Pluto: Discovered 1930, Reclassified 2006

Pluto was found on 18 February 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona, during a search for a further body thought to be perturbing Uranus and Neptune; the perturbations turned out to be errors in the mass of Neptune, so the discovery was in a real sense accidental. The name was suggested by Venetia Burney, an eleven-year-old in Oxford. In August 2006 the International Astronomical Union adopted Resolution B5, which defined a planet as a body that orbits the Sun, has sufficient mass for its self-gravity to make it nearly round, and has cleared the neighbourhood of its orbit; Pluto meets the first two conditions and not the third, and was reclassified as a dwarf planet. The decision followed the discovery of Eris and other large trans-Neptunian objects, which made it clear that Pluto is one member of a populous Kuiper belt rather than a ninth planet of the old kind.

Three further facts fill out why Pluto sits awkwardly among the planets. Its orbit is inclined about 17 degrees to the ecliptic, far more than any planet's, and eccentric enough that it runs from 29.7 astronomical units at perihelion to 49.3 at aphelion, so that between 1979 and 1999 it was inside Neptune's orbit. The two never approach, because they are locked in a 3:2 resonance: Pluto completes two orbits for Neptune's three. And Pluto's largest moon, Charon, is more than half its diameter, so the pair orbit a common centre of mass that lies outside Pluto's surface, a configuration no planet and moon share.