When is a planet not a planet? When it's a dwarf planet.

I have already made a few posts about definitions, discussing persuasive definitions and why properly defining terms is important. Today I want to extend this discussion by looking at Theoretical Definitions. A Theoretical Definition is a definition that seeks to create a theoretically or scientifically useful description of the objects to which the term applies. In other words, it is an attempt to formulate a definition that serves some theoretical or scientific function or has some theoretical or scientific use. Particularly in the realm of science, a dictionary definition just isn't good enough and our scientific theories require more precision and clarity.

As a case in point, we can look at the redefinition of the term "planet" by the International Astronomical Union (IAU) in 2006. Prior to this date, there was no formal definition of the word "planet" and people generally understood that our solar system had nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus, Pluto). However, there was a problem here in that as astronomical skills and observations improved, we found ourselves discovering more and more objects that would also seem to qualify as planets. In particular, astronomers discovered a number of objects in our solar system that were larger than Pluto (such as Eris). If Pluto was a planet, then these other large bodies would also need to be called planets. In addition, it seemed likely that more such bodies would be discovered and that the number of planets would have to be increased significantly.

This all came to a head at the 2006 meeting of the IAU when the group met and debated various potential definitions (as well as the option of doing nothing). Ultimately, the decision was made to change the definition of planet to the following:

RESOLUTION 5A
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" 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".

As a result of this redefinition, Pluto was reclassified as a "dwarf planet." Again, this was all done in the interests of making the jobs of astronomers easier, and providing them a set of terminology that is more precise and more useful.