Evolution of Definitions – The Problem of Pluto

In early 2006 I had the privilege of seeing NASA’s New Horizon’s mission blast off on its way to Pluto.  At that time, Pluto was a planet.  By August of the same year it was not.

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? 

Definitions can change, and should as we get to know reality better.  Yet the evolution of definitions seems to be little dealt with in the traditional literature on definitions.  It would be nice to have some rules about it, and some thought about how it should be done.

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.