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In order of increasing average distance from the sun, the planets are Mercury, Venus, earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. 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 39 AU for Pluto.
Many astronomers maintain that Pluto is not a planet at all. It is unlike the terrestrial planets : Mercury, Venus, Earth, and Mars : which are rocky, and it is unlike the gas giants : Jupiter, Saturn, Uranus, and Neptune. Its orbit, which is tilted from the plane in which the other eight planets travel about the Sun, its size, and its composition more closely resemble that of a comet residing in the Kuiper belt than a planet in the solar system.
See the table entitled Major Planets of the Solar System.
Planetary Motion
The motion of the planets was first described accurately by Johannes Kepler at the beginning of the 17th cent.; he showed that the planets move in nearly circular elliptical orbits. Isaac Newton later showed that the laws of planetary motion discovered by Kepler apply also to all other bodies in the solar system and are based on the force of gravitation. The sun's gravitational pull is the dominant force in the solar system; the forces exerted by the other celestial bodies on one another produce small shifts and variations, called perturbations, in their orbits. With the exception of Pluto, the planets orbit the sun in approximately the same plane (that of the ecliptic) and move in the same direction : counterclockwise as viewed from above the earth's North Pole. A planet's year, or sidereal period, is the time required for it to complete one full circuit around the sun. Mercury's year is 88 earth days, while Pluto's year is 248 earth years. All the planets rotate about their own axes as they revolve around the sun; their periods of rotation vary from just under 10 earth hours for Jupiter to 243 earth days for Venus. The rotation of Venus is from east to west (see retrograde motion). The equatorial planes of the planets are tilted to various degrees with respect to their orbital planes, giving rise to yearly seasons. The smallest tilt, that of Jupiter, is 3°, whereas that of Uranus is 98°, causing its axis of rotation to lie nearly in the plane of ilt of the earth's equatorial plane is 23 1/2 °.
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