Betting On The Tenth Planet

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By 2003, it seemed obvious that someone was going to find a world bigger than Pluto. Mike Brown was betting on it. Literally.

The Caltech astronomer had five bottles of good champagne—Veuve Clicquot—riding on his bet with a fellow astronomer, Sabine Airieau. If an object more massive than Pluto was found beyond Neptune's orbit by the end of 2004, Brown would win the bet. If not, Airieau would get the champagne.1

If anyone was in a good position to find the planetary prize, it was Brown himself. In 2002, he and his colleagues happened upon the biggest solar system object discovered since Clyde Tombaugh spotted Pluto. In 2003, they outdid themselves by finding an icy world that was even larger and farther away. But neither of those objects quite measured up to Pluto, and nothing bigger came to light as 2004 was winding to a close.

Brown was starting to think he was on the wrong side of the bet. "Given that our survey has covered almost the entire region of the Kuiper Belt, I'm willing to bet these days that nothing larger than Pluto will be found in the Kuiper Belt," he told a reporter.2

Little did he know then that his computer's memory banks already held the imagery that would win him the sparkling wine—and spark one of the strangest turnabouts in scientific history.

Champagne wasn't the only thing at stake in Brown's bet. He was also gambling with the first several years of a promising career.

The bet really began back in 1992, when Brown was a graduate student at Berkeley, studying the volcanoes of the Jovian moon Io. One day he was walking down the hall when a postdoctoral researcher in the office next door called him over and showed him a picture of the thing she and another researcher had just discovered. The student was Jane Luu, and the thing Brown saw was 1992 QBp the first Kuiper Belt object seen beyond Pluto.

"The day before that discovery, the idea that there were large objects out there simply hadn't occurred to most people," Brown recalled years later. "And when it came time to think about what to do next, this was obviously the place to look."3 That time came in 1997, after Brown received his Ph.D. and was settling into his own professorship at Caltech. Once again, he happened to be walking past the right place at the right time. During a visit to Caltech's Palomar Observatory, he noticed that a 48-inch telescope was just sitting idle—and realized that this would be the perfect instrument for a planet scan. Within the year, he began searching the night sky for far-off worlds, going further down the trail blazed by Clyde Tombaugh, David Jewitt, and Jane Luu.

The trail started out along the traditional route: For three years, Brown and his Caltech colleagues slid 14-inch-square photographic plates in and out of Palomar's Samuel Oschin Telescope, capturing deep images of swaths of sky until the plates ran out. The plates' images were digitized. Then a computer sifted through the bits, looking for the telltale motions of Kuiper Belt objects. The software flagged the most promising candidates for further inspection by human eyes.

"We found absolutely nothing, but it didn't matter," Brown said. "I knew that we had the chance to find something really big and significant out there."4

That's when Brown bet the champagne. And that's when he took yet another gamble. Some researchers in his position

Mike Brown Caltech
Caltech astronomer Mike Brown led the team that found the dwarf planet Eris, which displaced Pluto as the ninth-largest object known to orbit the sun.

might have taken the time to write up the negative results they spent three years of their life on, just to salvage something from the disappointment. In contrast, Brown put aside all those results and started over.

This time, he worked out a deal with NASA's Near Earth Asteroid Tracking team to pair up Caltech's 48-inch telescope with a top-of-the-line, cryogenically cooled 50-megapixel CCD camera. To assist with the computer analysis of the imagery, he brought in Chad Trujillo—the astronomer who wrote the groundbreaking software for Luu and Jewitt's Kuiper Belt search for his Ph.D., and who was now a postdoc at Caltech.

Just three months after Brown, Trujillo, and their teammates began the new search, they recorded their first hit: an icy world that appeared to be about 550 miles wide—just a little smaller than Ceres.

Six months after that, in June 2002, they found the biggest Kuiper Belt object detected up to that time. Based on its motion and brightness, the object was estimated to be more than half Pluto's size.5 It spent most of its orbit considerably farther from the sun than Pluto, and by custom such far-flung objects are named after creation deities. Trujillo consulted with the local Tongva tribe, who lived in the area around Caltech long before the Europeans came, and the result was that the icy world was named Quaoar, in honor of "the great force of creation" in Tongva mythology.

As the largest solar system body discovered in more than seventy years, Quaoar captured the world's attention, and reenergized the public debate over Pluto's planethood. One Australian newspaper asked the question in a headline over its story about the discovery: "Quaoar, the Newest Planet . . . Or Is It?"6

Brown himself ventured an answer to that question: "Quaoar definitely hurts the case for Pluto being a planet," he told reporters. "If Pluto were discovered today, no one would even consider calling it a planet, because it's clearly a Kuiper Belt object."

Of course, the debate over Pluto's place in the solar system had been simmering among astronomers long before Quaoar was found. Scientific theories and observations were converging on the view that Pluto, Quaoar, and other Kuiper Belt objects were pushed out of their original orbits during the early days of the solar system.

The idea that planets didn't have to stay within well-defined orbital lanes dated back to 1984, when astronomers Julio Fernandez and Wing-Huen Ip ran a set of computer simulations that could, in effect, fast-forward or rewind the solar system's history. They found that the current shape of the solar system could best be explained as the result of a great gravitational migration that began soon after the giant planets were formed. Other astronomers fleshed out the theory over the two decades that followed.

The simulations showed that Saturn, Uranus, and Neptune likely formed closer in to the sun than they are today but moved outward because of their gravitational interactions with a thick ring of Kuiper Belt objects. Angular momentum was transferred back and forth between the outer giants and the tiny ice worlds in a complex game of orbital billiards.

The result? As the planets swept out the inner edge of the Kuiper Belt, many of the smaller objects were diverted inward, toward Jupiter. Giant Jupiter swung those interlopers back out to the farthest edge of the solar system, and since every action has its reaction, Jupiter itself moved slightly inward, shaking up the asteroid belt.7

Some of the Kuiper Belt objects stayed outside Neptune's gravitational grasp, although their orbits may have been disrupted. This could explain Pluto's current, fortunate orbit. Pluto most likely started out following a far more circular orbit, closer to the solar system's main plane—but was gradually pushed into an eccentric, inclined orbit that allowed it to survive for its discovery by Clyde Tombaugh.

The computer models indicated that there should be more objects beyond Neptune that were about Pluto's size. Some of the simulations even suggested that the "ice dwarfs" could get as big as Mars.8 It was only a matter of time before telescope power and computer power rose to the level at which such objects might be found. And that's why Brown's bet seemed like such a safe proposition.

After Quaoar's discovery, the search for mini-worlds ratcheted up another notch. Another team of researchers, led Yale's David Rabinowitz, one of Brown's longtime collaborators, developed the world's largest astronomical CCD imager, a 161-megapixel camera called QUEST. The acronym stood for "Quasar Equatorial Survey Team," and one of the principal purposes for building the camera was to look for distant quasars and other curiosities far beyond our own galaxy. But it was also an unparalleled instrument for seeking dim objects inside the solar system—and in mid-2003 it was paired up with Brown's old friend, the 48-inch Samuel Oschin Telescope at Palomar.

The search process became even more automated, to the point that machines took care of the entire process of opening the telescope dome, pointing the telescope, taking pictures of the same patch of sky at three different times, digitizing the images, and sending the data to Caltech's computers. Then a computer program looked for points of light that moved just the right amount to spark a human's interest. Every morning, Brown would review the ten to twenty candidates that were flagged from the night before.9

The prospects would usually turn out to be false alarms—for example, bright stars that threw off tricky glints of light—but as 2003 gave way to 2004, the team identified dozens of Kuiper Belt objects, including some that rivaled Quaoar's size.

And then there was the Flying Dutchman. That was the nickname Brown and his teammates gave to an object that was first spotted in 2003 but seemed to elude further study. It was on the very edge of visibility, and faded in and out of view like the fabled captain and his ghostly ship.

The amazing thing about the Dutchman was how slowly it moved across the sky. It moved just barely fast enough to trigger the software that checked the QUEST imagery for objects worth a second look. "I just stared at it," Brown said later. "I'd never seen anything moving that slowly, and so very far away, that it was still big enough to be seen. I didn't think it could possibly be real."10

The slower an object moved, the farther out it had to be. And if the Dutchman was real, it promised to be the farthest-out solar system object ever found.

To nail down the object's position and orbit, the Brown-Trujillo-Rabinowitz team checked their own backlog of imagery as well as Palomar's archives. When they finally ran the numbers, they found to their amazement that the Flying Dutchman was way beyond even the Kuiper Belt—at a distance of 88 AU, or more than twice as far as Pluto. And that was nearly as close as the object would get. In six thousand years or so, the Dutchman would be at its farthest point from the sun, about 975 AU away.11

Temperatures on the lonely mini-world would never rise above 400 degrees below zero Fahrenheit. Brown's team played off that chilly theme by naming the object after the Inuit goddess of the sea, Sedna, whose frozen fingers were broken off and transformed into Arctic whales, seals, and walruses.

The error bars on the estimates of Sedna's size and mass are still large, but the current best guesses range between 750 and 1,100 miles for diameter, and no more than half of Pluto's mass. The most interesting thing about Sedna isn't how big it is, although its discovery did spark another rash of "tenth planet" reports. Instead, it's the mystery surrounding Sedna's strange orbit.

Astronomers were hard put to explain how Sedna got to where it is, in the inner Oort Cloud rather than the Kuiper Belt. In their paper announcing Sedna's discovery, Brown and his colleagues suggested that it could have been scattered by a yet-to-be-detected Earth-sized planet, or perhaps by a passing star.12 Other astronomers have suggested that Sedna was actually formed around a brown dwarf, and then captured into our own solar system when the brown dwarf passed through.13

Brown considered Sedna's mystery to be far more scientifically intriguing than the "bigger than Pluto" controversy, and he still does. But even after Sedna was discovered, that champagne wager was still hanging over his head—and time was running out.

Sedna made Brown and his team think twice about the software they were using to cull through the QUEST imagery. If Sedna had been a little farther out, it might have been moving so slowly that the software wouldn't have flagged it. So in mid-2004 the program was tweaked to add more sensitivity. There might be more false alarms, but the planet hunters would also be less likely to miss something big.

Sure enough, the software turned up lots more far-off objects in the old image files. A particularly bright one was found three days after Christmas, in imagery that was captured on May 6, 2004. The object was given a serial number based on the date (K40506A), as well as a catchy nickname (Santa, to mark the holiday). Santa could be as big as Sedna— but it wasn't bigger than Pluto. Close, but no champagne.

On the evening of December 31, Brown e-mailed Airieau and told her she had won the bet. He went out to buy the five bottles of champagne. After the first of the year, he went back to work as usual, rechecking the archived QUEST imagery with the revised software. On January 5, he was flipping through pictures taken on October 21, 2003. Flip, flip, flip . . . then he stopped. There, in the center of the screen, was a bright spot that moved slowly—so slowly that the old software hadn't noticed it.

An object that slow-moving had to be very far away. And a faraway object that bright had to be big. Brown clicked on a button to have the computer calculate just how far away the object was, and came up with a distance farther than Pluto, even farther than Sedna: about 97 AU. Then he ran some quick calculations to estimate how big the object was, assuming that it was as reflective as Sedna. The result gave him a jolt: It could be 4,375 miles wide. That would be wider than Pluto. Wider than Mercury!14

"I grabbed the phone and called my wife," Brown recalled. "'I just found a planet,' I said. She was pregnant at the time, and she replied, 'That's nice, honey. Can you pick up some milk on your way home?' "15

Brown also fired off an e-mail to Airieau: Could he have an extension on the bet? Airieau said okay—and Brown knew he would have plenty of champagne for the celebration.

That celebration had to wait a while longer, however. Brown needed to make sure that what he was seeing was real. If it was, he wanted to learn more about this object. Did it stay completely outside the Kuiper Belt, like Sedna? How big and bright was it, really? What was it made of? Brown, Trujillo, and Rabinowitz decided to keep quiet about this blockbuster until they could find more observations, nail down more of the details, and write up what was sure to be a landmark scientific paper.

The object was given its internal serial number (K31021C) as well as a sly nickname: Xena, which was borrowed from Xena: Warrior Princess, a syndicated TV show with a busty sword-wielding war maiden as the lead character. Brown's team had picked out that name in advance for any tenth-planet candidate that came along—partly because the "X" hearkened back to Planet X, and partly because the name took a humorous jab at the whole "name a planet after a goddess" tradition.

While the review of QUEST imagery continued, the world-hunting team came across yet another biggie in a fresh batch of observations: an object brighter than Santa, but closer than Xena. This one was designated K50331A, and because it was found just a few days after Easter, it was nicknamed Easterbunny.

The team found out much more about what Brown called the "Kuiper Belt Triumvirate" during the early months of 2005. First of all, Xena had to be downsized: It clearly wasn't bigger than Mercury. Xena's brightness had fooled Brown momentarily, because its surface was far more reflective than Sedna's. Nevertheless, it still had to be bigger than Pluto, even if its disk was made out of a perfect mirror. It also had an orbit more eccentric and inclined than Pluto's, straying farther than 97 AU and coming closer than 38 AU. That meant

Xena didn't belong to the classical Kuiper Belt but instead had been knocked into an unconventional orbit through gravitational interactions with other celestial bodies. Thus Xena is most often classified as a "scattered-disk object."

Santa was a little more conventional when it came to its orbit, but less conventional in its shape. Its variations in brightness suggested that it was rapidly spinning, which would probably give the spheroid a football-like shape. Here was one world that was as fat around its middle as the jolly old elf it was named after. What' s more, Santa had a tiny moon, which the team nicknamed "Rudolph."

Both Santa and Easterbunny were about a third as massive as Pluto, but Easterbunny was smaller and brighter than Santa, perhaps because it had an icier surface composition. Easterbunny was so bright, in fact, that it could conceivably have been found during Clyde Tombaugh's sky survey decades earlier—if only it hadn't been lost in the glare of the Milky Way's celestial thoroughfare.

In mid-2005, Brown and his teammates laid out the schedule for telling the world about their Kuiper Belt Triumvirate. The findings about Santa would be shared during presentations at a planetary conference in September. That would set the stage for the big splash over Xena, about a month later. Easterbunny, which was still a work in progress, would come last.

To get the ball rolling, the team members wrote up their abstracts for the Santa presentations, short descriptions that were a traditional way to highlight a coming attraction for fellow researchers. The abstracts provided a few details about the object they called K40506A, just to whet scientific appetites, but they held back on publishing the coordinates to keep other researchers from rushing out and staking their own claims.

By July 20, all the arrangements were taken care of, the abstracts were published, and Brown eased back on his work schedule to spend some time with his wife and their newborn daughter, Lilah.16

That's exactly the time when the team's best - laid plans went astray.

Astronomers at the Andalusian Astrophysics Institute in Granada, Spain, had their own occasion to celebrate on July 25, 2005. An analysis of three images from the Sierra Nevada Observatory, archived since 2003, had just turned up the track of a bright object—so bright, in fact, that the astronomers suspected it could be the biggest Kuiper Belt object ever reported. After giving the images and their calculation a thorough review, they e-mailed a report on the object to Brian Marsden at the Minor Planet Center.

They also asked an amateur German astronomer, Reiner Stoss, to look for further images of the object so that its orbit could be defined more precisely. Stoss came through with pictures from other sky search programs, and eventually the object was spotted on images going back to 1955.

On July 28, Marsden sent out the center's traditional announcement about the discovery of a new minor planet, designated 2003 EL61 . For Brown, the e-mailed announcement came as a huge letdown: This was Santa, the football-shaped iceball that he and his colleagues were planning to unveil as the first of their Kuiper Belt Triumvirate. Brown took the news philosophically nevertheless, and e-mailed his congratulations to the Spanish team on that summery Thursday evening.

A few minutes later, Brown got another e-mail from Marsden, and this one was even more worrisome. Marsden had been hearing from other astronomers that this might be the ice dwarf that Brown's team called K40506A. Were they the same? Could the Spanish astronomers have been tipped off to the location of the mysterious find?

Brown's mind raced. He was pretty sure there were no leaks from the team members themselves, or from the astronomers who had been working with them. And the key information couldn't be gleaned from the abstracts. Or could it? Brown typed the serial number "K40506A" on a Google search page, just to see what came up. He was horrified to find that the number—plus Santa's coordinates—came up in a database listing from the Kitt Peak Observatory. Brown's team had recruited the observatory to help check for other sightings of Santa, as well as Easterbunny and Xena. The location data for all three objects was in the clear, if someone figured out the code.

After a sleepless night, Brown got on the phone to Marsden the very next morning and told him the whole story. One secret was out, and Brown decided he had to reveal the other secrets to the world before someone else beat him to it. He provided everything he had on the orbits of Xena and Easterbunny to Marsden's office, so that the Minor Planet Center could issue the discovery announcements. Xena was given the provisional designation 2003 UB313, and Easterbunny was called 2005 FY9.

Brown also made arrangements with NASA's Jet Propulsion Laboratory for a teleconference with reporters. And that's how the revelation that Pluto was no longer the ninth-biggest object orbiting our sun spilled out, helter-skelter, on a Friday evening—unquestionably the worst time of the week for announcing big news.

The question on everyone's mind was whether 2003 UB313 could be considered the solar system's tenth planet. When Quaoar and Sedna were found, Brown's take was that if he were defining planets, he would reserve that term for celestial bodies that were much more massive than anything else orbiting at roughly the same distance from the sun. By that definition, he said, none of the objects beyond Neptune— including Pluto—would be planets.

But when the subject was Xena, Brown took a slightly different view: "Pluto has been a planet for so long that the world is comfortable with that," he told reporters. "It seems to me a logical extension that anything bigger than Pluto and farther out is a planet."17

The debate wasn't purely philosophical. If Xena was considered to be like every other object that had been found beyond Neptune over the past twelve years, the discoverers would submit their suggested name to the International Astronomical

Union's Committee on Small Body Nomenclature. That committee clears the names of asteroids, comets, and anything else that is not a planet or a moon. But if Xena was a planet, the name would be reviewed instead by the IAU Working Group for Planetary System Nomenclature.

The IAU had a long list of rules for naming various objects and features. Craters on Venus, for example, should be named after famous women if they are wider than 20 kilometers (12 miles), but given common female first names if they are smaller than that. The eight planets other than Earth were named after Roman or Greek deities, but trans-Neptunian objects were named after creation deities from outside Greek or Roman lore.

The one rule the IAU did not have was: How do you decide whether a particular object is a planet or not? Astronomers had been mulling over that question ever since Marsden's assault on Pluto in 1999, and the question took on a bit more urgency when Sedna was discovered. The IAU appointed a nineteen-member panel—including Marsden and Stern as well as other experts on planets, comets, and asteroids—but they were basically deadlocked.

As long as no new object was bigger than Pluto, an answer to the question could be put off. But now, with Xena hanging over their heads and the glare of media attention shining in their faces, the IAU's top officials decided that something had to be done, and fast. The world organization's triennial assembly was coming up in less than a year, and they created a fresh panel of experts to come up with a definition by that time.

This panel would be smaller, seven rather than nineteen, so that there'd be less chance of breaking down into squabbling factions. This panel would be chaired by an eminent historian of science and include a best-selling science writer as well, so that their proposal would benefit from the historical and cultural dimensions. They would do their work in secret, so that they'd remain free from the media's meddling as well as researchers' rivalries.

This panel would come up with a way to resolve the nagging question of the past few years—well, actually, the past few decades—without discord. Or so the members of the IAU's Executive Committee hoped.

How wrong they were.

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