(Greek planetes, "wanderers"), any body (except a comet, meteoroid, or satellite) revolving in an orbit around the Sun or around some other star. The nine major planets known to revolve around the Sun are (in order of increasing distance from it): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The inner four are sometimes called the terrestrial planets; and the term giant planets denotes those planets from Jupiter to Neptune. Between these groups is a belt of numerous, very small bodies called asteroids, or sometimes the minor planets.
In primitive astronomy the term planet was applied to the seven celestial bodies that were observed to move appreciably against the background of the apparently fixed stars. These included the Sun and Moon, as well as the five true planets (Mercury, Venus, Mars, Jupiter, and Saturn) readily visible as celestial wanderers before the invention of the telescope. The names of these seven bodies are still connected, in some languages, with the days of the week.
In astrology great importance is placed on the positions of the various planets in the 12 constellations of the zodiac, the belt around the sky in which the movements of Sun, Moon, and planets are confined.
Deviations in the proper motions--motions relative to the background of apparently fixed, distant stars--of several nearby stars indicate that they may be accompanied by dark, planetlike bodies too faint to be seen directly from the Earth.
Excerpt from the Encyclopedia Britannica without permission.