The Pluto Files: The Rise and Fall of America's Favorite Planet Read Free Page B

astronomy classes I taught, which totaled thousands of students by the time I left Texas. I also introduced pizza for the planet mnemonic in my first book, Merlin’s Tour of the Universe , published in 1988. And I have not once heard prunes associated with Pluto since the early 1990s.
    The perennial classroom exercise of memorizing planets in sequence from the Sun allowed the enumeration of the nine planets to take on mythical significance in the minds of students and educators alike. Every printed introduction to the solar system, no matter the grade level of the curriculum, began with a list of the nine planets, in order from the Sun, accompanied by a table or diagram of their relative sizes. This tradition became the pedagogical equivalent of eating comfort food. You somehow knew that all was right with the universe as you learned the planetary sequence, with little Pluto rounding out the list of nine. Even the Planetary Society, an organization founded in 1980 by Carl Sagan and two colleagues, Lou Freidman and Bruce Murray (both from NASA’s Jet Propulsion Laboratories in Pasadena), chose as its toll-free phone number 1-8 0 0-9 W O R L D S.

    Figure 1.9. Cartoon postcard by Paul McGehee. Although he drew one for each of the planets, Pluto’s cultural popularity surpasses them all.
    Meanwhile, the Voyager 1 and Voyager 2 spacecraft, launched in the 1970s but executing their outer-planet flybys in the 1980s, revealed that the moons of Jupiter, Saturn, Uranus, and Neptune may be as interesting as the planets themselves—maybe more so. It was soon clear that the number of intriguing worlds in the solar system vastly exceeds nine, including seven moons that measure larger than Pluto itself: Earth’s Moon; Jupiter’s Io, Ganymede, Callisto, and Europa; Saturn’s Titan; and Neptune’s Triton. The grade school tradition to rote memorize planet names (usually one’s first encounter with the solar system) unwittingly concealed a staggeringly rich landscape of objects and phenomena.



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Pluto in History
    B EFORE THERE WAS P LUTO THERE WAS P LANET X.
    Planet X was the “undiscovered” object in the outer solar system whose gravity was needed to fully account for the motions of the known planets. Heard about it lately? Probably not. That’s because it’s dead. But widespread belief in the existence of Planet X is what led directly to the systematic search and discovery of what would become Pluto.
    The rise of Planet X begins with the German-born English astronomer Sir William Herschel, who more or less accidentally discovered the planet Uranus on March 13, 1781. The episode was an exciting moment in eighteenth-century astronomy. Nobody in recorded history had ever actually discovered a planet. Mercury, Venus, Mars, Jupiter, and Saturn can each be seen relatively easily with the naked eye, and all were known to the ancients. The bias against finding additional planets was so strong that Herschel, even in the face of contrary evidence, assumed he discovered a comet. He even titled his discovery paper “Account of a Comet.” 8 Other astronomers were in denial as well. Charles Messier, the eighteenth century’s king of comet hunting, noted on April 29, 1781, “I am constantly astonished at this comet, which has none of the distinctive characters of comets.” 9
    Archival records of star positions show that several observers had seen Uranus before Herschel did, but each one had mistakenly classified the planet as a star. In an embarrassing example from January 1769, the French astronomer Pierre Charles Lemonnier did not discover Uranus six times. When Herschel finally noted that the mysterious object moved, the avail-ability of nearly a century’s worth of “prediscovery” data on its position in the sky enabled astronomers to calculate its orbit with good precision. Those calculations showed that the object’s orderly, near-circular path, far from the Sun, had nothing in common with the eccentric trajectories of all