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
Eris (dwarf planet) retains an oblique reference to Eris's old informal name "Xena", portrayed on TV by Lucy Lawless. In the 2010s, on the heels of the successful Pluto flyby there were multiple studies for follow-on missions to explore the Kuiper belt, and Eris was evaluated among the candidates. It was calculated that a flyby mission to Eris could take 24.66 years using a Jupiter gravity assist, based on launch dates of 3 April 2032 or 7 April 2044. Eris would be 92.03 or 90.19 AU from the Sun when the spacecraft arrives. Eris (dwarf planet) Eris (minor-planet designation 136199 Eris) is the