Pluto before Charon was discovered. Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in the asteroid belt. It is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005, though Pluto has a larger diameter of 2376.6 km compared to Eris's approximate diameter of 2326 km. Determinations of Pluto's size had been complicated by its atmosphere, and hydrocarbon haze. In March 2014, Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with a Plutonian diameter greater than 2360 km,
IAU definition of planet IAU definition of planet The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which: Among other things, this definition caused Pluto to no longer be a planet, a change from how it had been widely considered until that point. A non-satellite body fulfilling only the first two of these criteria (such as Pluto) is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling only the first criterion is termed a "small Solar System body" (SSSB).