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 considered planets. On September 13, 2006, the IAU placed Eris, its moon Dysnomia, and Pluto into their Minor Planet Catalogue, giving them the official minor planet designations (134340) Pluto, (136199) Eris, and (136199) Eris I Dysnomia. Other possible dwarf planets, such as 2003 EL, 2005 FY, Sedna and Quaoar, were left in temporary limbo until a formal decision could be reached regarding their status. On June 11, 2008, the IAU executive committee announced the establishment of a subclass of dwarf planets comprising the aforementioned "new category of trans-Neptunian objects" to which Pluto is a prototype. This new class of objects,