are a way to bring the vastness of the earth down to a human scale.

  And so with planets. The planets are our way of organizing the universe beyond the earth. In fact, they are the grandest organizational scheme that most people know. Ask someone to describe what is around them and they’ll describe their neighborhood. Press further and they might talk about their town and region. If you keep pressing, perhaps they will mention their country, next their continent (that word again!), and finally the world. But if you don’t give up there and you ask for more, you will ultimately be led down the descriptive path of the solar system. You’ll be told about the planets. And after the planets? What next? More often than not, you will be left with blank stares.

  When people describe their neighborhoods, they don’t care about the scientific meaning of the words they’re using; they care about recognizable landmarks to specify the points and the boundaries of their lives. The planets are these landmarks. That is what people mean when they say the word planet.

  Is the word planet, then, specific or descriptive? When people say the word planet, do they mean precise places—Mercury and Venus and Earth and the others—or do they mean those places and anywhere else like them?

  I found history to be a useful guide. When Uranus was accidentally discovered, it was quickly accepted as a planet; Neptune, likewise. Even Pluto, whose status was at stake in all of this, was accepted into the club with only a little grumbling. Sure, it was thought to be much bigger when it was originally accepted, much more like the other planets, but with its acceptance the bar was accidentally lowered, and most people—except for me and a few other nitpicky astronomers—meant Pluto, too, when they said planet.

  All of this planet-or-not-a-planet business would eventually be decided by the International Astronomical Union, which, by international agreement since 1919, has the right and the responsibility to make sure that everything in the sky is categorized, named, and filed in its right place. Before the IAU came along, the skies were filled with objects named by whatever system the astronomers categorizing them chose. The reddish star in the upper right of the spectacular constellation Orion is known not just by its common name Betelgeuse, which in Arabic means “armpit of the giant,” but also by HD 39801, for its place in Henry Draper’s catalog from the 1920s, and by many more names, including PLX 1362, PPM 148643, and my favorite, 2MASS J05551028+0724255, in other catalogs. The IAU now has procedures and policies for what to do about almost every type of discovery in the sky. A new supernova explodes? It gets a year and a letter. Supernova 1987A was the closest and brightest in living memory, and those five characters can still provoke a glowy sigh in an astronomer of a certain age. The procedure is much more systematic, though the names are not quite as evocative as giants and armpits.

  Only with the solar system does the IAU require history and poetry when it comes to naming things in the sky. By IAU decree, the moons of Jupiter are to be named after the consorts (voluntary or otherwise) of Zeus, craters on Mercury are to be named after poets and artists, and features on Saturn’s gigantic moon Titan (and I can only call them “features” because we have no idea what they really are) are named after mythological places in literature.

  The IAU was quick to act after the discovery of the first objects out beyond Neptune in the Kuiper belt. Things way out are named after creation gods in world mythology, though as the number of objects in the Kuiper belt grew faster than new creation gods, the rule began to be applied more and more loosely. Recently someone even got away with calling something in the Kuiper belt Borasisi, which is a god from a fictional story by Kurt Vonnegut.

  For all of its preparedness for endless contingencies and countercontingencies, the IAU had never actually contemplated a question that was suddenly on everyone’s minds: What do you call something new that is bigger than Pluto? How do you recognize a new planet when you see it?

  Like any good international organization, it knew what to do when faced with such emergencies: It needed to form a committee. Better yet, it already had one. At about the same time that Xena was discovered and astronomers (and everyone else) began to try to figure out how small the smallest planet could be, astronomers were also struggling with the question of how big the biggest planet could be. We had it easy in the solar system; it was unlikely that I would ever find anything bigger than Jupiter that needed to be categorized, but discoveries of things larger than Jupiter orbiting around distant stars were beginning to become routine. Some were about as massive as Jupiter or only a little more so. They were clearly planets. Some were only somewhat less massive than the sun. Those were clearly stars. Some were in between. What to do? The IAU formed a committee to decide. This same committee was now charged with figuring out the small end of planets, too.

  When the reporters called to talk about the discovery of Xena, they wanted to know where it was, how we had found it, and how big it was. We didn’t yet know the size for sure but thought it could be as much as half again the size of Pluto. And then they asked: When is the IAU going to make a decision about planets?

  “I hope they’ll decide before my daughter starts crawling,” I joked when Lilah was three weeks old.

  When a bevy of new reporters called after the story about the Spanish snooping broke, they asked what was going to happen to the Spaniards, how the conflict was going to be resolved, and how this might change the way astronomers interacted and protected their data. I explained that many scientists worldwide suddenly realized that they, too, could be vulnerable to unintended snooping, and many were scrambling to find solutions. And then they asked: When is the IAU going to make a decision about planets?

  “I hope they’ll decide before my daughter learns to stand,” I joked a few months later, since Lilah was already crawling by that point.

  When we discovered that Xena was not alone at the edge of the solar system, that it had a tiny moon going around it, reporters called again and wanted to know how the moon got there, what it looked like, and what we were planning to call it (Gabrielle, of course, after Xena’s spunky TV sidekick). And then they asked: When is the IAU going to make a decision about planets?

  “I hope they’ll decide before my daughter says her first words,” I joked the next winter, since by then Lilah was already standing and taking increasingly assured strolls around the periphery of the room.

  In the spring, when we finally were able to use the Hubble Space Telescope to figure out just how big Xena really was, reporters called again and wanted to know what Xena was made of, how it had gotten so big, and how much larger than Pluto it was (only 5 percent, it seemed, which was uncomfortably close to not even being bigger than Pluto at all, particularly when you include approximately 4 percent uncertainty in the measurement). And then they asked: When is the IAU going to make a decision about planets?

  “I hope they’ll decide before my daughter goes to college and takes an astronomy class,” I finally reverted to joking when it became clear that no decision was going to be made anytime soon.

  People kept asking me when the IAU was going to make a decision because they thought I should know. But I didn’t know anything. During this entire period, no one officially connected with the decision making—and I didn’t even know who that might be—ever once contacted me to ask a question or to tell me what was going on. I assumed th
at I was going to wake up one morning, open the Los Angeles Times, and see that I was suddenly the official discoverer of a planet. Or that there were only eight planets. Or that I had discovered many planets. Or that I had discovered the only thing in the solar system larger than a planet that wasn’t a planet.

  In the face of this uncertainty, I figured it best to be prepared for all options. I called up the person in media relations at Caltech who had—months earlier now—pressed me to decide whether or not to call Xena a planet in that original press release. I told him that we needed to prepare another press release, this time for the IAU decision.

  “Great!” he said. “What did they decide to do?”

  “Well, actually, they haven’t decided anything yet.”

  “But they’re deciding soon, right?”

  “Well, actually, I have no way of knowing what they are doing. They might decide tomorrow, and they might decide a decade from now.”

  “So …” He paused. “What are we going to say in a press release?”

  I knew that I wanted to have the opportunity to tell the full scientific story to the public. I had missed that chance, I felt, back in the original hurried rush when we had to make the announcement right away. I wanted the beauty and subtlety and essential order of the solar system to be at the center of the discussion after a decision was finally made. I cared less what the IAU decided—within limits, of course—than that the science got explained correctly.

  “We’re going to write four different press releases,” I explained.

  Ten planets made sense if you wanted your planets to have more emotional resonance than scientific significance. We very quickly wrote that press release, hailing Xena as the tenth planet. It made me proud to think of my tenth planet, but even from early on I admit that it also made me feel a little fraudulent. The discovery of Uranus was a big deal, and that of Neptune was amazing. But Xena? Little Xena? The tenth planet? Still: I channeled my inner geologist. If it mattered emotionally, that was all that counted. I was ready.