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
Quasi-satellite known to have accidental quasi-satellites. In the case of Pluto, the known accidental quasi-satellite, 15810 Arawn, is, like Pluto, a plutino, and is forced into this configuration by the gravitational influence of Neptune. This dynamical behavior is recurrent, Arawn becomes a quasi-satellite of Pluto every 2.4 Myr and remains in that configuration for nearly 350,000 years. Quasi-satellite A quasi-satellite is an object in a specific type of co-orbital configuration (1:1 orbital resonance) with a planet where the object stays close to that planet over many orbital periods. A quasi-satellite's orbit around the Sun takes exactly the same time as the