But for those generally unfamiliar with Solar System Astronomy, the recent IAU Resolutions were succinct and punctilious, as most astronomers themselves, are, and may be helpful...
RESOLUTION 5ANow as to the WHY of this, I am persuaded by the outstanding facts:
The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:
(1) 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.
(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 except satellites 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.
IAU Resolution: Pluto
The IAU further resolves:
Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
- All the other planets in the outer solar system are gaseous, giant planets whereas Pluto is a small solid object
- Pluto is smaller than any other planet by more than a factor of 2.
- Pluto's orbit is by far the most inclined with respect to the plane of the solar system, and also the most eccentric, with only the eccentricity of Mercury's orbit even coming close
- Pluto's orbit is the only planetary orbit which crosses that of another planet (during 1999 Pluto will again cross Neptune's orbit, thus regaining its status as the most distant planet)
- Pluto's satellite, Charon, is larger in proportion to its planet than any other satellite in the solar system.
Hayden Planetarium Director Neil de Grasse Tyson who says at SPACE.COM
"I argued strongly that Pluto, being half ice by volume, should assume its rightful status as the King of the Kuiper Belt of comets. "It may not be so bad for Pluto, which for years was rumored to have wondered in from the Kuiper Belt of comets anyway, or was a moon of Neptune that escaped! I prefer the former theory, which at least makes Pluto the greatest of the Ice Planetoids, the King of the Comets.