The DiscoveryIt was American astronomer Percival Lowell who first thought there might be another planet somewhere near Neptune and Uranus. Lowell had noticed that the gravitational pull of something large was affecting the orbits of those two planets.
However, despite looking for what he called "Planet X" from 1905 until his death in 1916, Lowell never found it.
Thirteen years later, the Lowell Observatory (founded in 1984 by Percival Lowell) decided to recommence Lowell's search for Planet X. They had a more powerful, 13-inch telescope built for this sole purpose. The Observatory then hired 23-year-old Clyde W. Tombaugh to use Lowell's predictions and the new telescope to search the skies for a new planet.
It took a year of detailed, painstaking work, but Tombaugh did find Planet X. The discovery occurred on February 18, 1930 while Tombaugh was carefully examining a set of photographic plates created by the telescope.
Despite Planet X being discovered on February 18, 1930, the Lowell Observatory was not quite ready to announce this huge discovery until more research could be done.
After a few weeks, it was confirmed that Tombaugh's discovery was indeed a new planet. On what would have been Percival Lowell's 75th birthday, March 13, 1930, the Observatory publicly announced to the world that a new planet had been discovered.
Pluto the PlanetOnce discovered, Planet X needed a name. Everyone had an opinion. However, the name Pluto was chosen on March 24, 1930 after 11-year-old Venetia Burney in Oxford, England suggested the name "Pluto." The name denotes both the assumed unfavorable surface conditions (as Pluto was the Roman god of the underworld) and also honors Percival Lowell, as Lowell's initials make up the first two letters of the planet's name.
At the time of its discovery, Pluto was considered to be the ninth planet in the solar system. Pluto was also the smallest planet, being less than half the size of Mercury and two-thirds the size of earth's moon.
Usually, Pluto is the planet farthest from the sun. This great distance from the sun makes Pluto very inhospitable; it's surface is expected to be made up of mostly ice and rock and it takes Pluto 248 years just to make one orbit around the sun.
Pluto Loses Its Planet StatusAs the decades passed and astronomers learned more about Pluto, many questioned whether Pluto could really be considered a full-fledged planet.
Pluto's status was questioned in part because it was by far the smallest of the planets. Plus, Pluto's moon (Charon, discovered in 1978) is incredibly large in comparison. Pluto's eccentric orbit also concerned astronomers; Pluto was the only planet whose orbit actually crossed that of another planet (sometimes Pluto crosses Neptune's orbit).
When bigger and better telescopes began to discover other large bodies beyond Neptune in the 1990s, and especially when another large body was discovered in 2003 that rivaled the size of Pluto, Pluto's planet status became seriously questioned.
In 2006, the International Astronomical Union (IAU) officially created a definition of what makes a planet; Pluto did not meet all the criteria. Pluto was then downgraded from a "planet" to a "dwarf planet."