The IAU has published a press release giving details of the proposed definition of what is and isn't a planet. The outline is:
1. 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.At first glance it looks like a reasonable attempt at solving the (non-?)problem but I can't help but think it's got some problems of its own.
2. We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet."
3. We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons".
4. All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
Rather than go into details I'd suggest a read of the Bad Astronomer's take on it as he seems to cover all of the concerns I had when I first read it.
The other thing I'm thinking is: isn't it just a little overly concerned with the solar system? Yes, yes, I know, it's the only system we've got to hand that we can study in any real detail but wouldn't it make sense, if you're going to come up with a definition, to have one that we can use everywhere?
File Under: IAU, Pluto, Definition of a Planet.