The Pursuit Of Pluto

Astronomers sift. They sift through galaxies, through stars, through planets, through asteroids, looking for both the expected and the unexpected. They search for the evidence that will bolster one theory or topple another, and they also hunt for the big discovery, the prize jewel that has slipped through other sieves: an exploding star, a phantom galaxy, or—best of all—a new planet.

The discovery of a new planet is especially rare. It has happened just three times, and only once in America. In 1930, after taking dozens of photographic plates and searching through over a million faint stars, Clyde Tombaugh discovered the most mysterious world in the solar system—Pluto. The search for Pluto had begun even before Tombaugh was born. For many of the scientists involved, the project was a long struggle to find the tools that could find the planet—a good enough telescope and the means to examine the photographic plates it produced.

The astronomer Percival Lowell launched the hunt for Pluto in 1905. He hoped the discovery of a new world would win him the credibility all his previous labor had not. The wealthy Bostonian was under attack for his staunch belief in intelligent life on Mars. He had built an observatory in Flagstaff, Arizona, to study the red planet and had come to believe that Mars was a dying world, a planet running out of water; through his telescopes he thought he saw canals the Martians had built to carry precious water from the polar caps to the equator. In three books, hundreds of articles, and scores of lectures, he conveyed his ideas to nonscientists everywhere. As a fervent pacifist he pointed to Mars as a civilization whose inhabitants had to cooperate with one another or die. Blood-red Mars, named for the god of war, was actually a world of peace.

Lowell’s theories excited the public imagination but met with ridicule from other astronomers. In response he initiated a secret search for a new planet, which he hoped to find on the fringe of the solar system. He called it Planet X. If he could predict where Planet X was and then find it, he would at last attain unassailable credibility in the eyes of even his harshest critics.

Such a thing had happened once be- fore. In the 184Os astronomers had predicted the existence of Neptune before the planet was actually discovered. Because Neptune is a giant planet, its gravity tugs at Uranus. By analyzing the deviations in the motion of Uranus, astronomers determined not only that Neptune had to exist but also where it had to lie. Lowell sought to duplicate the feat. In 1905 he initiated a trial-and-error, on-again, off-again search that would continue until the year of his death. But when he began the search, he had neither evidence for a planet past Neptune nor the tools to find it; he had only the hope that such a planet existed and his stubborn determination to discover it.

The first phase of Lowell’s search, from 1905 to 1907, utilized a five-inch telescope at Lowell’s Flagstaff observatory. Since Lowell did not know where the planet was, he had his astronomers take photographic plates all along the zodiac, the part of the sky that contains the other planets (the stars of the zodiac are those that lie near the plane in which the planets circle the Sun). Each plate required a three-hour exposure to register faint stars. The exposed plates were shipped to Boston, where Lowell inspected them himself. Stars do not move, but a planet wanders slightly from night to night. By comparing two plates of the same region of the sky taken on two different nights, Lowell hoped to find an object that had shifted position. To do this he lay one plate atop its mate and then studied the pair with a hand-held magnifying glass.

The search got off to an embarrassing start. One of the first plates revealed what looked like two comets, and Lowell reported the discovery to the astronomical community. The comets turned out to be nothing more than defects in the plate, and the incident prompted Robert G. Aitken, of the Lick Observatory, in California, to write: ‘The discovery of a comet by photography is unusual enough to be noteworthy, but to find two on a single plate is a unique achievement. … It might be suggested that photographic defects often look wonderfully like comet tails; but it is of course assumed that Professor Lowell took precautions to guard against such deception before announcing the discoveries.”

Lowell’s five-inch telescope held part of the blame for this first search’s failure to produce a new planet: it could barely pick up Pluto’s faint light, and it took in only five degrees of the sky at a time—the distance between the two “pointer stars” in the bowl of the Big Dipper.

What would make a better telescope for the search? If Planet X existed, it was dim and distant, so the instrument would have to reach faint stars. The key element dictating a telescope’s ability to see faint objects is the area of its objective lens or mirror. The bigger the lens, the more light it can take in. A ten-inch telescope can acquire stars one-fourth as bright as a five-inch can. But the telescope needed more than a large lens. It also had to provide a wide field of view. The greater the field of view, the more sky the telescope can cover with one exposure—but the less clearly an astronomer can see any particular object. For some work, such as discerning the purported canals on Mars, one would want a small field of view, but for the Planet X search it’s just the opposite—the need was to see a lot of sky with one exposure. Lowell’s telescope was lacking on both counts. Its five-inch lens was not sensitive enough to faint light, and its field of view was too small.

Lowell’s method of examining the plates by simply superposing them was also inadequate. In early 1906 Carl Lampland, one of Lowell’s Flagstaff astronomers, suggested that the observatory order a German-made device called a blink comparator. Built by Carl Zeiss Works, the machine would hold two different plates of the same part of the sky and alternate them rapidly, like two successive frames in a movie. To an astronomer looking at a tiny portion of the plate through the comparator’s eyepiece, stars would remain stationary as the plates alternated, but any moving object—comet, asteroid, or planet- would appear to jump back and forth from one plate to the other. The astronomer would then move the eyepiece to a new position and examine the next area the same way. Blinking plates could be a powerful if tedious way to detect a planet, and the comparator would ultimately be the machine to reveal Pluto. But on a visit to Europe in 1906 Lowell rejected the device.

Lowell suspended his search for Planet X in 1907 and resumed it four years later with several differences. In 1911 he went ahead and finally ordered a blink comparator for the observatory, and he abandoned the fiveinch telescope in favor of the observatory’s new forty-inch reflector. This big telescope could capture very faint stars. But its field of view was tiny- less than a degree across—and so it too missed any new planets. In 1914 Lowell borrowed the Sproul Observatory’s nine-inch telescope and pressed it into service. Though far smaller than the forty-inch, it took in a larger field.

This time Lowell was trying to ascertain the position of Planet X before searching for it. He and his assistants in Boston plunged full-force into nearly year-long extensive calculations to pinpoint the unseen planet from its influence on Uranus.

The task was exceedingly difficult- far more so than the one that had confronted the astronomers who located Neptune from its gravitational pull on Uranus. Neptune had not yet completed a full circle since its discovery in 1846, so its orbit was not well enough known to determine whether there were perturbations in it. Lowell turned to Uranus, but it lay farther from Planet X and so would be less affected by the unseen planet’s gravitational pull. He and his assistants tried to ascertain the mass, distance, and position of Planet X from its influence on Uranus, just as one might try to deduce the nature and location of an object from a fragment of its shadow.

Lowell’s final results, which he published in 1915, indicated that Planet X was a giant planet 6.7 times as massive as Earth and about half the size of Uranus or Neptune. Lowell marked off two areas on opposite sides of the sky that he deduced might contain Planet X. One of them was near the constellation Gemini.

By now, however, he had lost interest in the actual search. Although he had pur- sued the project with great ambition, the observatory’s telescopes never did find the planet, and his prediction of Planet X’s size and location produced no excitement among other astronomers. Lowell was a defeated man, his planet undiscovered and his Martian theories unaccepted. Furthermore, he felt great anguish over the outbreak of war in Europe. On November 12, 1916, 54he suffered a massive stroke and died. His brother called the failure to find Planet X “the sharpest disappointment of [Lowell’s] life.” Lowell’s friend George Russell Agassiz said it “virtually killed him.”

Yet the year before Lowell died, the nine-inch telescope actually caught a faint world that lay far beyond Neptune. On two different nights—March 19, 1915, and April 7, 1915—the photographic plates recorded Pluto buried in the stars of the Milky Way between Gemini and Taurus. No one noticed.

Lowell’s death ended the search for Planet X. The observatory’s astronomers wanted to keep looking, and Lowell had left the observatory more than a million dollars to continue his work, but his widow challenged the agreement, initiating a legal battle that lasted until 1927. As a result, as Lowell’s nephew, Roger Putnam, the observatory’s trustee, wrote, “Lowell Observatory is not rolling in wealth, and it has to conserve its resources as much as possible.”

The hiatus caused by the lawsuit nearly handed Pluto to one of Lowell’s rivals, the Harvard astronomer William Pickering. Pickering had helped Lowell establish the Flagstaff observatory in 1894 but later became a critic of Lowell’s theories about Mars. Like Lowell, he had an interest in undiscovered planets; he predicted that several existed. In 1919, after analyzing the motions of Uranus and Neptune, he concluded that an unseen world, “Planet 0,” lay in Gemini, and he persuaded astronomers at California’s Mount Wilson Observatory to search for it. In late December four different plates taken there recorded the image of Pluto. Once again, no one noticed.

For Lowell Observatory this was a close call. The Lowell astronomers knew they would need a new telescope to carry on the Planet X work; even Lowell himself, the year before he died, had concluded that the various telescopes used so far were not up to the job. What was needed was a good photographic telescope that could both penetrate to still fainter magnitudes and cover still more sky.

In the years following Lowell’s death, Carl Lampland tried to obtain large objective lenses for such a telescope. He sought a sixteen-inch lens from Germany and considered twelveto eighteen-inch disks from two English companies. The solution, though, turned out to lie closer to home, with the great American telescope maker Joel Metcalf. Metcalf died in 1925 while at work on a thirteen-inch telescope; Lowell Observatory bought the unfinished disks from Metcalf’s widow.

In 1927, as the legal battle over Lowell’s estate was ending, Putnam met with Vesto Slipher, director of the Lowell Observatory, to discuss the observa- tory’s future. They agreed to renew the search for Planet X as soon as a new telescope could be built. Money would not be a problem: Putnam persuaded Percival Lowell’s brother, President A. Lawrence Lowell of Harvard, to donate ten thousand dollars to build it.

The most difficult task would be finishing the thirteen-inch objective lens, a job for which Putnam sought C. A. Robert Lundin of the telescope makers Alvan Clark and Sons. The Clark firm had built the observatory’s twentyfour-inch telescope in 1896; Lundin had overseen the construction of the forty-inch telescope in 1909; and he had supervised the conversion of the forty-inch into a forty-two-inch in 1925. Lundin estimated that the work would cost thirty-five hundred dollars.

This left more than half of Lawrence Lowell’s money to construct a mount and dome for the telescope. The mount had to be heavy and stable enough so that strong winds couldn’t interfere with long exposures; smooth guiding of the telescope would require a worm wheel four feet across. To build the mount, the astronomers turned to the observatory’s own Stanley Sykes, an expert instrument maker. Sykes started his part of the project in early 1928 and completed it by autumn.

Lundin was then still hard at work shaping the lens. One of Metcalf’s disks was so thin he had great difficulty polishing it, and he didn’t finish the task until January 1929. His bill: $5,667.41, far higher than his original estimate. Putnam decided to offer the optician a compromise sum.

Slipher telegraphed Putnam directing that no money be paid until the observatory could test the lens, but Putnam had already sent off a check. When the lens reached Flagstaff in February, the astronomers bolted it to the upper end of the five-foot-long telescope tube and made their first tests. The new telescope’s performance quickly erased any fears: the results were excellent. It produced crisp star images over such a large field of view that the astronomers decided to start using photographic plates measuring fourteen by seventeen inches. With a one-hour exposure these plates captured very faint stars covering a region twelve degrees wide and fourteen degrees long. The telescope was indeed ideal for the search for Planet X.

With the thirteen-inch telescope ready to produce good plates and the blink comparator ready to examine them, Lowell Observatory had two of the three key elements for success. The third would be a thorough, conscientious observer to guide the telescope and expose the plates. They found such an observer in Clyde Tombaugh.

Tombaugh, a young man living with his family on a farm in western Kansas, had no formal training in astronomy, but he had built three telescopes and observed the planets through them. In late 1928 he sent drawings he had made of Mars and Jupiter to Lowell Observatory. Slipher was so impressed he offered him a job, saying merely that Tombaugh would be in charge of a new telescope. Only when Tombaugh reached Flagstaff did he learn he would be hunting for a new planet at the edge of the solar system.

Tombaugh arrived at Lowell Observatory in January 1929 and exposed his first plate with the new telescope on April 6. His tenth plate, which he took of Gemini just five nights later, actually snared the world he was looking for, but once again neither he nor anyone else noticed it. (Pluto, with its large orbit and slow speed, remains in one constellation for years.) Part of the reason no one saw Pluto was that Gemini was not well placed for viewing in April. Tombaugh had photographed the constellation because it was one of the two areas Lowell had thought might contain Planet X.

By the time the summer rainy season suspended further observations, Tombaugh had taken about a hundred plates of the sky. Hoping to make a quick find, the other astronomers blinked the plates on the comparator, which had been modified to hold the large plates. The scientists, rushing to justify all the money spent on the telescope, made cursory inspections, which, to their disappointment, turned up nothing.

In June Slipher asked Tombaugh to begin blinking the plates himself. The announcement startled Tombaugh; the plan had always been for him to shoot the plates but leave the blinking to older astronomers. Blinking plates was difficult and tedious. Each plate held tens or hundreds of thousands of stars, and those taken of the densest regions of the Milky Way had as many as a million. Blinking a pair of plates thoroughly required several days to several weeks, depending on the number of stars. As Tombaugh set the plates on the comparator, he found that many of them were worthless. One plate of a pair might record more stars than the other, rendering impossible any attempt to blink the two. Furthermore, the plates were much too good at picking up asteroids, whose movement masqueraded as the motion of a planet. How was he supposed to tell them from Planet X?

Tombaugh began to despair, and a visiting astronomer from an Eastern observatory made matters worse. “Young man,” he told Tombaugh in late June, “I am afraid you are wasting your time. If there were any more planets to be found, they would have been found long before this.” But during the rainy season Tombaugh devised the strategy that would finally apprehend Pluto. First, he would expose all plates uniformly, so that one plate of a pair did not acquire more stars than the other. One-hour exposures would be the norm; if sky conditions were poor he would extend the exposure another fifteen to thirty minutes to compensate. In addition, he would take not two but three plates of each part of the sky. A third plate could provide a check on any suspicious objects he discovered while blinking the first two.

Tombaugh also solved the asteroid problem. If he took plates of the sky exactly opposite the Sun, he could distinguish asteroids from Planet X simply by the size of their motion. Because the Earth moves fast around the Sun, all objects in the solar system lying farther from the Sun appear to move a bit from night to night—if they lie on the opposite side of the Earth from the Sun. But the size of this movement depends solely on distance: nearby objects like asteroids seem to move faster than distant ones like Pluto, just as nearby road signs whiz by a moving car much faster than do distant trees.

When autumn came and the rains ended, Tombaugh implemented his new strategy. In September he took plates of the constellations Aquarius and Pisces, which in early autumn lie opposite the Sun. During October, November, and December of 1929 he worked his way into Aries and Taurus, all the time getting closer and closer to Gemini. He exposed his plates during the two weeks of each month when the Moon was dark and then blinked them on the comparator the following two weeks, when the Moon’s light would fog any plates he tried to take.

In January Tombaugh reached Gemini. On January 21, 1930, he pointed the telescope toward the star Delta Geminorum. The sky was clear, but just minutes after he began to expose the plate a fierce northeast wind assaulted the dome and distorted the images of the stars. He knew the plate he was taking would be terrible. On January 23 and January 29, under better conditions, he took additional plates of the same region.

On the morning of February 18 Tombaugh started examining the plates with the blink comparator. Since the January 21 plate was not good, he chose to blink the plates from January 23 and January 29. By afternoon he had scrutinized about a fourth of the area the plates covered. Then, at 4:00 P.M., he moved the comparator’s eyepiece to a new region and saw it: a tiny speck jumping back and forth from one plate to the other.

“That’s it!” he said to himself. “That’s it!” The object’s shift was small—too small for an asteroid, just right for Planet X. With growing excitement Tombaugh checked the January 21 plate and found the blurred image of the far-off world there too. At 4:45 he called to Lampland in his office across the hall, and Lampland joined him at the comparator. Then Tombaugh walked down the hall to SIipher’s office and announced, “Dr. SIipher, I have found your Planet X.”

Slipher rose from his chair, hurried down the hall toward the blink comparator, and saw for himself the image of a new world. The next step would be to view the planet through the observatory’s larger telescopes. But the night was cloudy, and Tombaugh spent his evening watching not the newest known member of the solar system but Gary Cooper in The Virginian . The next night was clear, and Tombaugh took another plate that captured the planet. On February 20, using the new plate as a guide, the astronomers located the planet in the twenty-four-inch telescope. It was much fainter than Lowell had predicted and did not appear as a disk, suggesting that it was very small indeed. Lampland used the forty-two-inch telescope to look for satellites circling it but found none.

For three weeks the astronomers tracked the planet as it crept through Gemini. They were as cautious as Lowell had been flamboyant. Finally, on March 13— the 75th anniversary of Lowell’s birth and the 149th anniversary of the discovery of Uranus—a brief telegram informed the world of their achievement: “Systematic search begun years ago supplementing Lowell’s investigations for trans-Neptunian planet has revealed object which since seven weeks has in rate of motion and path consistently conformed to trans-Neptunian body at approximate distance he assigned. Fifteenth magnitude. Position March twelve days three hours GMT was seven seconds of time west from Delta Geminorum, agreeing with Lowell’s predicted longitude.”

Today we know Pluto is a tiny world that requires 248 years to revolve once around the Sun. Its average distance from the Sun is 3.7 billion miles, but its orbit is so elongated that when closest to the Sun this planet cuts inside Neptune’s orbit. (Pluto will never collide with Neptune; its orbit is tilted relative to Neptune’s, and whenever their distances to the Sun match, the smaller world lies far above the plane of Neptune’s orbit.) Ever since its discovery Pluto has been approaching the Sun. It equaled Neptune’s distance in 1979. Pluto reaches perihelion—its closest point to the Sun—every 248 years. It occurred this past September.

In 1978 James Christy of the U.S. Naval Observatory discovered a moon of Pluto as he examined photographs of the distant world. The moon appeared as a small bump on Pluto’s side. Even though the moon lies very close to the planet, it takes several days to circle it, indicating that Pluto must have little mass. Indeed, Pluto’s mass is just 0.2 percent of Earth’s, and its diameter is less than our Moon’s. It is so small, in fact, that some scientists consider it an asteroid rather than a true planet.

Whatever Pluto is, it is not Planet X. Lowell expected Planet X to be 6.7 times as massive as Earth; Pluto is 500 times less massive than Earth and is far too little to perturb the other planets. That it was found near one of the two locations Lowell predicted for Planet X is nothing more than a remarkable coincidence. Today there remain unexplained irregularities in the motions of both Uranus and Neptune, irregularities that suggest to some astronomers the existence of a large planet far beyond Neptune and Pluto that continues to elude scientists.

Lowell never found Planet X, and neither did Tombaugh. But Pluto is surely as strange a world as any still awaiting discovery. Without Lowell’s inspiration, Tombaugh’s skill, and the equipment that made the work possible, Pluto might still be circling the Sun in secrecy, hiding among the millions of faint stars in the sky.