How do they classify planets?
By Makiya Sanders
In 2006 we developed and implemented an official classification of the celestial bodies in our solar system. There are 8 planets, a small number of dwarf planets, and a large number of minor planets. The decisive criterion in the definition of a planet is based on dynamics (the science of forces and motions). This is an intelligent decision, and a decision that we can feel good about, for several reasons.
- Planets have always been defined by their dynamics. The word "planets" comes from the Greek word for "wanderer", an object that moves across the background of the fixed stars.
- Moons have always been defined by their dynamics. A moon is an object that moves around (orbits) a planet. There are moons that look round and there are moons that don't look round at all. Their shape is irrelevant to the classification.
- A classification based on dynamics is far easier to implement than a classification based on physical properties. Newly discovered objects can be classified immediately, long before the details of their physical properties are known.
What happened when Pluto was discovered?
In 1930 staff at the Lowell Observatory issued a circular entitled "Discovery of a solar system body apparently trans-neptunian" for distribution to astronomers around the world. The announcement describes a new "object" and makes no claim of a planet discovery. This object later became known as Pluto.
- Pluto has a tiny atmosphere. Several moons of the Jovian planets have an atmosphere, so the presence of an atmosphere is not a distinctive feature of planets.
- Pluto has satellites. Many small asteroids and trans-Neptunian objects have satellites, so the presence of a satellite is not a distinctive feature of planets.
- Pluto is round. Many moons are round, so roundness is not a distinctive feature of planets.