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
Definition of planet Pluto, such as Quaoar and Sedna, continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On July 29, 2005, Mike Brown and his team announced the discovery of a trans-Neptunian object confirmed to be more massive than Pluto, named Eris. In the immediate aftermath of the object's discovery, there was much discussion as to whether it could be termed a "tenth planet". NASA even put out a press release describing it as such. However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum