Classification

Classification of the Major Planets

The major planets are classified either as inferior, with an orbit between the sun and the orbit of Earth (Mercury and Venus), or as superior, with an orbit beyond that of Earth (Mars, Jupiter, Saturn, and Uranus, Neptune. Pluto, long regarded after its discovery in 1930 as the ninth planet, was gradually recognized as a Kuiper belt, or transneptunian, object (see comet), and in 2006 was reclassified by astronomers as a dwarf planet. Any dwarf planet beyond the orbit of Neptune is now classified as a plutoid.

On the basis of their physical properties the planets are further classified as terrestrial or Jovian. The terrestrial planets—Mercury, Venus, Earth, and Mars—resemble Earth in size, chemical composition, and density. Their periods of rotation range from about 24 hr for Mars to 249 days for Venus. The Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are much larger in size and have thick, gaseous atmospheres and low densities. Their periods of rotation range from about 10 hr for Jupiter to 15 hr for Neptune. This rapid rotation results in polar flattening of 2% to 10%, giving the planets an elliptical appearance.

An artist's image shows Pluto (center) and its large "moon" Charon (right) from the surface of one of Pluto's two smaller satellites. The other small moon can be seen on the far left.

Pluto is no longer a planet, according to a new definition of the term voted on by astronomers, because it does not dominate the neighborhood around its orbit. Charon is half the size of Pluto and orbits along with it; the eight true planets are all far larger than their moons.

The planets of the modern solar system are grouped into several different and sometimes overlapping classifications, as illustrated in the following figure:

1. The planets inside the orbit of the earth are called the Inferior Planets: Mercury and Venus.
2. The planets outside the orbit of the earth are called the Superior Planets: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
3. The planets inside the asteroid belt are termed the Inner Planets (or the Terrestrial Planets): Mercury, Venus, Earth, and Mars.
4. The planets outside the asteroid belt are termed the Outer Planets: Jupiter, Saturn, Uranus, Neptune, and Pluto.
5. The planets sharing the gaseous structure of Jupiter are termed the Gas Giant (or Jovian) Planets: Jupiter, Saturn, Uranus, and Neptune.

Source:
 http://csep10.phys.utk.edu/astr161/lect/classification/classification.html

  http://www.infoplease.com/ce6/sci/A0860415.html