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THE SEARCH FOR PLANET X

The nineteenth century spawned a long list of rich eccentrics—and if you were to alphabetize that list, you'd find Percival Lowell noted somewhere between W. K. Kellogg (cereal czar and health faddist) and Bavaria's King Ludwig II (builder of fairy-tale castles and patron of composer Richard Wagner).

Lowell possessed a rare combination of conventional connections and unconventional aspirations. On one hand, he was the scion of one of Boston's oldest, most established families. There's a whole city in

Massachusetts named after the Lowell clan. His brother was the president of Harvard, for heaven's sake.

On the other hand, Lowell was about as far removed from the staid society of nineteenth-century Boston Brahmins as you could get. At one point, he dabbled in psychic research in Japan. He took much of the fortune he amassed through his early business dealings and spent it on the astronomical venture that consumed his attention in later life: the Lowell Observatory, which was founded in 1894 and is still thriving today.

The most eccentric thing about Lowell was his campaign to convince the world that aliens were building canals on Mars, canals he thought he was uniquely suited to see. It was a campaign that would set him at odds with the scientific establishment, but would also lead by twists and turns to the discovery of Pluto fourteen years after his death.

Lowell was part of a grand generation of millionaires who backed astronomical projects in the late 1800s and early 1900s.1 His family's fortune rested on the cotton trade, and after graduating from Harvard with distinction in mathematics, he put his skill with numbers to work in the family business. Through the years, Lowell diversified his portfolio, making shrewd investments in railroads and other utilities to build up his personal estate to more than $2 million—and that was back when a million dollars really meant something.2

But Percival Lowell didn't aim to make his mark as an industrialist or an investor. Instead, exploration was his passion. Starting at the age of twenty-eight, Lowell traveled extensively to the Far East. He played a diplomatic role on a Korean mission to the United States, and later studied the trance states of Shinto believers in Japan—a project that paralleled the "scientific" studies of spiritualism undertaken in the West by his contemporary William James.

Drawing upon his travels, Lowell gave lectures and wrote books about the mystic Eastern psyche, which he held to be inferior to the Western scientific mind-set. His fascination with exotic frontiers, and his high regard for the scientific frontier, hinted at the turn he took toward astronomy in the 1890s.

It all started with Italian astronomer Giovanni Schiaparelli and his observations of canali on Mars, features that looked like lines crisscrossing the Red Planet's disk. Lowell saw Schiaparelli as a nineteenth-century Columbus who had discovered a new civilization through the sights of his telescope, and he resolved to see the canals for himself when Mars made a close approach to Earth in 1894. Thus was a gentleman astronomer born.

Lowell scouted out a number of locations for good "seeing"—that is, the atmospheric conditions that were most conducive for telescope observations. He settled upon the dry, high, remote area around Flagstaff, Arizona, as the site for his own observatory. When Lowell peered through the telescope he borrowed from Harvard, he saw what he hoped to see: straight lines, even double sets of lines, that he held up as proof that an extraterrestrial civilization had built canals to channel Martian water.

The canals, of course, were an artifact of limited telescope technology and the almost unlimited human ability to make patterns out of fuzzy phenomena—the same sort of visual trickery that sparked all the fuss over the "Face on Mars" in more recent times. Pluto's eventual discoverer, Clyde Tombaugh, saw the canals for himself through the Lowell Observatory's 24-inch telescope when he reproduced the parameters that Lowell used years before.

"They were not figments of Lowell's imagination. I'll vouch for that; he was being honest with what he saw," Tombaugh said.3 When he upped the power on the telescope, the seeming canals broke up into less regular patterns of dark and light.

Lowell's books and lectures about Mars's canal builders made a big splash with the general public, but they put him on a collision course with other researchers. Eminent scientists accused Lowell of bending the facts to fit his far-out preconceptions. In response, Lowell insisted that his "acute eye" allowed him to spot faint features unseen by his critics.

His scientific assistants were sometimes caught in the crossfire. One of them, A. E. Douglass, wrote a letter to the observatory's acting director, complaining that other astronomers didn't give Lowell any credit "because he devotes his energy to hunting up a few facts in support of some speculation instead of perseveringly hunting innumerable facts and then limiting himself to publishing the unavoidable conclusions, as all scientists of good standing do."

When Lowell heard about that, Douglass was promptly fired.

As the scientific establishment moved toward a consensus that Mars's canals were merely a mirage, the embattled Lowell quietly turned his attention to a campaign that he hoped would shore up his reputation: the search for a planet beyond Neptune.

The sensation over Le Verrier's discovery of Neptune in 1846 made the planet search into a cottage industry. After his Neptunian triumph, Le Verrier turned his attention to a discrepancy in the calculated orbit of Mercury. He figured that there should be yet another unseen planet, or perhaps a swarm of asteroids, circling the sun inside Mercury's orbit. Astronomers spent decades looking for the theoretical planet, which came to be called Vulcan.

"For many astronomers, Vulcan and Neptune both existed because Le Verrier's calculations demanded that they exist," Vanderbilt University astronomer David Weintraub wrote.4 Every once in a while someone would report seeing it as a dark spot moving across the sun's disk, but they eventually concluded that these were sunspots.

The Mercury mystery wasn't truly solved until after the turn of the century, when the discrepancy was explained as a consequence of Albert Einstein's general theory of relativity. Einstein figured out that the gravitational mass of the sun was warping the fabric of space-time where Mercury made its rounds. He wrestled with the mathematics for years, and in 1915 came up with a set of equations showing that the warp factor would throw Mercury's orbit off by just the right amount.5

Astronomers were also looking for additional planets on the solar system's far frontier: Even when Neptune was taken into account, some astronomers still saw a discrepancy between Uranus's observed orbit and the mathematical calculations for where the planet should have been. Some of the calculations suggested that one or two extra planets might be skulking around, perhaps two or three times as far away from the sun as Neptune. William Pickering—who was one of Lowell's Harvard chums until he became a scientific rival—claimed that a complete resolution of the discrepancy required as many as seven unseen planets, known as Planets O, P, Q, R, S, T, and U.

This challenge looked appealing to Lowell, because the solution relied on cold, hard calculations and observations. If Lowell found the missing planet, which he called Planet X, all the unpleasantness over the Martian canals would give way to the sort of acclaim that Le Verrier enjoyed. So, starting in 1905, he devoted more and more of his observatory's resources to the planet search.

He turned his own mathematical skill to the problem of working out Planet X's position in the sky. Lowell figured that the planet he was looking for had to be at least as big as Earth, and more likely several times bigger. To figure out where to look, he employed as many as four "calculators"—in those days, the term meant people, not adding machines.6 The most likely spot was determined to be somewhere in the constellation Gemini . . . or was it Libra? Over the course of a decade, Lowell pressed his assistants to make three separate photographic surveys of the Planet X hunting grounds, using progressively better equipment.

In retrospect, Lowell's search suffered from fatal flaws. He tended to concentrate on the places that were identified through mathematical calculations, and when those places didn't pan out, he'd move on to new calculations. Once again, he was looking for a few facts that would prove his case, rather than seeing the big picture.

Ironically, Pluto was captured on a couple of photographic plates that were made in 1915, but it was fainter than the kind of object that Lowell expected to see, and so it remained undiscovered.

An even bigger irony was that there was actually no discrepancy in Uranus's orbit, and thus no need for a Planet X. The discrepancy showed up in the mathematical calculations only because the estimates of Neptune's mass and position were wrong. The conclusive evidence for those past errors came in 1993 when E. Myles Standish analyzed data from NASA's Voyager mission.7

Even before Lowell began his search, some of the experts were beginning to suspect that Planet X wasn't really there. Lowell persevered nevertheless, hoping that his redemption would be found amid the stars.

Lowell never did find his planet. He died of a cerebral hemorrhage in 1916, reportedly just after blowing up in anger at a servant.8 In his will, he left his personal effects, his car, some investment income, and $150,000 in cash to his widow, Constance—but he put the bulk of his multimillion-dollar fortune in a trust to support the Lowell Observatory. That didn't sit well with Constance Lowell, and her legal challenges tied up the will for a decade.9 For all that time, the planet search had to be put on hold.

By the time the will was sorted out, hardly anyone—except perhaps for William Pickering—was still actively involved in the search for new planets. But the Lowell Observatory's director, Vesto M. Slipher, was at last free to resume the quest that his late boss started. The observatory purchased a brand-new 13-inch astrograph, a telescope equipped with a camera that could spot objects much fainter than anything Lowell could have seen.

Slipher and the observatory's trustee, Roger Lowell Putnam, decided they needed to hire someone to help out with the search. Someone who didn't already have a research agenda of his own. Someone, perhaps, like the "young man from Kansas" who had sent samples of his astronomical drawings to Slipher in hopes of getting some pointers.

So it was that twenty-two-year-old Clyde Tombaugh, who worked on the family farm in western Kansas by day and gazed through his homemade 9-inch telescope by night, came to the Lowell Observatory in late 1928. Tombaugh was born on a farm near Streator, Illinois, a year after Lowell began the search for Planet X. He was a ten-year-old stargazer when Lowell died in 1916. He moved west to Kansas with his family in 1922, while Lowell's widow was wrangling over her late husband's will. Now Tombaugh was being groomed to continue Lowell 's legacy.

Like everyone else around the observatory, Tombaugh was expected to help out with the chores: giving tours, stoking the furnace, even climbing up onto the canvas-covered telescope dome and shoveling off the snow in the wintertime. But his primary job was to stay up during the night and take a series of one-hour photographic exposures, capturing patches of the star field on 14-by-17-inch glass plates.

The same patch of sky would have to be photographed at different times—and that's when the real work began. The point of the exercise was to compare the plates meticulously, in hopes of finding spots that changed their position over time. By precisely measuring the distance between the two images of a moving object, astronomers could calculate how far away the object's orbit was.

A fast-moving object would be closer than a slow-moving object. The closer spots were most likely asteroids or comets, and the observatory's astronomers saw a few of those. But what they were really looking for was an object moving slowly enough to demonstrate that it was beyond the orbit of Neptune.

Decades earlier, when Lowell started his search, he would lay two glass plates on top of each other, slightly offset, and then peer through a magnifying glass looking for the spots that moved. Then, in the latter years of his search, the observatory acquired a device known as a blink comparator, a setup that moved a microscope apparatus quickly back and forth between one plate and another—clack, clack, clack—for a visual comparison of the star field.

When the search resumed in April 1929, the observatory's senior researchers were supposed to take turns "blinking" the sets of photographic plates that Tombaugh made. But the researchers became distracted by other duties, and the plates started to pile up. In June, less than a year after Tombaugh arrived in Arizona, Slipher told the Kansas farm boy that he would be in charge of blinking the plates as well as making them.

Clyde Tombaugh peers into a blink comparator at the Lowell Observatory, circa 1950. Such devices were used to check photographic plates for tiny, faint objects that moved between one exposure and another—objects such as Pluto.

"I was overwhelmed," Tombaugh recalled. "It had become evident to me that the one doing the blinking carried the heavy responsibility of finding, or not finding, the planet."10 The hunt for Planet X thus became something radically different from the hunt for Neptune. For Le Verrier, the main task was to refine the mathematical calculations, time after time, until he came up with a solution that fit the data so precisely he could tell any astronomer willing to listen where to look. Lowell tried the same strategy, but the observations stubbornly refused to conform to the calculations.

By necessity, Tombaugh's approach was a throwback to the good old days of Herschel's time: It turned into a wider, more painstaking survey of the ecliptic, the equatorial zone in the night sky where planets wandered. "The project in a sense passed out of the mathematics of Percival Lowell and into the observational technique of Clyde Tombaugh," astronomer David Levy, codiscoverer of a famous comet that smacked Jupiter in 1994, wrote in his biography of Tombaugh.11

Today, astronomers are finding Pluto's kin on the edge of the solar system using the same basic technique, supercharged with automated telescopes, high-powered cameras, and sophisticated software to sift through gigabytes' worth of imagery. But in 1930, the telescope, the camera, and the analysis were all guided by one young man: a country boy who went out west because he didn't want to go into farming and couldn't afford to attend college.

Fortunately, this country boy was a dogged observer, unlike Lowell. No one could accuse him of failing to hunt down "innumerable facts." He fell into a workday routine that was as grueling as a Kansas farmer's schedule.

On a typical day, Tombaugh would get up in the morning, eat a light meal, and check the weather. If the skies looked favorable, he' d make his plans for a night of observations. Some of the day might be spent doing the chores, or developing plates, but the real workday began after dark. Sitting beside the Lowell Observatory's 13-inch triplet-lens telescope, Tombaugh would take pictures of the area of the night sky directly opposite the sun. If he was lucky, he could get in several hourlong plate exposures during the course of the night, and then fall into bed before dawn at the end of a fourteen-hour workday.

If clouds looked likely to spoil the night's observations, or if the moon was due to interfere, Tombaugh would spend the day blinking plates. This involved sitting down at the comparator, looking through the microscope eyepiece, and checking every square inch of star-filled photographic plates. The task required so much concentration that Tombaugh had to take a break every half hour. During the breaks, he might write down lab notes about what he saw, have lunch, read a journal, or chat with the other astronomers about Planet X, Mars, or other matters.

On the morning of February 18, 1930, Tombaugh could tell it was a day for blinking. The skies over the observatory were cloudy—and the glare from a last-quarter moon would probably spoil the picture-taking even if the skies cleared up. He hunkered down for what he reckoned would be a nine-hour day of checking photographic plates.

As it happened, the plates he selected had been made the previous month, when the telescope was pointed at an area of the sky that Lowell targeted all those years before. The familiar clack of the comparator sounded as Tombaugh slowly made his way across the surface of the plate. He put in several hours of blinking, broken up by short breaks at the office and a lunch hour at a downtown café.

By four o'clock, Tombaugh had examined about a quarter of the plate, but there was nothing to see except for faint flaws in the photographic emulsion. He clicked past Delta Geminorum, a bright star about six degrees away from a point Lowell had once favored for the location of Planet X. Then he saw it.

As the comparator clicked, a dark spot popped in and out of his field of view. When Tombaugh moved the microscope a bit to the side, another spot appeared and disappeared— clack, clack, clack. He turned off the automatic blinking. The spots were several times fainter than Lowell had predicted for Planet X, but they were definitely there. And the object recorded by those spots was definitely moving in the right direction.

How far away was the object? Tombaugh took a plastic ruler and measured how far apart the spots were. The distance was 3.5 millimeters, or about an eighth of an inch. That meant the object couldn't be a close-in asteroid. If the object was genuine, it had to be farther away than Neptune.

For the next forty-five minutes, Tombaugh's mind raced through a flurry of questions: Could the spots be just a fluke? Were they just smudges on the glass? Or maybe they were two variable stars that just happened to flare close together? To answer those questions, he pulled out a third photographic plate from January and took a close look. Sure enough, there was a faint extra spot just where he expected it to be.

"I was walking on the ceiling," Tombaugh recalled later. "I was now 100 percent certain."

Small sections of photographic plates show the discovery images of Pluto moving across the night sky. Pluto is indicated by a white arrow superimposed on each plate. The left plate is from January 23, 1930. The right is from January 29, 1930. Lowell Observatory astronomer Clyde Tombaugh found Pluto as he examined these plates on February 18, 1930.

Small sections of photographic plates show the discovery images of Pluto moving across the night sky. Pluto is indicated by a white arrow superimposed on each plate. The left plate is from January 23, 1930. The right is from January 29, 1930. Lowell Observatory astronomer Clyde Tombaugh found Pluto as he examined these plates on February 18, 1930.

Clyde Tombaugh thus became the first man in history to take notice of little Pluto. After checking a few more plates, just to make sure, it was time to tell the world. Tombaugh walked across the hall to astronomer Carl Lampland's office and filled him in. Lampland immediately ran over to look at the plates through the comparator. Meanwhile, Tombaugh continued down the building's long hallway and walked into Slipher's office.

"Dr. Slipher," Tombaugh announced, "I have found your Planet X."12

It has often been said that if Pluto were discovered today, it would not have been considered a planet, due to its lack of mass or breadth or orbital dominance. But it doesn' t seem right to gauge Pluto's status—or the status of anyplace else, for that matter—without taking the historical circumstances into account. How would the Tigris River rank among waterways if it weren't part of the cradle of civilization? How would Mount Sinai rate as a mountain if it weren't mentioned in the Bible?13

The tale of Pluto's discovery has history galore. The search for Planet X took longer than the search for Neptune, and eventually thrust an unlikely hero from Kansas into the scientific spotlight. The strangest part of the story is that the highly eccentric world was found because of a highly eccentric millionaire's mistake. It turned out to be pure coincidence that Pluto was found in the same part of the sky that Lowell started searching twenty-five years earlier. Lowell might never have started that search if he had had the right numbers for the orbits and the masses of Uranus and Neptune.

But if Lowell had not embarked on his quest for redemption, Pluto might have gone unnoticed for years longer. And who knows? The tale of the glittering realm on the edge of our solar system might have spun off in a completely different direction.

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