solar system: The Planets

The Planets

In order of increasing average distance from the sun, the planets are Mercury, Venus, earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The planets orbiting nearer the sun than the earth are termed inferior planets; those whose orbits are larger are called superior planets. The unit for measuring distance in the solar system is the astronomical unit (AU), the average distance between the earth and the sun. The mean distances of the planets from the sun range from 0.39 AU for Mercury to 30.04 AU for Nepture.

Pluto, regarded for many years after its discovery as a planet, was reclassified in 2006 as a dwarf planet, which is a planetlike celestial body that does not clear or dominate the region of its orbit. In addition, Pluto is unlike the terrestrial planets—Mercury, Venus, Earth, and Mars—which are rocky, and it is unlike the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). Its orbit, which is tilted from the plane in which the eight planets travel about the Sun, its size, and its composition more closely resemble those of the objects residing in the Kuiper belt (which were first discovered in 1992; see under comet) than those of a major planet, and Pluto is now recognized as a Kuiper belt, or transneptunian, object.

See the table entitled Major Planets of the Solar System.

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