This is Sandi's other half who has been "encouraged" to make a post here after a discussion we had about the demise of Pluto from being the ninth planet.

For many years I've discussed this issue with people and I've always been convinced that Pluto wasn't really a planet. For some reason, with a PhD in physics, people seem to expect me to be an expert on astronomy too.

It has been extensively reported how despite a move to make Pluto and a few other objects achieve the classification of planet, the International Astronomical Union (IAU) General Assembly voted on a series of resolutions about the definition of what is a planet.

From a purely scientific perspective, it doesn't really matter what you classify as a planet, it is really only of direct relevance to books and school science lessons.

The resolutions voted on are listed here.
The resolution (5A) is:
RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet1 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Pluto is classed as a dwarf planet by virtue of it not passing rule 1c as its orbit crosses the orbit of Neptune and therefore it hasn't cleared the neighbourhood around its orbit.

However nobody has applied the logic to Neptune, whose orbit is crossed by Pluto and therefore it too has not cleared the neighbourhood around its orbit.

Logically if object A crosses the orbit of object B then neither A nor B are planets.

This seems to have been a very poorly defined definition of what is a planet.

Now the above issue is not the only problem, there are many other possible orbital arrangements where you could have very large objects which by any definition should be a planet violate the orbital clearing requirement.

To give you an idea of just how complex orbits can be, there is the amazing example of how two moons orbiting Saturn swap their orbits every four year in a complex orbital dance which is described here. For further descriptions, just search on "Saturn Epimetheus Janus" in Google.

The more you understand of orbital mechanics, the less you understand of the rational of the definition selected and approved by the IAU.

Mike