Mike Brown

by How I Killed Pluto;Why It Had It Coming

  When I hired Chad and set him to work, he was so good at it that I had spent most of the previous year or so enjoying my life. Most nights I even left work at an almost reasonable hour and went home and made dinner for Diane—and the nights I didn’t were usually because she was working late, not me. In the year before the new camera had been connected, Diane and I had married, gone on a monthlong honeymoon to South America, been on vacations, fixed up our little house. In short, we were behaving like normal people. I had never quite behaved like a normal person before.

  I could do all of this because Chad was hard at work every night scanning the skies. And he periodically let me know how it was going. But, really, I didn’t know much about the details of what he was doing.

  Now Chad had moved on to a new job, and I was left with a big complicated system that was suddenly mine alone. And a major component of the system had just changed, and everything needed to be fixed, and no one knew how to do it.

  Antonin and I were still drinking our coffee. “Keep looking,” Antonin said. “How could there be nothing left to find?”

  I had used that same argument myself. How could there be nothing left to find? How could this really be the end of the solar system?

  I drank more coffee. I stared into space. How would I do it? There was no way I could get someone up to speed quickly enough to keep going. We were still scanning the skies every single night. I didn’t have the time to wait months or years for someone new to come on board to get things going. I needed someone right now.

  And then I thought of someone who was actually pretty good at this sort of thing and even knew a bit about it already. Me. It would mean an end to being normal, to going home most nights and cooking dinner, but it would mean that the solar system didn’t have to end.

  I finished my coffee, and Antonin and I headed back toward campus, but I took a quick detour to Diane’s office. She was between meetings. I told her about the problems and the 37,000 objects and about the solar system that I didn’t want to end and how the only solution was to start doing all the work myself. She looked at me, smiled, and said, “Go find a planet.”

  In the end, the solution to what to do with 37,000 objects in one night turned out to be deceptively simple: I let it go. After a few more nights of collecting data and finding 33,000, 50,000, 20,000, and 42,000 objects, patterns began to emerge. Almost all of the camera junk turned out to be in a few places on the images. If I just threw some parts of the pictures away, ignoring what was there, then everything else was suddenly manageable. That meant, of course, that if something real was there I had to throw it away, too. But it was a price I was willing to pay. I finally settled on throwing away about 10 percent of the sky to get rid of 99.7 percent of the camera junk. For the first night’s worth of data, I went from 37,000 potential objects to look at to about one hundred. I could handle one hundred.

  I spent a couple of long days and nights at my computer going through the two months’ worth of pictures that had accumulated while I had been figuring out what to do. Of that very first night’s one hundred objects, one turned out to be a real object out there in the Kuiper belt. It wasn’t the biggest we had ever seen—it was only about one-third the size of Pluto—nor did it really distinguish itself in any other major way, but there it was, a tiny little needle that I had found by throwing away only 10 percent of the haystack.

  On one of those late nights when I was sorting through recent data, I found a bright Kuiper belt object; and then five minutes later, one more; and then five minutes later, a third. Again, they were not the biggest or the brightest objects, but it was clear we were in business. I let out a little shriek, which caused Emily Schaller—my graduate student who was working on Titan’s methane clouds—to stick her head in my office to see if everything was okay.

  The objects I found didn’t look that special—the postage-stamp-sized picture just showed a single faint point of light moving slowly across a patch of sky full of stars. I don’t know if it was the fact that no one had ever seen this little world before, that something in the sky was moving, or that this thing I was seeing was near the edge of the solar system, but each discovery of one of those moving dots on my screen gave me a charge of adrenaline and a jolt of excitement. Even today, when I see one I want to grab whoever is in the hallway and sit him or her down in my chair and point. Look!

  Over the next months, I barely kept my head above water. I was refining the software, making sure the telescope looked in the right places, flipping through a hundred or more images every morning, and still spending most of my time on the class I was teaching. My class that fall was called The Formation and Evolution of Planetary Systems, which taught graduate students current thinking on how the solar system is constructed. A lot of the time, the lectures focused as much on what we don’t know as on what we do. One of my favorite lectures was titled “The End of the Solar System”; it was where I got to talk about my own work in relation to the rest of the solar system. One of the mysteries I had been working hard on for the past few years was why the solar system seemed to end so abruptly. Yes, it continued on farther past Pluto than anyone had initially guessed, but about 50 percent farther than Pluto’s current distance from the sun everything came to an exceedingly abrupt end. Nothing had ever been found beyond this distance, and no one knew why. It is a mystery that still dogs and excites me today. I’ve gotten pretty good at ruling out almost any idea that anyone ever has. But I am just as good at ruling out my own ideas.

  I’d prepared my lecture more quickly than usual the morning of November 15, 2004, since I knew the subject intimately. I had a few extra minutes before class, so I decided to look at the images from the night before. As usual, almost everything that showed up on my screen was an obvious mistake the computer had made. But after a few minutes, I stopped my quick flipping through images, because I had found one that confused me. A faint object moving slowly across my screen—more slowly, in fact, than anything I had ever seen before.

  The speed with which an object moves in our pictures is directly related to how far away it is, in precisely the same way that when you’re looking sideways out the window of a speeding car, the things nearby zoom by quickly while the mountains in the distance appear to be just barely crawling along. The fact that this thing that I was looking at was moving at about half the speed of anything else I had ever seen meant that, if it was real, it would have to be twice as far away as anything anyone had ever found.

  Most of the time when I find a real object, I know it right away. Most of the time, the thing that I see moving across the screen is unmistakably real. But this one was moving so slowly and was so faint that I couldn’t decide whether or not it was real. It could have been just a series of slight smudges that had coincidentally lined up but meant nothing. If you look at the sky for long enough, you’re bound to find such things. But what if it was real? What would it mean to find something so far away? I didn’t have any more time to think, because it was time for my class.

  I gave my normal lecture. But at the end I couldn’t resist. After I told my students all about what we understood to be the edge of the solar system, I stopped, looked up, and added, “Maybe.” I told them that I had perhaps just found something that had changed all of that. But I wasn’t sure. And I would keep them posted.

  I went back to my office and sent an e-mail to Chad and David. I tried to downplay the potential discovery:

  Subject: amusement

  I just found something that, if real, is at 100 AU. Wouldn’t that be fun?

  Something at 100 AU—a hundred times the distance from the earth to the sun—would be more than three times the distance of Pluto and well beyond anything ever found in the Kuiper belt. Chad wrote back almost immediately:

  If that one is real I’ll be buying the champagne.

  Chad and I eventually drank that champagne. We were sitting on a beach on the Big Island of Hawaii, with the sun setting over the ocean in front of us, a pig roasting in a pit beh
ind us. By entirely appropriate chance, Antonin, who had convinced me not to quit my search for a new planet, was there, too. We raised our plastic cups to an unending solar system.

  • • •

  This was it; something so far away that we could nonetheless see had to be big—almost certainly bigger than Pluto. It’s true we had been fooled by Quaoar at first—since it had had a much shinier surface than we had anticipated and was thus unusually bright without being as large as Pluto—but even if this new object had a surface as shiny as Quaoar, it would still have to be bigger than Pluto. Because it had been so elusive, we gave this new object the code name Flying Dutchman. The Flying Dutchman is, of course, the ghost ship of folklore that can never go home and is instead destined to sail the seas forever. We had no idea at the time what an appropriate name this was.

  Since the Flying Dutchman—or Dutch, for short—was farther away than anything anyone had ever seen before, it certainly seemed to be part of a new, previously undiscovered part of the solar system. But I knew that there was another possibility. Even though Dutch was currently far beyond the Kuiper belt, it could still really be part of it. Sometimes objects in the Kuiper belt come a little too close to Neptune and get flung out onto long, looping orbits.

  We do the same sort of flinging whenever we want a spacecraft to get somewhere in a hurry; we send it by Jupiter first to get a slingshot off the planet. The trick is to aim the spacecraft almost at Jupiter. The spacecraft gets closer and closer to Jupiter and is pulled faster and faster by the gravity of the giant planet, and then it just misses, skims the clouds, and now zips along at high speed toward its final destination. Jupiter is so massive that it has enough gravity to give an object a slingshot that will take it clear out of the solar system. The Pioneer and Voyager spacecraft went past Jupiter, took pictures, got the slingshot, and will never be seen again. Neptune, however, is too small to give a strong enough slingshot to propel something out of the solar system, so when it tries, the objects always come back. Many objects in the Kuiper belt thus have orbits that take them close to the orbit of Neptune but then much, much farther away from the sun. These objects have been called “scattered” Kuiper belt objects, as Neptune appears to have scattered them to those looping orbits.

  Only small things get scattered. The large planets are on nice circular orbits because there is nothing big enough to kick them around. The objects in the Kuiper belt—including Pluto—have tilted, elongated orbits because they are too small to resist the bullying of Neptune. Dutch could well have been a scattered Kuiper belt object rather than something on a circular orbit like a planet. Maybe we just happened to be seeing it so far away because it was at the most distant point in its scattered orbit and would soon be making its way back toward the sun to show that it really belonged to the Kuiper belt region. Its orbit would be a clue to its potential planethood.

  As we had with Quaoar before, we eagerly looked for pictures of Dutch that had been inadvertently taken by previous astronomers. Dutch was much fainter than Quaoar had been, so there weren’t nearly as many on which it showed up, but after a few days of careful searching we found it back a few years, which was enough to calculate what sort of orbit it had.

  What was it going to look like? Circular, the way the orbits of massive planets should be? Scattered, like the orbits of many of the other smaller objects in the Kuiper belt? At first it was hard to tell. Although it is true that you need to figure out only where an object is and how fast it is moving to know an orbit, Dutch was so far away and moving so slowly that every time we measured it we came up with a slightly different answer. First we thought its orbit was circular; then we thought it was moving in a straight line and not even in orbit around the sun (that would be a first!). But after more care and measurement, we finally got the answer: Dutch was definitely not moving in a circular orbit, and it was definitely not moving in a straight line. The orbit was extremely elongated. So was Dutch at its farthest point in its orbit and moving inward like a normal scattered object would? No: just the opposite. It turned out that Dutch was at almost its closest point and moving outward. And its orbit around the sun appeared so elongated that it was going to take eleven thousand years to go all the way out and come back in again. It was the most distant object that humans had ever seen in the solar system, but it was eventually going to be even ten times farther away. Nothing was supposed to act like this in the solar system. It was neither a normal-seeming planet nor a normal scattered Kuiper belt object. There was nothing like it known anywhere else in the universe.

  It’s sometimes hard to picture all of these orbits and what they mean. So try this. Take a sheet of copy paper, a pencil, and a quarter (or just follow along on the diagram on the next page). Put the quarter in the middle of the paper, trace its outline, and put a little dot at the center of the circle you have just drawn. This little dot is the position of the sun, while the outline of the quarter is the nice circular orbit of Neptune. Inside this circle is everything in the solar system that was known until the moment that Pluto was discovered in 1930. If you would like to put Pluto on your drawing, put your pencil at the four o’clock position of the circle of Neptune’s orbit and now draw an oval that starts and ends there, but while it goes all the way around the sun it reaches a distance almost but not quite twice the diameter of Neptune’s circle from the sun at the ten o’clock position (okay, if you’re being precise, get out your ruler and make Pluto go 19/16 inches from the center of your circle). Now you can draw the outer edge of the Kuiper belt: Sketch a rough circle all the way around the sun at the farthest distance of Pluto. Finally, shade in all of the space between Neptune and this outer circle. Now it is time to add a few scattered objects. Place your pencil at, say, a point halfway inside your Kuiper belt at the eight o’clock position. Now draw an oval all the way around the sun that starts and ends here but gets to a distance two or three times farther by the two o’clock position. Feel free to draw as many scattered objects as you like, just always make sure to start and end in the middle of the Kuiper belt before zipping off to the edges of the solar system.

  Now you will need to draw Dutch. Draw a little dot about three times as far from the sun as the orbit of Neptune at, say, the one o’clock position (again, you precision freaks, put that dot precisely 2⅜ of an inch from the sun). You’re forgiven if, at this point, you would like to now draw an oval around the sun by coming into the Kuiper belt before going back out to your one o’clock position. But don’t do it. Dutch never gets much closer to the sun than where you drew it. Instead, take your pencil and draw an oval around the sun that starts and ends at the position of Dutch; but at its most distant point, at the seven o’clock position, the oval needs to be farther away. How much farther? Almost 33 inches—three times the full length of your 8½-by-11-inch paper! Dutch never touches the Kuiper belt. It never comes close to Neptune. And it spends most of the time so far away from the comparatively tiny region that is the Kuiper belt that from Dutch, the sun would be just an extrabright star in the sky. There is nothing else like Dutch.

  Now take your paper and put it in a safe place for later study. It will be on the final exam.

  Even though nothing like Dutch had ever been seen before, I had an idea about what it was immediately.

  One of the benefits and joys of teaching a comprehensive class on something like The Formation and Evolution of Planetary Systems is that you learn an awful lot about the formation and evolution of planetary systems. Much of my day (and late nights and early mornings) is spent with the concepts that I want to teach spinning in my mind. I see and continuously rearrange the outline for whatever is my next lecture as I am lying in bed or driving home or cooking dinner or eating breakfast. I mentally go through all of the connections and logic and calculations to make sure they make sense.

  On the very day I realized that Dutch was unlike anything else known in the universe, I was mulling over my next class lecture, which was about the origin of comets. Dutch had an orbit almost lik
e that of a comet. Comets are tiny balls of dirty ice that come from far out in the solar system, quickly swing by the sun, and return again. Dutch did the same, but it never came nearly as close to the sun as a comet, nor did it go nearly as far away from the sun as a comet. Comets acquire their distinct orbits through a complicated dance with giant planets and passing stars, and—I quickly calculated—Dutch never comes close enough to any of the planets to be a partner in any such dance. But while working on my lecture for the day, I quickly realized that Dutch could have acquired its odd orbit if, when the sun was born 4.5 billion years ago, the sun was not an only child but, rather, simply one in a litter of many stars. Before all the other stars went their own ways, they could have pushed Dutch around and put it exactly where it is now. Astronomers had speculated about such things for decades and had argued back and forth about whether it was true, and I had just found the thing that was going to answer all of those questions for good.

  Discovery is exciting, no matter how big or small or close or distant. But in the end, even better is discovering something that is capable of transforming our entire view of the sun and the solar system. Dutch was not just a chunk of ice and rock at the edge of the solar system. It was a fossil left over from the birth of the sun. And as surely as a paleontologist can take a fossilized bone of a T. rex and learn what the earth was like 70 million years ago, I was pretty sure that we could examine this fossil in space—this object that could have been put in place only near the very moment of the sun’s birth—and learn more about the sun’s earliest childhood than ever before.

  That class was the most astounding I have ever taught. I carefully explained the steps and the calculations that show why comets are where they are and why something like Dutch—which they still didn’t know about—could not possibly exist, at least given the standard picture of the formation of the solar system. And then I showed them Dutch. Finally, I went through the same calculations but now with different conditions 4.5 billion years ago and showed that it would lead precisely to things like Dutch. QED. The students in the classroom dutifully took their notes, probably thinking nothing more profound than whether or not this would be on the final exam. Of course it was.