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
Naming of chemical elements planets, dwarf planets, a few asteroids, our star, our planet, and our Moon. Uranium, neptunium, plutonium, cerium, and palladium were named after Uranus, Neptune, Pluto, Ceres, and Pallas respectively, which were at the time of their naming all considered to be planets. (Today, Pluto and Ceres are considered to be dwarf planets, and Pallas is considered to be an asteroid.) The name of the element selenium came from the Greek word for the Moon (Σελήνη, "Selene"). The name helium comes from the Greek word for the Sun (Ἢλιος, "Helios"). This is because the first evidence for helium was in distinctive,