In the dark, outer reaches of the Sun’s realm lies a world unlike any other. Discovered in the spring of 1930, Pluto—named by an Oxford schoolgirl, who was fascinated by classical mythology—was for three-quarters of a century considered the ninth planet in the Solar System, before a major reclassification in 2006 demoted it to its present status as a dwarf planet, a trans-Neptunian object, and the largest known body in the Kuiper Belt. Since then, it has proven the butt of much cruel humor, with fierce debate on whether it should be reinstated as a “planet” or if it should retain its somewhat less lofty descriptor. Thirty-eight days from now, on 14 July, our final first-time, close-up glimpse of the last of the “classical” nine planets will be made, as New Horizons reaches the climax of its 9.5-year voyage. Over the coming weeks, AmericaSpace’s New Horizons Tracker and a series of articles by Mike Killian, Leonidas Papadopoulos, and myself will cover the discovery and exploration of Pluto to date, the many trials and troubles faced by those who desired to send a spacecraft there, and the unfolding developments as New Horizons seeks to make this unknown world known.
For many of us, born and raised long before 2006, it was simple and taught through nursery rhymes and schooltime songs: Pluto was the ninth planet in the Solar System, although it was known that the dynamics of its highly elliptical orbit—which carries it out of the ecliptic, the plane on which the other major planets circle the Sun—caused it to periodically draw closer to its parent star than Neptune, moving into eighth place for a while, then receding back to ninth. And since the dawn of the Space Age, humanity has long desired to visit each of these worlds, which had hitherto appeared as little more than points of light in telescope eyepieces. Mercury, the innermost planet to the Sun, was first imaged by Mariner 10 during a pair of flybys in March and September 1974; Venus by Mariner 2 in December 1962; Mars by Mariner 4 in July 1965; Jupiter by Pioneer 10 in December 1973; Saturn by Pioneer 11 in September 1979; and Uranus and Neptune, both by Voyager 2, in January 1986 and August 1989. And now, in the summer of 2015, New Horizons will pass and observe Pluto for the very first time.
Whichever way one looks at it—whether you believe Pluto to be a planet or not—our first visit by a machine fashioned by human hands promises to be an epochal moment in the history of our species; an illustrator of how far we have come, figuratively and literally, in just a handful of decades. Yet the discovery of tiny Pluto came about through a peculiar set of circumstances, influenced by the unexpected motions of its far larger sibling, Neptune. Discovered in September 1846, it soon became clear that Neptune did not appear to follow its predicted orbital path, even after experts in celestial mechanics had taken the combined motions and perturbations of the other planets into account. In the early part of the last century, the U.S. astronomer Percival Lowell tasked himself with finding an unseen “Planet X,” a new world beyond Neptune, and it is an unfortunate footnote of history that the man who founded the Lowell Observatory in Flagstaff, Ariz., and proclaimed that artificial canals existed on Mars never realized how close he came to finding Pluto.
Lowell conducted several telescopic and photographic searches of the heavens from 1905 onward, all within the level of the Solar System’s ecliptic plane and all without success, but a few years later he happened to attend a lecture by his friend, William Pickering of Harvard University, who described a graphical plot of the motion of Uranus as a predictor of a trans-Neptunian planet. Pickering nicknamed the hypothetical object “Planet O” and speculated that it might lie 52 Astronomical Units (AU)—about 5 billion miles (8 billion km)—from the Sun and take 373 Earth-years to accomplish a full revolution of its parent star. Drawing on Pickering’s theory, Lowell refined his own calculations and by 1909 was in a position to predict with reasonable confidence that an unknown planet lay 47.5 AU, or 4.4 billion miles (7.1 billion km), from the Sun and took 327 Earth-years to complete an orbit. He also suggested that it was less than half as massive as Neptune itself.
Lowell understood that Neptune, discovered just a few decades earlier, had simply not been observed for long enough in its 165-Earth-year orbit for full perturbations of its motion by another planet to be readily apparent. As a result, he relied upon the smaller and less obvious perturbations on Uranus—discovered in 1781 and whose 84-Earth-year orbit had therefore been fully observed during at least revolution of the Sun by Lowell’s time—to assist with his calculations. In July 1910, Lowell started a second telescopic search for Planet X, whilst in tandem Pickering published estimates for three more trans-Neptunian objects, which he labeled “P,” “Q,” and “R.” Unfortunately for Pickering, his prediction that Planet Q was 20,000 times more massive than Earth drew much scorn and criticism from his contemporaries. Sadly, his prediction of where Planet O may reside actually turned out to be tantalizingly close to where Pluto was ultimately found in February 1930.
As for Lowell, he spent the remaining years of his life, until his death in 1916, searching without success for the mysterious new world. He labored under the assumption that Planet X carried a mass seven times larger than Earth—approximately half as massive as Neptune—and took the form of a low-density object with a high albedo, similar to the giant gaseous planets and revealing an easily visible disk with a diameter of one arc-second and an apparent magnitude of 12-13. During his endeavors, he identified 515 asteroids, 700 variable stars, and—without realizing—in March 1915, two of his photographic plates revealed a faint object, somewhere between 15th and 16th magnitude, which later turned out to be Pluto. With the benefit of hindsight, it seems unlikely that Lowell even looked at the two plates, for the faint object was far dimmer than anything like the kind of magnitude he expected for Planet X. His brother, George, subsequently wrote with a hint of sadness: “That X was not found was the sharpest disappointment of his life.”
Thirteen years after Lowell’s death, in January 1929, a 22-year-old Kansan farmboy named Clyde Tombaugh joined the staff of the Lowell Observatory. He had no formal training in astronomy, but had so impressed the observatory’s director, Dr. Vesto Slipher, with his remarkable sketches of Mars and Jupiter that he was offered a job. Tombaugh’s first assignment was to search for Planet X, and, to achieve this end, he utilized the Lowell Observatory’s 13-inch (33-cm) telescope to acquire 14 x 17-inch (35 x 43 cm) photographic plates by night, in tandem with a “blink comparator.” The latter alternately shone a light through one plate, then another, so that any object which appeared in both plates—like an apparently motionless star—would appear steady and unmoving. On the other hand, if an object turned up on one plate, but not the other, or in different places on both plates, it would “blink” noticeably.
By thus enabling Tombaugh to flip backwards and forward betwixt plates, the effect was akin to perusing a picture-book and carried obvious benefits for finding a new planet, which would be expected to “move” against a seemingly stationary backdrop of stars. Significantly, he focused on the entire zodiac, rather than the regions suggested by Lowell, and over the course of his first year at the observatory he observed almost two million stars, before reaching the constellation of Gemini. His attention was drawn to a pair of photographic plates taken six days apart, on 23 and 29 January 1930, which revealed a faint speck, moving gradually across the sky, close to the star Delta Geminorum. A third plate, of somewhat lesser quality, served as confirmation of the new planet’s motion. “On 18 February 1930, I suddenly came upon the images of Pluto,” Tombaugh later wrote. “The experience was an intense thrill, because the nature of the object was apparent at first sight.”
He walked straight into Slipher’s office with the news. “Doctor Slipher,” he announced triumphantly, as described by Ken Croswell in Planet Quest: The Epic Discovery of Alien Solar Systems, “I have found your Planet X!” It lay just six degrees from one of two locations suggested by Lowell. After several more nights of observations, the discovery was verified and on 13 March—what would have been Percival Lowell’s 75th birthday—it was telegraphed to the Harvard College Observatory (HCO) at Harvard University in Cambridge, Mass., and formally announced to the world.
In spite of the euphoria, the diminutive new world was far from what had been anticipated by Lowell, Slipher, Tombaugh, and the bulk of the astronomical community who fervently believed in the existence of Planet X. Occupying a highly elliptical orbit, which carries it out of the ecliptic plane, ranging as close as 30 AU (2.7 billion miles or 4.4 billion km) and as far as 49 AU (4.6 billion miles or 7.4 billion km) from the Sun, Pluto proved so tiny and so dark that it revealed no visible disc and was six times dimmer than Lowell had predicted. Right from the outset, it had a tough time earning scientific recognition as a fully fledged “planet.” The German-born U.S. astronomer Armin Leuschner suggested as early as 1932 that its dimness and high orbital eccentricity made it more likely to be an asteroid, its course severely perturbed by a close passage to Jupiter, or even a long-period comet, whilst others published mathematical presumptions that irregularities in Uranus’ orbital motion were probably not induced by a mysterious outer planet after all.
Notwithstanding its nature, the discovery of the new world prompted the search for a name. And not just any name, for it resided further from the Sun than any other known planet in the Solar System, in a dark and gloomy realm, where incident sunlight was the tiniest fraction of the quantity that we receive on Earth. It was this location, this darkness and this gloom, which led to the name “Pluto” being suggested: though not by a panel of academics, but, as will be described in tomorrow’s AmericaSpace article, by an 11-year-old schoolgirl from the English city of Oxford.
The second part of this article will appear tomorrow.
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