The Planets

The relatively large natural bodies that revolve in orbits around the sun, and presumably around other stars as well, are called planets.

The term does not include such smaller bodies as comets, meteors, and asteroids, many of which are little more than pieces of ice or rock. (See also Comet; Meteor and Meteorite; Asteroid. ) The sun, the nine planets, their satellites, and all the smaller bodies, particles, and dust that circle the sun form the solar system.

The sun, near the center of the solar system, governs the planets orbital motions by gravitational attraction and provides the planets with light and heat. In order of increasing mean distance from the sun, the nine planets of the solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. (See also Gravitation; Solar System; Sun. ) Mercury, Venus, Mars, Jupiter, and Saturn can be seen without a telescope. The ancient Greeks called them planetes, or wanderers, because the objects appeared to move across the background of the apparently fixed stars. Although Uranus is also sometimes visible without a telescope, ancient astronomers were unable to distinguish it from the true stars. The planets may be divided into groups in several ways. In one scheme Mercury and Venus, the planets that revolve around the sun in orbits smaller in diameter than that of the Earth, are classified as inferior planets.

The so-called superior planets are those that revolve around the sun in orbits larger in diameter than the Earths orbit. The planets may also be classified into two groups according to their gross physical characteristics. The terrestrial, or Earth-like, planets are close to the sun and are composed primarily of rock and metal. They include Mercury, Venus, Earth, and Mars. The terrestrial planets are also called the inner planets.

The Jovian, or Jupiter-like, planets are very large compared to the terrestrial planets and are much farther from the sun. They are also called the outer planets.

They include Jupiter, Saturn, Uranus, and Neptune. These planets are composed mostly of hydrogen and helium in gaseous and liquid form. Pluto, the outermost planet, is usually considered neither a terrestrial nor a Jovian planet. It is composed of ice and rock and is much smaller than the other planets.

Although the origin of the solar system is uncertain, most scientists believe that it began to develop about 4 1/2 billion years ago from a large cloud of gas and dust. The cloud began to contract. As the material within the cloud became compressed, it grew hot. Most of this mass was drawn toward the center of the cloud, eventually forming the sun. The remaining material, less than 1 percent of the original, formed a spinning disk, called the solar nebula, around the center. The planets and satellites evolved from the nebula as it cooled. (See also Solar System, Past and Future of the Solar System.

) Close to the center, the material in the disk condensed into ...