Mike Brown

by How I Killed Pluto;Why It Had It Coming

Living on the outskirts of Los Angeles with a clean sweep of the sky to the south of us, we have a very nice view of the standard flight path for arrivals and departures at LAX. Those things moving through the daytime sky that turned into lights brighter even than the stars at night held a special fascination for Lilah. Her sign-language symbol for airplane (arm held aloft with hand parallel to the ground) got much use. First it was just for those little moving dots in the sky, then it was for pictures of airplanes in books, and then, one very exciting day when Lilah was thirteen months old, it was for an airplane that she actually got to fly in. I spent the whole morning trying to prepare her for the mental transition:

  “Look! Airplanes in the sky!” I said, as we got close to LAX.

  “Look! The airplanes are driving around on the ground!” as we were moving through the airport.

  “Look! That is the tunnel people take to get on to the airplane!” as we were at the gate.

  “Look! We’re now inside!” as we sat down.

  For what I had assumed would be a difficult cognitive leap, Lilah took it all in stride. Of course we’re inside the airplane and now flying in the sky, Daddy. What else would we be doing?

  We were taking our first family vacation, two weeks on Orcas Island, the largest of the San Juan Islands, northwest of Seattle. Diane had lived on Orcas Island for her high school years, and her mother still lived there. It was the first place Diane and I had ever gone together on an official vacation (Hawaii together? That was just work). And now we were bringing our daughter there. Diane has an affinity for the annual Library Fair, where used books get sold on a Saturday morning in August at the otherwise sleepy farmers’ market. For reasons that I, as a non–native islander, can never fully understand, the Library Fair is an unspoken homecoming day; previous island inhabitants show up, as if by accident, to stroll around on Saturday morning, peruse the used books, and snack on barbecued oysters.

  I love the Library Fair and—out of sheer exuberance—always buy used books that I would not otherwise even glance at. And then I sit on Diane’s mother’s porch on a midsummer twilight lasting until well past ten o’clock and read.

  But during this very first family vacation with Lilah, I had no opportunity to relax and read on the porch. My vacation coincided with the once-every-three-years meeting of the International Astronomical Union, this year in Prague. And at this meeting, for the first time in history, they were finally going to vote on the definition of the word planet.

  Why wasn’t I there? Why was I vacationing half a world away from where the astronomical action was?

  It’s a good question.

  There are four answers to that question. First, I love the Library Fair. Second, it was our first family vacation. Third, no one—not even the astronomers who found themselves in Prague—had been warned that the vote about planets was imminent. I admit, had I known ahead of time that this vote was going to take place, I might have felt obligated to be there rather than to abscond to a small island in the Pacific Northwest. Luckily, I didn’t know. Fourth, and probably most important, I am not a member of the International Astronomical Union. I would have been ineligible to vote. I’m embarrassed to admit that I can’t get myself to fill out the paperwork to join. It’s all because of Question 12 on the form. After the form asks for your academic qualifications and awards, both of which I can handle, it then quizzes you about your miscellaneous special distinctions. And then I stop. Well, I think, I’m no William Herschel (the discoverer of Uranus, which is indisputably a planet). I’m no Adams or Leverrier or even Johann Galle. (Adams and Leverrier predicted the existence of Neptune, and Galle confirmed it.) Really, I wonder, do I have any special distinctions? And each time I get to that spot in the application, I have to stop and put down my pen. There was no reason to bother going to the International Astronomical Union meeting in Prague because I would not have been let in the doors.

  • • •

  I was sitting in Diane’s mother’s house on Orcas, watching the sailboats navigating Westsound out the window, when an e-mail arrived from the other side of the world telling me the details of what, precisely, the IAU was going to vote on. I read it to Diane in my excitement.

  “Diane, Diane, it says right here that the planets are to include the big eight, of course, and then also Pluto and 2003 UB313—that’s Xena—and wait a second, there are a few more.” I was confused. Xena and Pluto were nine and ten. But there was also to be Ceres—the asteroid discovered in 1801 that was declared not-a-planet sometime around 1850. And in a surprise that I had never anticipated, Charon, the moon of Pluto, which was about half the size of Pluto, was to be number twelve. Twelve planets. Not eight, nine, or ten, or even two hundred, which I would have understood. And Charon? The e-mail didn’t make any sense. I didn’t recall any discussion in which naming Pluto’s moon a planet had ever come up before. What was the committee thinking? Who in their right mind would declare Charon a planet?

  I reread the e-mail carefully. The committee, which had met in secret, was adhering to the notion that “all round things are planets,” which I had thought was a bad idea to begin with but which I at least understood and could support as a scientifically rational and consistent definition, even if a poorly chosen one. If the assembled body of astronomers thought that that was the right way to define the word planet, I would be disappointed personally, but I would get over it. After all, I would still get a few planets out of it.

  The secret committee had its reasons, which were passionately stated. First: The word planet should have a scientific basis. Who was I to argue against that point? I had been willing to go along with a cultural definition instead of a scientific one, but if the astronomers were going to insist on science, I could hardly say no. And second, they suggested that in deciding whether an object is a planet or not, you should be able to tell just by looking at it—in other words, you shouldn’t have to know anything about where it is and what it is doing and what else is around it. The committee didn’t buy the idea that planets should be the small number of unique important dominant objects in the solar system.

  And then they discussed the newly proposed twelfth planet, Charon.

  Charon is the biggest of Pluto’s three moons. It was discovered, accidentally, in 1978 by James Christy, an astronomer at the United States Naval Observatory who was examining old photographs of Pluto and noticed a slight bulge coming and going, first on one side and then the other. Though Charon is smaller than our moon, than four of Jupiter’s moons, than one of Saturn’s moons, and than one of Neptune’s moons, making it only the eighth largest moon in the solar system, it is big proportionally to Pluto. And because it is big in proportion to the planet around which it orbits, it alone of all of the moons in the solar system deserved to be a planet.

  What?

  In the proposal from the committee, Charon was considered to be a planet for two reasons. First, it was big enough to be round, which was in itself a good enough reason to be considered a planet if you’re inclined to think of planets that way. But there are many round objects in the solar system that no one considers a planet. My nemesis the moon, for example. In fact, the proposal from the committee specifically excluded moons from being called planets. But it made a special exception for Charon—smaller than our own moon by a factor of about sixty—for one reason: Pluto and Charon go around a center of mass that is a bit outside Pluto.

  Here a bit of quick physics is necessary (and I would like to point out that the fact that we need a physics lesson to explain the proposed definition of the word planet is already a bad sign). Whenever an object orbits another object (the moon about the earth, the earth about the sun, for example), it is not that the bigger object is stationary while the smaller object goes in circles. Instead, both objects go in circles around what is called the center of mass. You can find the center of mass of the earth and moon, for example, by finding a really large seesaw, putting the earth on one end and the moon, which has only 1 percent the mass of the earth, o
n the other end, and trying to make them balance. In the case of the earth and moon, you would have to move the pivot point to a location about a quarter of the way inside the earth. The seesaw is now balanced, and you have found the center of mass. In the twenty-nine days it takes for the moon to go in its big circle around the earth, the earth, too, in addition to traveling around the sun, goes in a tiny circle that is smaller than the earth itself. Rather than the moon circling the earth, both objects really circle around the point inside the earth that is the common center of mass.

  There is nothing particularly special about the location of the center of mass. If you were to find yourself at the precise spot that is the center of mass of the earth-moon system, the only thing unusual that you would notice is that there would be one thousand miles of rock on top of your head.

  Pluto is only about twice the size of Charon, so if you put Pluto and Charon on the cosmic seesaw you would find that the balance point is a little bit outside Pluto, rather than inside it. Again, there is nothing particularly special going on there. If you were to find yourself at that precise spot, you would only notice that you were very, very cold and could no longer breathe.

  According to the IAU proposal, though, the obscure fact that the center of mass of the Pluto-Charon system sat a bit outside Pluto rather than a bit inside it made all the difference. It suddenly turned Charon into a full-fledged planet, and Pluto-Charon into the solar system’s only double planet. Pluto lovers everywhere would be thrilled. Pluto’s status was about to change from imperiled to wildly distinctive. It would suddenly be the only place in the solar system you could go and get two planets for the price of one.

  Except, of course, that the proposed definition was crazy. The members of committee first argued that only the object itself, and nothing else nearby, should be considered in determining whether the object was or was not a planet. Then they changed their minds and argued that satellites, though round, were not planets, because they were in orbit around larger round things instead of the sun. And then they changed their minds again and said that Charon, though small in comparison to other satellites in the solar system, was a planet because the common center of mass of the Pluto-Charon system was outside Pluto rather than inside it, so that, technically, Charon orbits an empty spot in space rather than Pluto. Because it doesn’t orbit a planet, it was therefore not—by this argument—a satellite.

  So here is how you tell, in the committee’s opinion, that something in the vicinity of the sun is a planet. Look at it and see if it is round. If it is, then it might be a planet. Next, check to see if it orbits around something else instead of the sun. If it does, then it’s probably just a moon and not a planet. But before you know for sure, calculate the center of mass (if you even know the masses of the bodies in question, which you usually don’t) and see if it is inside or outside the larger body. Then you know. It’s all quite simple.

  While the inclusion of Charon was the most jarring aspect of the proposal, there was one other oddity that I couldn’t make sense of. The committee said that all round things were planets (except for moons, which weren’t, except for Charon, which was). I had estimated that about two hundred objects in the solar system would fit that criterion, but the IAU had done its own estimate and come up with its own number: twelve.

  Why would Charon and the asteroid Ceres be added, but not the dozen known Kuiper belt objects that were larger than Ceres? And the hundreds that were smaller but almost certainly round? It was as if the International Arboreal Union were to tell you that all things with trunks and bark and branches and leaves were to be called trees, but then it told you that the only trees were oak trees, maple trees, and elm trees. You would be right to ask: How can you make a very precise definition of tree and then claim that things that very precisely fit your definition are not, in fact, trees?

  Why would the International Astronomical Union do such a thing? I have a theory that I strongly believe to be true, but which is strongly denied by everyone I’ve talked to who might have more intimate knowledge of how the decisions were made. My theory is that the IAU decided that keeping Pluto as a planet and adding three new planets—Xena, Charon, and Ceres—would seem like a minor change to the order of things. It knew that after the newspapers declared that the solar system now had twelve planets and it proudly exclaimed that its new definition was the first true scientific definition, the pro-Pluto crowds would be satisfied and no one would be terribly startled. Three new planets? Yeah, that happens every century or so. No need to get alarmed. Who could complain? It wouldn’t elicit anything like the reaction people would have to the headline “Solar System to Have 200 Planets!” Given the choice between scientific rigor that might cause protests and a scientific whitewash to conceal reality, the IAU chose the latter. The first scientific definition of the word planet was afraid of its own scientific shadow.

  From my increasingly stressful vacation spot on Orcas Island, I got ahold of the committee member with whom I had originally spoken, who was in Prague to present the committee report in the next day or two. I told him I thought the committee recommendation was a mess. How could Charon be a planet? How could it say there were only twelve round things? It made no sense.

  He calmly explained the committee’s reasoning and said that he would make sure that in the press release and the press conference it would be very clear that many, many more objects were on the way to being included as planets. And, he mentioned again, he had already talked to many of the astronomers in Prague, and there was nearly universal support for the new definition.

  There was clearly nothing I could do. I was on a remote island on the wrong continent; it was impossible for me to have any influence over what was happening in Prague. And in Prague the very next day, they were going to declare that I was one of only seven people in human history who had ever discovered a new planet in the solar system. Who was I to complain?

  That night, long after Lilah was asleep and Diane had crawled into bed, I walked (limped, really, but I was now in a walking cast, at least) down to the rocky shore. I could see north across the strait to the islands off the coast of Canada. I could see the deep twilight still casting its final red rays on one of the triangular volcanic peaks over toward the mainland. I turned around to look back at the island, back to the south, but my view of the southern sky was blocked by madrone trees growing close to the water. Farther down the beach, some rocks jutted out into the strait; I’d have a view from there. I hobbled down to the rocks and slowly made my way out to the point. From here I could see the unobstructed southern sky. Low to the south, masquerading as the brightest star around, was Jupiter, undisputed king of the planets.

  I sat on the rock and watched the sky and looked at Jupiter. Who was it who first noticed that Jupiter moves? You can sit and stare at it all night long and not tell a thing. You can come back the next night, and unless you’re looking very, very closely, you will still probably not notice anything different. But Jupiter moves. It’s a wanderer. A planet.

  I know we’re past the point that when people say the word planet they mean an object that wanders across the sky. In fact, we’re so far past that point that most people never even realize that the planets really are up there wandering night after night. Planets are, to most people these days, pictures from spacecraft, drawings on a lunch box, models in a museum. Meanings can change. After tonight the word planet would change again, adding to the pantheon a little point of light moving through the sky that almost no one other than I had even seen. But it was real enough to me. At any point, day or night, winter or summer, if you walked up to me unannounced and said, “Quick! Where is Xena?” I could point an outstretched finger somewhere in space and locate it, with an error of about a hand’s width. If you asked me, “How big is Xena?” I would point at the moon and say, “Imagine a frosty world about half that size.” If you asked me what it would be like to walk on the surface of Xena, I would ask you to image walking on a frozen lake in the dark of the new moon. Th
at was Xena. My tiny, frozen, nearly invisibly lovely planet. I looked off to the east, where Xena was just about to rise above Mount Constitution, and thought: So be it. I was ready for the next day.

  I stared back at Jupiter, wishing I had thought to bring along binoculars so that I could pick out its miniature solar system of icy moons circling it. I tried to pretend I could tell that Jupiter was moving across the sky. The earth rotated. The stars moved toward the west.

  I couldn’t accept it.

  The solar system does not consist of twelve planets and then everything else. That is simply a fundamentally incorrect description of it. And the next day in Prague, astronomers were going to stand up and encourage the world to think of the solar system incorrectly. As someone who spends much of my life trying to be not just a scientist but an educator, trying to explain the universe and show the excitement without resorting to science fiction or trivial simplification, the idea that astronomers would actively encourage people to have the wrong view of the solar system seemed almost criminal. The idea that I was going to, overnight, become one of the most famous astronomers in the world on account of this criminal activity made me an passive accomplice. I had to do something to stop it.

  I hobbled back from the rocky beach up to the house. I woke Diane and told her that when the press called tomorrow I was going to have to tell them why the new proposed definition of planet was no good and why, in the end, it made sense all along for there to be just eight planets. I told her that I was going to have to kill Pluto and that Xena would go down as necessary and important collateral damage.

  All along, Diane had been more practical than I was. “Just let it be a planet,” she would say. “Try not to worry about it so much,” she had told me all year. “Relax” was her usual advice.

  But this time, when I told her that I couldn’t support Xena’s becoming a planet, Diane simply said, “Of course not, sweetie. You always needed to do what’s right.” And then she gave me her usual advice: “Relax.”