On August 24, 2006, the International Astronomical Union (IAU) publicly defined a planet as "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.".
This raises questions such as:
- Can definitions change? Pluto had been called a planet until the IAU changed the boundaries of the definition.
- What authority has the IAU to define a planet?
- I still think of Pluto as a planet – am I wrong to do so?
- The IAU’s new definition seems a bit contrived. Will it stand up?
- What motivation did the IAU have to change the definition?
As to the IAU, it is quite free to come up with a definition for any term – just as the Red Queen did in Alice in Wonderland. And I too am free to have my own definition of a planet. Whether the IAU’s definition will stand up is a good question. New research suggests that extra-solar planetary systems are very diverse. The IAU’s definition may very well not stand up in the face of future discoveries – but surely such evolution is part of what science is. As to motivation, that is a discussion for another post – but it often matters for all kinds of reasons.