The International Astronomical Union (IAU) has made its decision on the definition of a planet. Resolution 5A: Definition of 'planet' states:
The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A "planet" 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 neighbourhood around its orbit.
(2) A "dwarf planet" 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 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
Thus, Pluto is no longer considered a planet. Similarly, Charon, Ceres and 2003 UB 313 (which is not called "Xena" by the IAU) are not classified as planets. The proposal to do so was rejected.
I note that they begged the definition of "satellite", but that's fine with me. Categorization can easily be taken too far.
The eight planets in our solar system are now Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
I am pleased to see that the IAU made this decision carefully. Although my suggestion for a resolution was never considered, the end result is similar. We are safe from the possibility of discovering hundreds of new "planets" in our system.
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