The story of Pluto's discovery begins with Percival Lowell, the founder of Lowell Observatory in Flagstaff, Arizona. Lowell was obsessed with the notion of a "trans-Neptunian" planet, which he believed could be detected from the effect it would have on Neptune's orbit. After all, the planet Neptune had been discovered in 1846 by examining irregularities in the orbit of Uranus. Astronomers reasoned that the mystery planet's apparent gravitational influence on Uranus and Neptune could be used to calculate where in the sky it should be found.
Lowell was one of several people (William H. Pickering was another) who hunted for Planet X by computing orbits and carefully searching the sky where they concluded the new planet ought to be. Lowell founded an observatory and funded three separate searches for Planet X. He died in 1916 without discovering it, but the search continued at the observatory. In 1929, a special camera-equipped telescope with a 13-inch objective lens was built specifically for this search.
Observatory director Vesto Slipher hired a young man from Kansas to conduct the third search — a move that led to Clyde Tombaugh becoming the first American to discover a planet. Amateur astronomer Tombaugh was hired to expose photographic plates with this new camera by night, and to carefully compare the plates by day using an instrument called a blink comparator. On February 18, 1930, Tombaugh finally found what he was looking for: a tiny spot of light moving slowly against the fixed pattern of stars in the constellation Gemini.
Discover Pluto for yourself using an interactive Blink Comparator here! (Requires Adobe Flash)
Tombaugh set about to search the ecliptic plane for a new planet. As it turns out, Lowell's calculations were based on flawed data about the perturbations of Uranus' orbit. Despite that, one of the two locations predicted by Lowell's calculation (the favored one, in fact) happened to be right where Pluto was found. Tombaugh was fortunate to find Pluto after only searching for a few months. According to Tombaugh and Moore's 1980 book, "Out of the Darkness: The Planet Pluto," he took pictures in pairs, a few days apart, and looked for anything that moved. Any planets would appear to shift against the backdrop of stars because Earth had moved to a new viewing angle over the intervening days. The discovery plates were taken only six days apart, on January 23 and 29, 1930. After Pluto's discovery, Tombaugh began a laborious search of the entire ecliptic and turned up no additional objects in the outer solar system.
Planet X was subsequently christened Pluto in 1930, a name suggested by Venetia Burney, an 11-year-old girl in Oxford, England. This name was favored by the astronomers of Lowell Observatory because its first two letters were the initials of Percival Lowell. In hindsight, the discovery had nothing to do with Lowell's calculations based on perceived perturbations to the orbits of Uranus and Neptune. We now know that those perturbations were not real, and that Pluto's mass is much too small to have produced such perturbations in any case. The discovery owes more to the remarkable persistence and diligence of Clyde Tombaugh in his careful search of the sky.
The New Horizons Science Operations Center is named for Clyde Tombaugh. Read the story here.
In February 1930, American astronomer Clyde Tombaugh discovered our solar system's ninth planet—Pluto. Yet not even Tombaugh and his mentors could have forecast how fascinating their new planet would turn out to be. In a 2005 article, New Horizons Principal Invesigator Alan Stern describes this amazing space discovery.
Read more here.