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

too small to account for Uranus’s and Neptune’s orbital oddities. Planet X still had to be lurking, undiscovered, in the outer limits of the solar system.
    That was the prevailing belief until May 1993, when E. Myles Standish Jr., of the Jet Propulsion Laboratory in Pasadena, California, published a paper in the Astronomical Journal titled “Planet X: No Dynamical Evidence in the Optical Observations.” Standish used the updated mass estimates for Jupiter, Saturn, Uranus, and Neptune that had become available from the Voyager flybys; in the case of Neptune, the mass difference amounted to nearly 0.5 percent—quite large by today’s standards. Assuming that the masses derived from the Voyager missions were accurate (a wise move), and discounting a single set of suspicious measurements made at the U.S. Naval Observatory between 1895 and 1905 (another wise move), Standish recalculated all the orbital parameters. The result? The misbehaving trends in the paths of Uranus and Neptune disappeared completely, and their orbits could be explained entirely within the gravitational landscape of the presently known solar system. In plain English: Planet X was dead. The inventory of large objects, as decided by the gravity budget of the solar system, was complete.
     
    It seems quite obvious what a planet is, or ought to be. If an object orbits the Sun but is not itself a comet and does not orbit another object the way moons do, then all is well. William Herschel discovered Uranus in 1781. And Johann Galle, of the Berlin Observatory, discovered Neptune in 1846. But few people know that on January 1, 1801, the Italian astronomer Giuseppi Piazzi discovered the planet Ceres happily and silently orbiting the Sun between Mars and Jupiter. The suspiciously large gap between Mars and Jupiter had finally been filled. But astronomers rapidly determined that Ceres was much, much smaller than any other planet. Then on March 28, 1802, the German astronomer Heinrich Wilhelm Olbers discovered the planet Pallas in the same orbital zone as Ceres. For these two new planets, William Herschel could not identify a visible surface, even through the optics of his powerful telescopes. Apart from their obvious motion across the field of view, the telescopic appearance of Ceres and Pallas was otherwise indistinguishable from that of a distant star. In an expression of sentiment that echoes modern-day debate over what to call Pluto, Herschel wrote, in an 1802 letter to his friend, physician and scientist William Watson:
    You know already that we have two newly discovered celestial bodies. Now by what I shall tell you of them it appears to me much more poor in language to call them planets than if we were to call a rasor a knife, a cleaver a hatchet, &c. They certainly move around the sun; so do comets. It is true they move in ellipses; so we know do some comets also. But the difference is this: they are extremely small, beyond all comparison less than planets…. Now as we already have Planets, Comets, Satellites, pray help me to another dignified name as soon as possible. 11
    In a research paper submitted to the Royal Society the following month, Herschel proposed “star-like” as a descriptor, which in Greek becomes the more familiar “aster-oid.”
    At 600 miles in diameter, Ceres is dwarfed by Mercury, the reigning smallest planet. But that won’t concern us just yet. By 1807, three more of these diminutive planets had been discovered: Pallas, Juno, and Vesta. By 1851, 11 more had been logged, and the solar system’s planet count reached 18—duly recorded this way in textbooks of the times. The spate of new planets were all small and traveled in orbits similar in size and location to that of Ceres. By 1853, it was clear that a new class of objects had been identified: the asteroids . These bodies occupied a new swath of real estate in the solar system: the asteroid belt . Practically overnight, the planet count dropped back to seven: Mercury,