Poor Pluto has suffered a couple of weeks of highs and lows, but this morning astronomers at the 2006 General Assembly of the International Astronomical Union defrocked Pluto, removing it from our list of planets in the Solar System.
Some astronomers are happy, some are mad, and some are ambivalent. But I promise that if you call Pluto a "planet," I will not call the IAU Police and have you arrested.
The passed definitions of planets are (copied shamelessly from the IAU):
- A "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) shape, and (c) has cleared the neighborhood around its orbit.
- 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) shape , (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
- All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
- The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
- An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
- "Small Solar-System Bodies" currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
Note one glaring omission -- this definition of the word "planet" means that planets orbit the sun. So what are the objects the size of Jupiter and Saturn circling other stars? I think those are planets, but why they are excluded from this definition, I don't know.