How I Killed Pluto and Why It Had It Coming

by Mike Brown

  We now knew where Object X had been twenty years before, which meant that we could compute a very precise orbit for it. Just as important, we demonstrated that our hunt was not in vain. There might be more things out there that Kowal had not seen on his plates.

  But first, we needed to get back to Object X itself. The orbit that we found was surprising. Object X goes around the sun every 288 years in an orbit closer to circular than even most of the planets, but it is tilted away from the planets by 8 degrees. Eight degrees might seem small, but compared to the planets it is enormous. What was Object X? How did it get its almost perfect but slightly askew orbit?

  Today we still don’t know the answer. We have elaborate theories of how the objects out in the Kuiper belt have been tossed around in their orbits by the giant planets, but all of this tossing both tilts and elongates the orbits. Tilted but circular? All but impossible. Finding out that something you have just discovered is considered all but impossible is one of the joys of science. It is an enormous clue to billions of years of the early evolution of the solar system. If only we knew what it meant. Eventually we’ll piece together enough other parts of the story so that the peculiar orbit of Object X will suddenly make sense.

  With the orbit and the position of Object X determined, we could finally try to answer the one question that had been burning in the backs of our minds. How big was it really? From the day of discovery we were convinced that it was bigger than Pluto. But we didn’t actually know that for certain. Object X was so far away that, from our telescope, we couldn’t tell that it was anything other than a point of light. It looked like a star; it was starlike, an asteroid by the literal meaning of the word, though that literal meaning had long ago been forgotten. Object X was bright, but all that “bright” means is that it reflects a lot of sunlight. An object can reflect a lot of sunlight if it has a shiny surface—because it is covered in snow, for example—or it can reflect a lot of sunlight if it has a darker surface but is really big. You would have the equivalent problem if you were on the ground and someone was signaling to you with a mirror high in the mountains. You wouldn’t be able to tell the difference between someone with a small but highly polished mirror and someone with a larger but dirty mirror. Both would reflect the same amount of light in your direction. Both would appear as simple points of light from your distant vantage point.

  There was, possibly, one telescope that could see the disk of Object X crisply enough that we might be able to directly measure its size. The Hubble Space Telescope orbits the earth high above the atmosphere and, now that the original defects in its mirror have been corrected, takes the sharpest pictures of anything around. Even the Hubble has fundamental limits—due not to defects but to the laws of physics—as to how tiny an object it can resolve, but I quickly calculated that if Object X was really the size of Pluto, then Hubble’s newest camera, recently installed by visiting astronauts, would have no problem seeing the tiny disk and allowing us to measure its size.

  To use the Hubble Space Telescope you have to submit a lengthy proposal—which is accepted only once a year—detailing what you would like to look at and why; then a committee of astronomers looks over all of the proposals and selects those they believe are the very best. The next due date for proposals was not for about nine months. The earliest we could possibly hope to get a picture from the Hubble was in about a year. We seemed to have only two choices. We could announce our discovery quickly, tell everyone that we thought it was likely bigger than Pluto, and then wait for a year to confirm. But our estimate of the size really was just an educated guess. What if our object was actually smaller than Pluto? We didn’t want to have to be in the position to come back a year later and say that the thing we had called a new planet was actually smaller than Pluto after all. Our other option, though, was to wait a year so that we could announce the correct size when we announced our discovery. But we couldn’t delay the announcement of our discovery for a year; someone else might find it in the meantime and not feel the need to know how big it was before making it public. And even if we did delay until after we got images from Hubble, we didn’t think the secret would keep. Once the proposal was submitted, it would be read by dozens of people, and while proposals are ostensibly confidential, we were pretty sure that word would leak out quickly. Luckily, there was a third option.

  It is understood that sometimes discoveries will be made that need pictures from the Hubble Space Telescope faster than the process will allow, so there is an official route by which you can appeal for data immediately. Even this route made me nervous. Many, many people would still be reading the request and learning about the object. So I went for an even more direct route. I sent a note to one person I knew who worked for the Hubble Space Telescope. I explained that we had just found something potentially bigger than Pluto and wanted to look at it with Hubble as soon as possible, but we were afraid to go through any of the official routes in case the information leaked. I attached a detailed proposal just like the one that I would have submitted, but requested that the fewest people possible know about it. I sent the note by e-mail and sat back to look at a few more images of the sky, but within about two minutes I had already gotten a reply: YES!

  I quickly set to work trying to figure out the right time to target the Hubble. We wanted to make a very precise measurement of the size, so we knew we wanted to take the pictures just as Object X was moving close to a distant star to which we could compare it. I called up archival images of the sky, had the computer draw in the path that Object X was going to take through the stars, and looked for a good time. I found that in only three weeks the object was going to skim past a bright star; the timing would be perfect. I designed the precise sequence of pictures for the Hubble telescope to take and then sat back to wait the three weeks.

  Normally that three-week wait would have driven me crazy, but I had a distracting trip planned. I was flying out to Hawaii to use one of the the Keck telescopes—the largest telescopes in the world—to take a first really good look at Object X. Just as with any of the other great telescopes in the world, getting to use a Keck telescope requires writing a detailed proposal explaining what you will use the telescope for and why it is a good use of the time. As usual, the proposal is read by other astronomers, and then three to nine months later you might find yourself assigned to a particular night at the telescope. Unfortunately for us, again, we didn’t know we were going to discover Object X ahead of time, so we couldn’t have already written the proposal. Luckily for us, though, I had written a proposal to do something else entirely at the Keck—to study the moons of Uranus for evidence of icy volcanoes—so I was scheduled to be at the telescope soon after our discovery. One of the unspoken rules of being at a telescope is that once you are there, the night is yours to do with what you want. Yes, I had planned to look for icy volcanoes, but looking at Object X would clearly be a much more interesting and pressing use of the time.

  The Keck telescopes sit atop the currently dormant summit of the giant Mauna Kea volcano on the Big Island of Hawaii. At nearly 14,000 feet above sea level, the summit looks more like the sterile surface of the moon than part of a fertile tropical island. The only sign of wildlife I have come across up there was a mouse who must have hitchhiked up in an equipment shipment and who lived on the crumbs dropped by astronomers or others working inside the dome. If the mouse ever got itself locked out of the telescope, it would find nothing to eat for miles around.

  While the majestic Hale Telescope at Palomar Observatory looks like part spotless battleship, part elegant WPA dam, and part nineteenth-century high-rise, the monster Keck telescopes look like nothing but high-strung engineering projects. The dome at Palomar is mostly empty space, with the smooth outlines of the telescope truss looming high above in the darkness. The domes at Keck are the same size, but the mirrors on the telescopes are four times as big, meaning that the telescopes are so tightly crammed into the domes that there is nowhere to stand to even get a good perspective o
n what the telescopes look like. If you take one of the elevators that goes midway up a dome and step outside onto the metal platform encircling the telescope, you can walk around and get some idea of the different components—white girders, sprawling wires and cables, massive industrial-sized cranes—and you will find yourself looking directly into one of the two biggest telescopic mirrors in the world. It’s not one mirror, though; it is a bug eye of thirty-six smaller hexagonal mirrors all arranged into a much larger, almost circular hexagon looking back at you. The mirror itself, all combined, has a square footage only slightly smaller than the house that I lived in.

  Later that night, when we pointed the telescope at the faint dot in the sky that was Object X, the mirrors would concentrate all of the light from that immense area onto a tiny spot about the size of the period at the end of this sentence. Our goal was to take that concentrated light and pass it through a system that acts as a prism, to spread the light out, and then look at the different components. By looking at this spread-out light—the spectrum—I hoped that I could determine what was on the surface of Object X.

  I was scheduled to be at the telescope for two nights. I arrived in Hawaii a day early to begin to shift my body to a nighttime schedule and to do final preparations far from the distractions of home (including planning a wedding that was now only seven months away). I stayed up late at the observatory’s headquarters refining calculations on the computer, and then I went to sleep with the hope that I would sleep until noon so I would be fresh for the long night ahead. Instead, I woke up before dawn. I tried to force myself back to sleep, but my mind was uncontrollably running through the plans for the night, how I would set up the telescope and instruments, what would be the best way to collect the most useful data possible. I gave up on sleep and walked over to the telescope control room to set up for the night.

  The control room is arranged as a dense ring of desks around the center of the room, with an even denser ring of computer screens. At last count the room had something like twelve computer screens, all of which might be in use during the night. I checked the weather reports, the telescope reports, how things had gone the previous night. All of the nighttime staff from the observatory were still asleep, but there was plenty of preparatory work to do. At lunchtime, I walked to the shopping center to get some fresh Hawaiian poke from the grocery store.

  Walked to the shopping center? No, there is not a shopping center on the desolate summit of Mauna Kea. I was in the little cowboy town of Waimea, only a couple of thousand feet above sea level and surrounded mostly by ranch land. To use the Keck telescope these days, astronomers rarely actually go up to the summit. Instead, we sit in the control room in Waimea and connect to the summit by a fast video and data link. We talk to the people there and control the instruments there, but we don’t go there ourselves.

  The first time I used a telescope like this while being in a control room miles away, I felt strangely disconnected from what was going on. I couldn’t walk outside to feel the wind and humidity. I couldn’t check for cloudy patches or impending fog. I couldn’t hear the reassuring clanking of the dome and rumbling of the telescope. How could I do astronomy this way?

  The answer is, nearly perfectly. Your brain doesn’t work very well in the sudden oxygen deprivation of 14,000 feet. Combine that with lack of sleep, and efficient work is extremely hard. Fish-eye cameras pointing at the sky are better at seeing clouds coming and going than your eye will ever be. Wind and humidity gauges work just fine. And the video link is so seamless that you almost forget that you’re not talking to someone sitting right next to you. Still, I always find it disconcerting when, on nights that I am working at the telescope and the sky at 14,000 feet is beautiful and clear and the humidity is low and we are collecting beautiful data, I think to look out the window and, outside the control room at 2,000 feet in Waimea, rivers of rain are being driven horizontally by gale-force winds.

  Object X was going to rise above the horizon at about 8:00 p.m. I had finished setting everything up and was waiting anxiously to get started for the night. The crew arrived at the summit around 5:00 p.m., and we chatted over the video about the plans for the evening. When the sun went down, the big dome swung open and the thirty-six little hexagonal mirrors pointed together to begin collecting the light from my first target in the sky.

  My first job was to do a very quick check of all of the systems. We swung to a nice bright star, focused the telescope, and put the light from the bright star down through the prism to see if everything worked. After a few minutes, the spectrum appeared on one of the computer screens in front of me. I typed a few commands to take a quick look; the spectrum of the star looked just as it was supposed to. I stored the data away to later compare it to Object X. Finally, it was time to find Object X. We turned the telescope in the right direction and took a picture to see what was there, and the picture that appeared a minute later on my screen showed that there were twenty stars more or less where I expected Object X to be. Which one was it? I knew how to find out: It would be the one that moved. We did a little more calibration, and then twenty minutes later we took another picture. At first glance, the picture looked precisely the same, but I lined up the two pictures on the computer screen and blinked back and forth between them. Nineteen of the twenty stars reappeared in exactly the same place. One of the stars had shifted slightly. It wasn’t a star. It was Object X.

  Though we had been studying it and tracking it for more than a month now, my first view of Object X through the giant Keck telescope—or at least on the computer screen twelve thousand feet below the giant Keck telescope—still amazed me. I was about to get the first peek at the composition of something that might be bigger than Pluto, something that only a handful of people on the planet even knew existed. I shifted the telescope slightly to direct the light of Object X into the prism, and we were ready. Though Object X was the brightest thing beyond Pluto that had ever been seen, it was still faint. Even with the biggest telescope in the world, we had to collect a large amount of light before we had enough to be able to make a sensible analysis. We stared at Object X all night long, stopping every once in a while to be sure that the light was indeed going into the prism. I watched the data come in and obsessively checked the weather reports. Everything went perfectly. No clouds, no fog, no telescope malfunctions. Everything went so perfectly that it was, to be honest, an incredibly tedious night. I occupied myself with loud music, junk food, double-triple-quadruple-checking that everything was going perfectly, and speculating about what I might find.

  The sky began to brighten with the rising sun at around 5:30 a.m., and I finally made my way back to my little room. I slept until almost 11:00 a.m., went back to the control room, and again began preparing for the night. The second night was almost exactly like the first. I went to sleep around 6:00 a.m., got up the next day at 10:30 a.m., and was on a flight back to LAX by 1:00 p.m., confident that I had collected exactly the data I needed.

  Two nights at the Keck telescope will provide weeks’ or even months’ worth of data to pore over. Though totally exhausted, I got started on the five-hour airplane ride back home, trying to use all of the pictures and data to create one coherent view of what we had seen. First, I had to carefully remove any effects that were caused by the telescope or the prism or the earth’s atmosphere rather than by Object X itself; second, I had to figure out what we were seeing; and third, I had to figure out what it all meant.

  It quickly became clear that we were seeing dirty ice. Perhaps that should not have been a big surprise for something so far from the sun. Ice was supposed to be one of the main components of Pluto, too, and it was on the surface of almost all of the big satellites of Jupiter, Saturn, Uranus, and Neptune. But in addition to the dirty ice, there appeared to be something that looked like frozen methane. Methane would perhaps not be surprising to find on the object’s surface, since it is one of the main components of the surface of Pluto, but it had never been seen anywhere else in the Kuiper
belt, and the signature of methane was not overwhelmingly convincing. If methane was there at all, it was in extremely small amounts. A few years later another astronomer would suggest that perhaps there was no methane at all on Object X, but that what I thought looked like methane was actually evidence for the same icy volcanoes on Object X that I was supposed to have been looking for on the satellite of Uranus to begin with.

  The methane on Object X (and it was methane, after all) never made sense until years later, when Emily Schaller, a graduate student of mine working on a Ph.D. dissertation about the methane clouds on Titan, walked into my office with an idea for why Titan and Pluto both had methane. Her final explanation was deceptively simple and explained not just these objects but the rest of the Kuiper belt as well. Object X, it turned out, formed with methane—as did Pluto and Titan—but Object X was just a little too small, so that its gravitational pull was not quite strong enough to hold on to the methane forever. With the Keck telescope we were seeing the very last remnants of frost on a cold, dying world.

  While I was still working to understand the data from the Keck observatory, the Hubble Space Telescope snapped its sequence of pictures and transmitted them to the ground, where they were sent to my computer in Pasadena. Because the Hubble is totally automated and you design the entire sequence ahead of time, you can very easily lose track of when the telescope is actually looking at your target. The Hubble pointed at Object X on a Saturday, as I was having a housewarming party to welcome Diane as a new resident of my—now our—home. The house, with a square footage only slightly larger than that of the Keck telescope, was a bit of a tighter fit now. I didn’t make it to work until Sunday afternoon, after a long cleanup from the party. The new data would immediately tell us how big Object X was. Much bigger than Pluto? Only a little bigger? A tad smaller? When I first opened up the file that contained the image, I immediately closed it and double-checked what I was looking at. Clearly this was not Object X, the object potentially larger than Pluto—how could it be? But yes, the tiny dot that surely couldn’t be the tenth planet was, indeed, Object X. Object X, in the end, turned out to be only about half the size of Pluto.