Planetary Bodies

A group of astronomers have decided to tackle the thorny issue of planetary definitions, a task that has grown more controversial in recent years. They have proposed a hard and fast definition for planets with three components: 1)Planets must be 500 miles in diamater or larger. 2)Planets must orbit a sun. 3)Planets must be circular with self-gravity. This last qualification is the toughest, surprisingly enough, disqualifying earth's moon (whose shared center of gravity with the earth is outside of the moon).

If this definition of planets is accepted, it would mean that there would be 12 currently recognized planets, with the possibility of many more in the future. Besides the eight planets recognized without controversy (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and my favorite, albeit controversial planet (Pluto), there would be three more planets: Ceres (the largest member of the astroid belt), Charon (Pluto's moon, which would be promoted to the solar system's first "double planet"), and Xena (a recently discovered Kuiper belt object--like Pluto and Charon--slightly larger than Pluto). Of course, any object in the solar system that met the three defintions above would also have to be added to this list.

This would mean that other definitions would have to change. Astroids and comets, for example, could no longer be called "minor planets" or "planetoids" as they currently are. As for whatever heavenly bodies met the definition of planet, they would fall into one of three categories. The first is the inner planets (Mercury, Venus, Earth, Mars, and Ceres (?)) which are rocky planets of small to moderate size that have a regular orbit and are kept warm to hot by the sun. The second group of planets would be made up of the gas giants (Jupiter, Saturn, Uranus, and Neptune) that are all of large size, with regular orbits, and have a large number of moons. Any of these moons that had self-gravity would also be planets as well, though this does not appear to be the case at all at the current time. Finally, the third group of planets in the solar system would be called "plutons," which are small, cold, distant, dark, and have eccentric orbits. Is it any wonder I like this type of planet so much? Included in this type of planet would be Pluto, Charon, Xena (or whatever they rename it) and any other large spherical Kuiper Belt Objects.


I don't know about you, but I have to salute the 17 year old blogger who mobilized a "save Pluto" campaign and thus influenced astronomers to come up with a consistent definition of a planet, even if it means that textbooks will have to be edited accordingly. At least my beloved Pluto remains a planet, with some new company to boot.