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Edge of Space


Plotting the Planet Pluto and Beyond

November 22nd 2010

Science - Clyde Tombaugh
Clyde Tombaugh

In September 2010, a group of astronomers at the Lick-Carnegie Exoplanet Survey announced the discovery of the first so-called “Goldilocks planet.” Gliese 581g (also dubbed “Zarmina's World” by its discoverer, UC Santa Cruz astronomer Steven Vogt, who has romantically named the planet after his wife) is the first rocky planet outside our solar system known to be situated “just right” for life—that is, within the habitable zone of its star.

While its possibilities for harboring life have attracted new attention to the continuing search for other worlds, Gliese 581g is just one of hundreds of exoplanets located during the last few years. More than 20 light years from Earth, its discovery calls to mind the amazing story behind the discovery in 1930 of what was, for a time, the most distant known planet—Pluto. This year is the 80th anniversary of Pluto’s discovery, and it's fair to say that the pace of discovery is, to say the least, accelerating.

The six planets in outward progress from the sun—Mercury, Venus, Earth (naturally), Mars, Jupiter, and Saturn—have been known from antiquity. The seventh planet, Uranus, was accidentally discovered in 1781 by William Herschel, who, over the course of a few days in March of that year, observed an object moving steadily against the unmoving background stars. Herschel initially reported his finding as a comet, but by 1783 his and others’ observations had confirmed the planetary nature of Uranus. This revolutionary discovery greatly expanded the understood size of the Solar System, and suggested that more worlds might remain to be found.

Indeed, careful study of the slightly wobbly orbit of Uranus suggested that its passage was being perturbed by a very large body even further from the sun. Something, it appeared, was exerting gravitational pull on Uranus from beyond. But it would not be until 1846 when German astronomer Johann Galle, using coordinates provided to him by the French mathematician Urbain Le Verrier, finally observed a large gas planet just where Le Verrier’s calculations had indicated the perturbing object would be.

Continued study of Uranus’s orbit suggested that not even Neptune was massive enough to have jostled that planet as much as it was being jostled, and a hypothetical ninth planet was proposed to account for what were believed to be the remaining wiggles. Planet X, as it would come to be called, would be very far from the sun indeed, and while it was predicted to be a gas giant—roughly as large as Neptune—it would be difficult to detect using the telescopes available. For decades, the existence of a further planet (or planets) was debated, but the question was more often taken up as a curiosity rather than as a matter for serious investigation.

The most sustained search for Planet X was conducted at Lowell Observatory by Percival Lowell from 1905 to his death in 1916. Percival Lowell was a gifted mathematician, Harvard-trained and rich enough to do what he liked. He was also, in the tradition of the eccentric millionaire, attracted to the more imaginative ends of his chosen field. It was Lowell, for example, who popularized the notion of Martian “canals,” and in fact he built his observatory largely to conduct research on the question of the Martian civilization he believed was thriving there.

The search for Planet X appealed to Lowell; it was sufficiently grand and appealingly speculative. With the help of a number of “computers” at Harvard College Observatory—that is, skilled human calculators—he attempted to deduce the most likely location of Planet X from the known perturbations of the outer planets. After more than a decade of elaborate calculation and erratic searching, Lowell died with Planet X still undiscovered, and with his reputation as a gifted crackpot more or less confirmed.

As tends to happen to wealthy eccentrics, as though in revenge for their earthly strangeness, Lowell’s wishes for his estate—as expressed in his final will—were contested by his widow. Constance Lowell, herself a rather peculiar lady, did not wish the Observatory to receive the bulk of her late husband’s wealth in order to continue what she regarded as a hopeless undertaking.

While probate questions were argued, the search for Planet X at Lowell Observatory was suspended. By the time the legal questions were concluded about a decade later, photographic equipment and telescopic instruments had improved. But with the observatory still somewhat underfunded (the legal battle had consumed more than one million 1928 dollars), the staff were forced to cut some corners when they resumed the search in 1928.

One of these cut corners could be imagined as being embodied in the unlikely person of Clyde Tombaugh, the high-school educated Kansas farmboy who was invited to Flagstaff on the strength of some line drawings he had made of Jupiter, which he had observed through his homemade 9-inch telescope.

Whether Tombaugh was really invited to Flagstaff as a cost-cutting measure is an open question. I think he was, and I think, too, that his hiring indicated that the director of Lowell Observatory had only faint confidence in his institution’s chances of success of locating Planet X, notwithstanding the late Founder’s settled belief.

An exhaustive 10-month photographic hunt ensued, during which Tombaugh, working largely alone after some brief on-the-job training, used a device called a blink comparator to hunt for one moving speck amid millions of background stars. The blink comparator held two nearly identical photographs of the night sky. With the two exposures made roughly a week apart (depending on weather), if anything had moved in the seven days between exposures, it could be seen to shift position from one plate to the next.

Tombaugh’s unfathomably tedious task was to examine, for what would be months on end, more than 150 plate-pairs, each of which contained images of upwards of 10,000 stars, looking for the one speck that would move in the way a planet would. It’s hard to know which is more remarkable: that this was considered a viable method for finding something, or that Tombaugh actually found was he was looking for.

When Tombaugh finally found his infinitesimal moving speck in February of 1930, it was very nearly where Lowell’s mathematics had predicted it would be. Lowell’s painstaking calculations and sky-eyed convictions seemed, against all odds, to have been confirmed.

But—and here’s the kicker—the object Tombaugh found was much too small to have exerted any kind of perturbative effect on Uranus. No gravitational bully like Neptune, the new object was in fact smaller than Earth’s moon. After some weeks of confusion and puzzled reconfirmation, it was deduced that in fact the discovery of a new solar-system body so near Lowell’s predicted location was nothing more than unearthly coincidence. (Decades later, using data from Voyager 2’s 1989 flyby, astronomers concluded that the mass of Neptune had been inaccurately measured all along—by a factor of just 0.5 percent—and that, in fact, the orbit of Uranus had never been unexplainably perturbed after all.)

In 1930, public reaction to the discovery of what was deemed a ninth planet was immediate and enthusiastic. Though the astronomers at Lowell Observatory hadn’t asked for them, hundreds of letters and telegrams arrived suggesting names for the new world, from the respectable (“Thanatos,” god of death) to the laughable (“Burdett,” Clyde Tombaugh’s home town in Kansas).

Times were hard in the US then—what would be later known as the Great Depression had just begun—and the story was seized upon as an example of home-grown American know-how and determination. Clyde Tombaugh went on to college, eventually to become a professor at New Mexico State University. Lowell Observatory continues to be known for its study of Pluto and related outer Solar System objects. The instruments Lowell and Tombaugh used are still in order, and you can look through them yourself on top of Mars Hill, in Flagstaff, Arizona, as I did, during the research for my novel about Clyde Tombaugh and the hunt for Planet X.

In 2006, Pluto was reclassified as a “dwarf planet,” joining a handful of recently discovered brethren (including Haumea, discovered in 2004, and Eris and Makemake, both discovered in 2005). While often perceived as a demotion, Pluto’s reclassification rather expressed the evolving scientific understanding of the variety of solid bodies in orbit around the sun. Many scores of trans-Neptunian objects, or bodies beyond the orbit of Neptune, have since been located, with more being found every year. And in addition to the Lick-Carnegie Exoplanet Survey, NASA’s Kepler Mission, which has placed a planet-hunting telescope in orbit, has already found more than 700 suspected exo-planets since its launch in 2009, a rate of nearly one per day. Some scientists suggest that there may be literally billions of earthlike planets within our galaxy alone.

More and more, when we look up, we see a universe that may eventually house our descendants, or whose inhabitants may one day come to visit. Using present technology, a trip to Zarmina’s World would take around 180,000 years—but if history is any guide, we’ll always be imagining our way into the stars.

Michael Byers is an associate professor of English at the University of Michigan. His most recent book is Percival’s Planet, a novel about the discovery of Pluto. This article is adapted from one at Michigan Today.

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