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
Ceres (dwarf planet) quadrangles. They are named after the first craters whose names the IAU approved in July 2015. The map image(s) were taken by the "Dawn" space probe. Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter, slightly closer to Mars' orbit. With a diameter of , Ceres is the largest of the minor planets, and the only dwarf planet, inside Neptune's orbit. It is the 33rd-largest known body in the Solar System. Ceres is composed of rock and ice and is estimated to comprise