planet: Classification of the Sun's 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, gas giant, or ice giant. 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 gas giants—Jupiter and Saturn—are much larger in size and have thick, gaseous atmospheres consisting mostly of hydrogen and helium and low densities. The ice giants—Uranus and Saturn—are not as large as the gas giants, have atmospheres that are not as thick, and consist mostly of elements that are heavier than helium and exist in the form of compounds such as water, ammonia, and methane that have freezing points near or above 100°K; they also have lower densities than the terrestial planets. The periods of rotation for the giant planets 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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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