PLANET

(Greek planetes, wanderers), any body (except a comet, meteoroid, or satellite) revolving in an orbit around the Sun or around some other star. (See also individual articles on the solar system's major planets, listed below.) 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 two main groups is a belt of numerous, very small bodies called asteroids, or sometimes the minor planets. See also solar system. 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. The planets of the solar system are thought to have formed when an interstellar cloud of gas and dust collapsed under its own gravitational attraction and formed a disk-shaped nebula. Further compression of the disk's central region formed the Sun, while the gas and dust left behind in the midplane of the surrounding disk eventually coalesced to form the planets. This process of planetary formation may well have accompanied the birth of other stars besides the Sun, and astronomers have long wondered whether other stars have planets circling around them. Such planets are virtually impossible to detect with Earth-based telescopes, however, because they are too small and dark to be discerned in the glare of the stars around which they orbit. Moreover, as seen from Earth, a planet and its parent star are too close together for a telescope to optically resolve, or separate, their respective images. Planets, if they exist, can only be observed indirectly, by noting their gravitational effects on the observed motion of their parent stars. An orbiting planet can be detected by the small periodic wobbles it produces in the parent star's position in space, or by deviations in the star's velocity as viewed from Earth. Using these cues, astronomers by 1995 had identified three planets circling a pulsar (i.e., a rapidly spinning neutron star) called PSR B1257 + 12 in the constellation Virgo. The first discovery of a planet revolving around a star like the Sun took place that same year, when the existence of a massive planet in orbit around the star 51 Pegasi was announced. By the end of 1996, astronomers had identified several more planets in orbit around other stars. In 199798 the Hubble Space Telescope provided what appear to be the first optical images of a planet outside the solar system.