A Planet by Definition

The International Astronomical Union is pondering a big question. What, exactly, is the definition of a planet? The answer could change how we look at our solar system:

The definition, which basically says round objects orbiting stars will be called planets, is simple at first glance:

"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."

"Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor," said Richard Binzel, an MIT planetary scientist who was part of a seven-member IAU committee that hashed out the proposal. "Nature decides whether or not an object is a planet."