DOYLC

In this post we continue to learn lessons from the International Astronomical Union's definition of "planet" in 2006 (http://www.iau.org/public_press/news/detail/iau0603/).  The question tackled here is whether the IAU's definition of "planet" is a quality definition.  After close examination, it seems it is not. Here is the definition: 'A "planet" [1] 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. [1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.'   And here is the analysis of the definition: (a) The definition is not actually of "planet" but of "planets in our Solar System".  This can be mined out of the text of Resolution 5A, which states:   "RESOLUTION 5A The IAU t...