Planetary Discoveries Before Neptune: From William Herschel to the “Celestial Police”

From time immemorial, Saturn had marked the outermost visible boundary of the Solar System. Then, in March 1781, the Solar System was suddenly doubled in size with the telescopic discovery of Uranus, by a German immigrant musician and independent astronomer, William Herschel. Naturally, doyens of celestial mechanics attempted to calculate an orbit for Uranus, based on the inverse-square law, whose prestige had been greatly enhanced by its success in solving problems such as the advance of the lunar apsides and the return of Halley’s comet.The calculation, however, proved to be more difficult than expected. Meanwhile, the interest of astronomers was seized by the discovery in 1772 of an empirical numerical relationship (the Titius-Bode law, or Bode’s law as it was known throughout the 19th century). This gave the distances from Mercury to Saturn according to a series of numbers, 4, 7, 10, 28, 52, 100. There was one exception—number 28 appeared as a gap between Mars and Jupiter. When Uranus was discovered, it too fit the scheme, but the gap between Mars and Jupiter remained, and inspired a number of astronomers in Germany to speculate about the possibility that a planet might be found in that location. They began to organize a cooperative search in an effort dubbed the “Celestial Police,” but were forestalled by Giuseppe Piazzi’s discovery on New Year’s Day 1801 of the new planet Ceres, now known to be the largest asteroid and the closest dwarf planet. Later, uncoordinated, efforts by two members of the Celestial Police additionally yielded Pallas, Juno, and Vesta.Meantime, back in the outer solar system, Uranus refused to follow the orbit calculated for it. After several decades, astronomers began to consider, among alternatives, the possibility that a massive planet even more distant than Uranus might be perturbing it from its course…