Moons of Pluto while searching for Plutonian rings. Styx, announced on 7 July 2012, was discovered while looking for potential hazards for "New Horizons". Charon is about half the diameter of Pluto and is so massive (nearly one eighth of the mass of Pluto) that the system's barycenter lies between them, approximately 960 km above Pluto's surface. Charon and Pluto are also tidally locked, so that they always present the same face toward each other. The IAU General Assembly in August 2006 considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned. Pluto's four small
Ceres (dwarf planet) major planets was widely accepted, though a precise definition of "planet" was never formulated. The 2006 debate surrounding Pluto and what constitutes a planet led to Ceres being considered for reclassification as a planet. A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet". Had this resolution