Supposedly, there’s going to be a new planet definition proposal made the IAU today. Everyone thinks “Pluto is a planet. It just is, so whatever we propose needs to keep it a planet”. So the new proposal also adds three new planets to the solar system: Xena (that’s 2003 UB313 to you), Ceres (the first discovered and largest known asteroid-belt object), and even Pluto’s moon Charon. Under the proposed definition, “A planet is 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.” What’s that you ask? Why does that make Charon a planet? Well, it’s because the barycenter for its orbit lies above the surface of Pluto. It would now be considered a double-planet system.
The rule also introduces the likelihood of even more planets (like Quaoar and Orcus) once we have a better idea of their size and mass. I definitely like some of the new terminology: a “Pluton” is any planet beyond Neptune. We need to start working on a new mnemonic for MVEMCJSUNPCX. Go.
Update 10:15: The IAU press release