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103 12 MORE THAN ONE PLUTO Is Pluto a planet or is it something less? For years this question has been raised, discussed, and arbitrated again and again. Pluto’s 1930 discovery stemmed from the observations that neither Uranus nor Neptune followed the orbits that Newton’s cosmos, even with Einstein’s alterations, derived for them. Uranus strayed from the orbit it should have complied with, and this led directly to Neptune’s discovery. But later observations confirmed that neither followed their orbits, hence another planet must be out there somewhere. This led Percival Lowell to his search, which succeeded only after he died in 1915. The discovery in 1930 by Clyde Tombaugh, a young staff observer at the time, of a starlike object, too small and too far away to be seen as a disk, was thought to satisfy the need for the restoration of Newtonian-Einsteinian order in the system. When Gerard Kuiper scanned the planet with the new 200-inch telescope at Mount Palomar soon after its completion in 1948, he measured its diameter at about 3,600 miles, placing it between Mars and Mercury in size, a respectable planet. Still, for years afterward, the three outer planets could not make up their minds to behave. For an outer planet to make the others follow the gravitational rules as computed at the time, an object of about the mass of the Earth was required. In order for Pluto to do the job, it had to be either by far the densest or the blackest body in the universe; the densest in order to pack an Earthlike mass into a body smaller than Mars, or the blackest with an absurdly low albedo that could make it appear so faint. But Pluto turned out to be neither the one nor the other. Thus another, tenth, planet, Planet X, must be still farther out there to handle the gravitation thing. The situation remained in a muddle as no new planet could be found until the second Voyager space probe passed near both Uranus and Neptune by 1989, providing the first clear images of them and their moons and redetermining their orbits with far greater preci- sion than earthbound observations could muster. Astronomers at the Jet Propulsion Laboratory in Pasadena used new observations made by the flyby and came to the conclusion that these two giants were indeed following the orbits we designed for them, and as a result the need for Planet X vanished! The discovery in 1978 of Pluto’s satellite, Charon, gave us Pluto’s mass with the familiar story applied from Kepler’s harmonic law with masses. A puny mass, one-sixth that of the Moon, and a diameter of somewhere around 1,400 miles raised the question: was this a proper planet or something less—an asteroid? The plot thickened when other Plutos were found out there, none quite so large but of the same constituents and in very similar orbits. In 2001 and again in 2002, we have found two more Plutos, or near enough. They have been known by their designations in astronomical parlance, 2001 KX76 and 2002 LM60, and as soon as their orbits have been calculated with a precision that enables us to recover them as we choose, they shall receive names. They are tentatively called Ixion and Quaoar, respectively, after gods, since names for these objects generally favor the deities of any of a number of cultures. Ixion’s size is uncertain, but it is probably larger than Charon, at 600 miles in diameter; Quaoar is certainly larger, being about 800 miles across, making it more than half the size of Pluto (1,400 miles) and the largest object discovered in the solar system since Pluto itself was found in 1930. Quaoar has a nearly circular orbit with a period of 288 years and an orbital inclination of about 8 degrees. Compare with Pluto’s 248 years and 17-degree inclination. As John Davies has written, the past decade has seen a doubling in the size of the solar system. The area beyond Neptune, now called the Kuiper belt, after Professor Gerard P. Kuiper, who was among the first to call attention to it, is home to a group of minor planets discovered since 1992. This recent development has dramatically broadened and altered our understanding of how the solar system was formed and has provided answers as to the origin and nature of Pluto. Theoretical physicists had decided that there...

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