IAU definition of planet who had theorized on the existence of the large trans-Neptunian object Planet X, and Tombaugh been searching for Planet X when he found Pluto. Almost immediately after its discovery, however, astronomers questioned whether Pluto could be Planet X. Willy Ley wrote a column in 1956 titled "The Demotion of Pluto", stating that it "simply failed to live up to the advance publicity it received as 'Planet X' before its discovery. It has been a disappointment all along, for it did not turn out to be what one could reasonably have expected". In 1978, Pluto's moon Charon was discovered. By measuring
Definition of planet IAU General Assembly meeting in Prague in August 2006, and on August 24, the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between "planet" and "rock" (or, in the new parlance, "small Solar System body"), called "dwarf planet" and placed Pluto in it, along with Ceres and Eris. The vote was passed, with 424 astronomers taking part in the ballot. The IAU also resolved that ""planets" and "dwarf planets" are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be