I _ olden anniversaries are traditionally a time for celebration—but for astronomer Brian Marsden, the golden anniversary of Pluto's discovery was a reminder of how much of a misfit the planet had become.
Marsden, a British-born expert on celestial mechanics, was head of the Central Bureau for Astronomical Telegrams, the same office that had sent out the news of Pluto's discovery fifty years earlier. Since then, the bureau had been relocated from Copenhagen to Percival
Lowell's old stomping grounds in Cambridge, Massachusetts, where Marsden was the primary gatekeeper for astronomical bulletins as well as the head of the International Astronomical Union's Minor Planet Center.
It was Marsden's job to help keep track of asteroids, comets, and natural satellites—in fact, anything that popped up in someone's telescope that didn't happen to be a major planet. Pluto was one of the nine major planets, and thus not part of Marsden's domain. That was a situation he wanted to change.
Marsden got his chance when Clyde Tombaugh invited him to deliver a talk during a gala symposium on "the Ninth Planefs Golden Year" at New Mexico State University on February 18, 1980—exactly fifty years after Tombaugh spotted Pluto.
When it was Marsden's turn to speak, he started out by tracing the 199-year history of planet-hunting since Herschel's day. Then he noted that astronomers were starting to find a few asteroids that, like Pluto, crossed the orbits of giant planets. One of them, called Chiron, was even trumpeted for a while as the "tenth planet." Eventually, however, Chiron was pigeonholed as an unusual kind of asteroid that happened to cross the orbits of Saturn and Uranus. In the end, astronomers classified it as a comet as well as an asteroid because it sometimes sprouted a glowing tail. Yet another asteroid, named Hidalgo, appeared to be a burned-out comet, tracing a highly eccentric, highly inclined orbit that took it close to Saturn.
Marsden suspected that there were still more orbit-crossing oddities out there, just like Chiron and Hidalgo. And like Pluto.1 "Is it therefore perhaps not time we dropped the appellation 'ninth planet' and classified Pluto with the two objects it most obviously resembles, as an unusual minor planet?" he asked the audience.2 He said he could even give Pluto a new identity as No. 330 on his list of minor planets—replacing Adalberta, a reported asteroid that had turned out to be a celestial will-o'-the-wisp.3
Marsden's suggestion was couched in the polite qualifiers and wit that reflected his British upbringing. "It was partly in jest that I did this at that time," he recalled years later. But to some of his listeners, it was as if someone had stood up at a couple's golden anniversary party and announced that the wedding was a sham. At least that's how Tombaugh felt.
"My dad was crushed," said his daughter, Annette Tombaugh-Sitze. "He was very angry, and he was crushed that Brian picked this time to bring it up."
Marsden said he meant no disrespect. "I never intended to be unkind to Clyde," he insisted. Yes, he was aware that members of Tombaugh's family were still angry with him, but he said any ill will was the result of a misunderstanding. "They're not astronomers," Marsden said. "They don't see it in quite the right way."
Marsden was of the opinion that Pluto should never have been designated a planet in the first place. He said the Lowell Observatory "bamboozled" the world into thinking Pluto was the giant Planet X that Percival Lowell had predicted. But
history showed that Lowell was totally wrong. In Marsden's words, he was "very much a member of the minor leagues."
And the way Marsden saw it, Pluto belonged in the minor leagues as well.
Some scientists might not have cared that much which league Pluto was in. But this was a matter that nagged at
Marsden, the man responsible for cataloging the miscellaneous bits of the solar system. His suggestion to classify Pluto as a minor planet, as a far-out kind of asteroid, was aimed at putting a misfit planet in its rightful place at last.
In the beginning, even Tombaugh had his doubts about Pluto—and there was nothing unusual in that. After all, William Herschel had to be convinced that what he saw back in 1781 was really a planet and not just a comet.
The biggest knock against Pluto was that it was so small. It takes about 25 Plutos to equal the mass of Mercury, the next largest planet on the solar system's scale, and 476 Plutos to match Earth's mass. But if you judge by volume rather than mass, the discrepancy isn't as great—largely because ice-covered Pluto is less dense than those rocky terrestrial planets. It takes just 8 Plutos to fill up Mercury's volume, and about 150 Plutos to equal Earth. In comparison, it takes 57 Earths to equal the volume of the next planet up on the size scale, Neptune.
When astronomers looked toward the other side of the size spectrum, as it was known in 1980, they saw no asteroid or comet that was nearly as big as Pluto: It's 14 times as massive as the biggest asteroid, Ceres. Even if you took Ceres and added in all the other asteroids in the main belt, you'd still have less than a quarter of Pluto's mass. And on the volume scale, it would take about 14 Ceres-sized objects to equal one Pluto.
Another knock against Pluto was its inclined orbit, which is at a 17-degree slant from the solar system's main plane.4 But that tilt actually works in Pluto's favor: Its orbit is angled in such a way that Pluto comes closer to the sun than Neptune for twenty years at a time, while never cutting through the bigger planet's orbital path.
That's one way in which Pluto's orbit is calibrated to keep the little guy out of the big guy's way. Another has to do with the clockwork of the two planets' orbital motions. For every three orbits that Neptune makes, Pluto makes almost precisely two. When Pluto nears the track that Neptune follows, Neptune is at least a quarter of an orbit away. And when the speedier Neptune overtakes Pluto, their separate tracks are so far apart that the gravitational effect is minimal. What little effect there is serves to correct variations in Pluto's orbit, keeping it close to the two-to-three resonance. In effect, Neptune keeps Pluto in a protected zone.
The result of all this is that Pluto never comes any closer to Neptune than 17 AU. That's about 1.6 billion miles, or roughly the distance from the sun to Uranus. Pluto actually comes closer to Uranus (11 AU) than to Neptune in the course of its celestial travels.5
The Pluto-Neptune clockwork completes one grand cycle every 496 years or so—two full Pluto orbits, or three Neptune orbits. The ever-so-slight gravitational shifts oscillate over a far longer cycle: about 300 full orbits of Pluto, or 70,000 years.6 This finely tuned timetable explains why Pluto has been able to stay the course for billions of years.7
The case for Pluto looked stronger once astronomers worked out all the implications of Charon's discovery. Here was a planet in an odd but stable orbit, with a moon and an atmosphere and surface variation. Marsden might have wanted to lump Pluto in with asteroids or comets, but when he spoke out in the 1980s, Pluto's qualities and its heft put it in a class by itself. The loneliness of the planet and its overgrown moon seemed to work in its favor.
Nagging questions remained, however. As scientists continued to study Pluto and Charon, they began to ask themselves how those two misfits ever came together. They considered a variety of scenarios—including the idea that they sprang from the same knot of gas and dust, or that Charon was a passing globe of ice drawn into its orbit by Pluto's gravitational pull. Based on all the evidence—including the differences in the makeup of Pluto and Charon as well as the characteristics of their orbits—the most likely scenario suggested that a celestial interloper slammed into Pluto billions of years ago. Pluto eventually recovered from the blow, and the lighter debris from the blast coalesced to form Charon.
Such a scenario isn't as crazy as it might sound: Our own moon is thought to have formed in the same way, as the result of a collision between a hot, infant Earth and a Mars-sized planet that got in its way. The difference, however, was that ancient Earth's cataclysm occurred in a place where plan-etesimals got in each other's way like bumper cars at a carnival. What were the odds that two solitary worlds—Pluto and the object it ran into—would cross paths on the solar system's very edge? When astronomers ran the numbers, the chances against that happening turned out to be . . . well, astronomical.
In order to provide a realistic probability for the kind of smash-up that gave Pluto its moon, there had to be thousands upon thousands of objects lurking beyond Pluto and the seeming boundary of the solar system. The way that Pluto and Charon were thought to have come together added to the growing suspicion that the edge of the solar system was once teeming with icy objects. If such worlds still survived, they were apparently beyond the power of telescopes to see. But not for long.
When Marsden spoke in 1980, astronomers couldn t see anything out there that came anywhere close to Pluto and Charon.
Long after Tombaugh found Pluto and got his college degrees, he kept on with his methodical search of the skies from the Lowell Observatory. He toiled over the observatory's comparator, blinking photographic plates until 1943. Over all that time he identified 3,969 asteroids, two comets, and a nova. But he also determined that there were no more Planet Xs to be found, at least within the observing capability of the telescopes at Lowell.
Was there anything at all beyond Pluto? Of course: The ancient Greeks counted comets among the wanderers of the sky, and by the time Tombaugh found Pluto, astronomers understood that the orbits of many comets swung far outside the orbit of Neptune. Marsden even suspected that Pluto might be a comet stuck in the deep freeze, occupying an orbit that never took it close enough to the sun for a tail to flare.
The most famous comets are the ones that have been seen coming back around on a regular schedule, such as Halley's Comet. Such objects are known as short-period comets, and in 1951 Dutch-American astronomer Gerard Kuiper proposed that the solar system's gravitational effects pried loose short-period comets from an icy disk beyond Pluto's orbit. This hypothetical disk came to be known as the Kuiper Belt.
Then there are the long-period comets, whose orbits extend so far out that they take thousands of years to make just one full circuit of the solar system. One of the best-known examples is Comet Hale-Bopp, which caused a sensation in 1997—and triggered the Heaven's Gate mass suicides in San Diego. In 1950, Dutch astronomer Jan Oort suggested that the long-period comets came from a vast reservoir of ice surrounding the planets, perhaps hundreds of billions of miles beyond Neptune. This hypothetical reservoir came to be known as the Oort Cloud.8
Even in the 1980s, these far-flung features of the solar system were still considered hypothetical, because no one had detected any evidence of the Kuiper Belt, let alone the even more distant Oort Cloud. But in 1986, David Jewitt, a British-born astronomer who had studied comets and asteroids for years, resolved to look for Kuiper Belt objects with the same kind of dedication that drove Tombaugh's search for Pluto.
To join him in the search, Jewitt recruited Jane Luu, who had fled Saigon as a child at the end of the Vietnam War and had become a graduate student under Jewitt's wing at the Massachusetts Institute of Technology. Jewitt thought looking for the elusive trans-Neptunian objects would make a fine project for Luu's postgraduate studies. She recalled asking Jewitt why they should take on such an unglamorous search. "Because if we don't, nobody will," Jewitt told her.9
Jewitt and Luu began the quest much as Tombaugh did: Images of the sky were captured on photographic plates at observatories in Arizona and Chile. Then Jewitt and Luu peered at the plates using a blink comparator. It was eye-straining, mind-numbing work, which could be done for only a couple of hours at a time. Examining just one plate took eight hours.
Soon, however, the pair brought advanced technology into play. After all, these were the 1980s, not the 1930s. Arrays of integrated circuitry, known as charge-coupled devices, or CCDs, were taking the place of photographic plates for recording digital images of the sky. Those digital pictures could be run through computer processing to smooth out the images and then line them up for a quick visual scan.
The two continued their search, year after year, with better CCD and computer technology at their disposal for each new campaign. In 1988, their base of operations shifted to the University of Hawaii's Institute for Astronomy, where they had access to an 88-inch (2.2-meter) telescope atop the dormant Mauna Kea volcano, one of the world's best vantage points for scanning the heavens. Eventually, that telescope was equipped with digital detectors capable of spotting objects thousands of times fainter than Pluto.
On the night of August 30, 1992, around midnight, Jewitt and Luu finally spotted their first Kuiper Belt object. It was a faint spot that moved ever so slightly when two computer images of that part of the sky were blinked. For the rest of the night, they kept watching that spot—and the next morning the ecstatic astronomers alerted Marsden to the news. They held off on making a public announcement for a couple of weeks, however, so that they could double-check the observations. Further sightings confirmed that the faint object was far beyond Pluto. And so, in mid-September, Marsden issued the IAU circular announcing the discovery and designating the object 1992 QB1.10
Once Jewitt and Luu figured out where and how to look with their powerful new tools, the search became easier. They found their second Kuiper Belt object six months after the first. Six months after that, they discovered two more. Other astronomers started sighting Kuiper Belt objects as well.
A whole new frontier was coming into view, thanks to improved telescopes, smarter software, and astronomers hungry for discovery. "Discovering the Kuiper Belt is like waking up one morning and finding that your house is 10 times as big as you had thought it was," Jewitt told one interviewer.11
Eventually, astronomers found enough Kuiper Belt objects, or KBOs, to notice patterns in their orbits. Some of the objects traced orbits in a lane ranging from 40 to 50 AU
from the sun, and came to be known as classical Kuiper Belt objects. Marsden, ever the classifier, suggested that these objects be named cubewanos (pronounced like "QB-1-o's," in honor of 1992 QB1, the first of its kind). Other icy worlds veered much farther out, on extremely eccentric orbits, and were called scattered-disk objects. And then there was a class of objects that, like Pluto, circled the sun in a two-to-three resonance with Neptune. Like Pluto, these ice dwarfs sometimes came closer to the sun than the giant planet, while staying a safe distance away. Jewitt suggested calling these objects plutinos—that is, "little Plutos."
These plutinos didn't trace exactly the same orbit that Pluto did. Their orbits were oriented and inclined at different angles, rather like the old pictures of electrons circling the nucleus of an atom. Nevertheless, the fact that there were other objects that followed Pluto's protected pattern—and maybe more yet to be discovered—l ed astronomers to the realization that Pluto wasn't such an oddball after all.
The fast-changing situation on the edge of the solar system brought the example of Ceres and the asteroid belt to mind. The latest calculations suggest that the Kuiper Belt could hold 70,000 objects wider than 60 miles (100 kilometers). That would be 300 times more than the number of similar-sized asteroids, and there might be more than a million additional ice dwarfs in the 1- to 100-kilometer range. If there were so many objects occupying the zone of the solar system through which Pluto traveled, what was so special about Pluto itself? Did it really deserve to be called a planet?
The wave of discoveries that began with Jewitt and Luu's sighting of 1992 QB1 greatly expanded the suburbs of the solar system's metropolis. And it didn' t take long for other astronomers to combine the power of more sensitive telescopes with more powerful computers as well.
A Ph.D. student who joined the Jewitt-Luu team in 1995, Chad Trujillo, wrote a software program that automated the blinking process for the digital imagery from the University of Hawaii's telescope. The days of sitting down at a mechanical blink comparator, as Tombaugh did in 1930, were finished. Automated blinking led to a quick upswing in the number of Kuiper Belt objects detected, not only by the Hawaii team, but by others as well.
Even Tombaugh, who was nearing his nineties, was aware of the rapid change in the celestial neighborhood he first charted more than sixty years earlier. "I'm fascinated by the relatively small 'ice balls' in the very outer part of the solar system," he wrote in a 1994 letter to the magazine Sky & Telescope. "I have often wondered what bodies lay out there fainter than the 17th magnitude, the limit of the plates I took at Lowell Observatory. May I suggest we call this new class of objects 'Kuiperoids'?"
Tombaugh was also aware of the looming doubts about Pluto's planetary status. Perhaps too aware. Ever since Marsden's talk in 1980, Tombaugh had worried about what would happen to his discovery, the brightest object in the solar system beyond Neptune. "Let's simply retain Pluto as the ninth major planet," he pleaded.12
In his latter years, the farm boy who became an astronomer suffered from congestive heart failure. "This controversy did not help his condition any," his daughter Annette said.
Finally, on January 17, 1997, at the age of ninety, Tombaugh's heart gave out. His obituary in the New York Times hailed him as the discoverer of the "ninth planet"—and said nary a word about the controversy over that title.13
As Pluto began its long swing away from the sun and back into the colder reaches of the Kuiper Belt, the planethood debate kept warming up. Once again, it was Marsden who brought the issue to a head: He never wavered from his view that the Lowell Observatory had put one over on the rest of the world, and that Pluto had an unjustly high status compared to his flock of minor planets. "Pluto has been a longstanding myth that's difficult to kill," the Atlantic Monthly quoted him as saying a year after Tombaugh's death.14
This time, Marsden mcDde a modest proposal to the IAU: Thanks to the improvements in telescope and computer power, the pace of discovery was beginning to quicken, and soon the ten thousandth object would be added to the list of minor planets Marsden kept. How about giving Pluto, the misfit among the nine major planets, that place of prominence on Marsden's list? Or how about starting up a whole new list of "Trans-Neptunian Objects"? Pluto could be classified as No. 1 on that list, just as Ceres was No. 1 on the minor- planet list.
As far as Marsden was concerned, it would be okay to keep listing Pluto with the solar system's eight bigger planets as well, at least for the time being. That way, there'd be no demotion or disrespect. Pluto would merely enjoy dual status as a major planet as well as a minor planet. How would that sound?
The IAU mulled over Marsden's modest proposal, but once word got out to the public, the idea went over like a plutonium balloon. Newspapers editorialized: "Send Those Scientists to Pluto," read the headline in the Peoria Journal Star.15 Letters were written by schoolchildren, including Elizabeth Bearss, a sixth-grader in Tampa, Florida: "I think Pluto should stay a planet," she wrote. "It kind of gives our solar system a personality. There's Earth, which everyone knows about; Mars, where we've sent robots; and then there's little old Pluto. He's cute and all the dogs love him, and he and Charon are inseparable."16
Astronomers were in the thick of the fray as well: About 135 of them quickly signed a petition opposing Pluto's designation as an asteroid, passed around by Mark Sykes, then at the University of Arizona's Steward Observatory. The American Astronomical Society's Division of Planetary Sciences issued a statement complaining that the IAU's actions would be viewed as a "reclassification" of Pluto. "We feel that there is little scientific or historical justification for such an action," the statement read.17
"I think Pluto's being impeached," said David Levy, the comet discoverer and Tombaugh biographer. "Pluto hasn' t done anything to deserve this."18
Some scientists were on Marsdens side: "For at least 20 years, it's been obvious that Pluto doesn't fit," said Michael A'Hearn, a University of Maryland astronomer who was leading consideration of the policy change at the IAU's division for planetary systems sciences.19
Other astronomers didn't have a strong feeling one way or the other, but just felt embarrassed that so much attention was being drawn to what seemed to be a long-settled issue. "We all grew up knowing Pluto as a planet. Why upset the solar system cart at this time?" Adler Planetarium astronomer Phyllis Pitluga asked.20
Marsden felt stung by the whole affair. " Maybe I 've been too democratic about it," he told one reporter. "Maybe I should have made the decision, and that's that."21
But it was too late for that. Faced with the public outcry, the IAU issued a statement denying that anyone was thinking about changing Pluto's status as the solar system's ninth planet, and announcing that Pluto would not be given a minor-planet number as Marsden wished. General secretary Johannes Andersen noted that the IAU's decisions and recommendations didn't have the force of international law, but gained acceptance only if they were "rational and effective when put into practice."
"It is therefore the policy of the IAU that its recommendations should rest on well-established scientific facts and be backed by a broad consensus in the community concerned," Andersen declared. "A decision on the status of Pluto that did not conform to this policy would have been ineffective and therefore meaningless."22
Plutophiles rejoiced at the news, and some of the loudest rejoicing was heard at St. Anthony's School in Streator, Illinois, the town where Tombaugh was born. Congressman Jerry Weller came by the school to praise the students' letter-writing campaign, delivering Certificates of Special Congressional Recognition. "Thanks to St. Anthony's and the student body here, you helped save the planet Pluto," he told the 189 students and their teachers.23
In reality, the planet wasn't in need of saving. Pluto wouldn't have gone poof if it were given a minor-planet number as well as a major-planet name. The highly public controversy did raise its profile, for a while, but the IAU's support of the status quo simply postponed the battle to come. And, unfortunately, it was shaping up as a battle. The controversy polarized the scientific discussion over the nature of the solar system' s diverse neighborhoods, just at a time when our ability to see and study the far suburbs was dramatically widening.
Some astronomers thought the correct course was to lower Pluto's public profile. For example, at the American Museum of Natural History's Rose Center for Earth and Space in New York, the Hayden Planetarium was rebuilt in the late 1990s as an eighty-seven-foot-wide sphere that could be seen as representing the sun. Jupiter, Saturn, and other planets are displayed next to the giant ball, in sizes that reflect their scale with respect to the sun. Earth, for instance, is a ten-inch-wide sphere—about the size of a basketball. Mercury is a bit less than four inches wide—the size of a softball.
Pluto would be a couple of inches smaller, about the size of a handball. But you won't find a ball-sized Pluto mounted in the Rose Center's Hall of the Universe. Instead, a plaque labeled "Where's Pluto?" stands along the museum's walkway. The plaque explains that a ninth sphere wasn't added to the lineup simply because the display was meant to highlight the solar system's bigger classes of planets—the gas giants and the terrestrial planets. (Still, it might not hurt to leave a handball lying around, just in case anyone asks).
Not even the plaque was there when the remodeled center opened in 2000. It was added only after news reports about the omission resulted in a flood of protest letters that filled up the in-box of the planetarium's director, Neil deGrasse Tyson.
One letter, from seven-year-old Will Galmot, started out with the salutation "Dear Natural History Museum" and enclosed a hand-colored picture of a blue disk in space. "You are missing planet Pluto," the letter read. "Please make a model of it. This is what it looks like. It is a planet. Love, Will Galmot."24
Tyson often joked about receiving stacks of "hate mail from third-graders" for leaving Pluto out of the display.25 But even he looked forward to the day when robotic explorers could snap the first up-close pictures of Pluto and the denizens of the Kuiper Belt.
Fortunately, other astronomers were working to do just that. Even before David Jewitt and Jane Luu found their first
Kuiper Belt object, a group of planetary scientists who called themselves the "Pluto Underground" dreamt about the first space mission to Pluto. The mission went through several name changes, and it suffered a couple of near-death experiences as well. If Pluto had somehow lost its planetary status along the way, the whole effort might have gone under. But it didn't, thanks to the Pluto Underground.
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