The Discovery Of Pluto
Speculations about the existence of a ninth planet arose soon after astronomers discovered that the planet Neptune (discovered in 1846) did not move in its orbit as predicted. The small differences between Neptune's predicted and actual position were taken as evidence that an unseen object was introducing slight gravitational perturbations in the planet's orbit. The first search for a trans-Neptunian planet appears to have been carried out by David Peck Todd, of the U.S. Naval Observatory, in 1877. Todd conducted a visual search during 30 clear nights between November 1887 and March 1888, but he found nothing that looked like a planet.
The first systematic survey for a trans-Neptunian planet, using photographic plates, was carried out by the American astronomer Percival Lowell, at the Flagstaff Observatory, in Arizona between 1905 and 1907. No new planet was found, however. A second survey was conducted at Flagstaff in 1914, but again, no new planet was discovered. On the basis of predictions made by W. H. Pickering in 1909, Milton Humason, at Mount Wilson Observatory, carried out yet another photographic survey for a trans-Neptunian planet, with negative results, in 1919.
A third photographic survey to look for objects beyond the orbit of Neptune was initiated at Flagstaff Observatory in 1929. Clyde Tombaugh was the young astronomer placed in charge of the program. The survey technique that Tombaugh used entailed the exposure of several photographic plates, of the same region of the sky, on a number of different nights. In this way, an object moving about the Sun will shift its position, with respect to the unmoving, background stars, when two plates of the same region of sky are compared. The object that we now know as the planet Pluto was discovered through its "shift" on two plates taken during the nights of January 23rd and 29th, 1930. The announcement that a new planet had been discovered was delayed until March 13, 1930, to coincide with the one-hundred-and-forty-ninth anniversary of the discovery of Uranus, and to mark the seventy-eighth anniversary of Lowell's birth. Humason, it turns out in retrospect, was unlucky in his survey of 1919, in that a re-examination of his plates revealed that Pluto had, in fact, been recorded twice. Unfortunately for Humason, one image of Pluto fell on a flaw in the photographic plate, and the second image was obscured by a bright star.
After its discovery, it was immediately clear that the Pluto was much smaller and fainter than the theoreticians had suggested it should be. Indeed, a more refined analysis of Neptune's orbit has revealed that no "extra" planetary perturbations are required to explain its orbital motion.
Science EncyclopediaScience & Philosophy: Planck mass to PositPluto - Basic Properties, The Discovery Of Pluto, Pluto's Characteristics, Charon, Pluto's Strange Orbit - Charon's characteristics