>>12568790IAU Resolution 5A: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets.” The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries led us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that “planets” and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:
1. A “planet”1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
While planets have to be nearly spherical, they usually bulge at the equator, thanks to the fact that planets rotate. The Earth's diameter at the equator is about 12,760 km (7,925 miles), while from pole to pole it is about 12,720 km (7,900 miles). Scientists estimate that for a rocky body, hydrostatic equilibrium will happen for an object that is about 800 km (500 miles) and has a mass about 1/12,000 that of the Earth (Ceres is about 950 km and has a mass about 1/7000 that of Earth).
