DOYLC

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....