planet [Gr.,=wanderer], a large nonluminous ball of rock or gas that orbits a star. The term, once limited to any of the eight solid, nonluminous bodies (major planets) that revolve around the sun, has been extended to include similar bodies discovered revolving around other stars. The term is sometimes used to include the asteroids asteroid, planetoid, or minor planet, small body orbiting the sun. More than 10,000 asteroids have orbits sufficiently well known to have been cataloged and named; thousands more exist.
..... Click the link for more information. (or minor planets) but excludes the other members of the solar system solar system, the sun and the surrounding planets, natural satellites , dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass.
..... Click the link for more information. : comets comet [Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun. Comets visible from the earth can be seen for periods ranging from a few days to several months.
..... Click the link for more information. and meteoroids (see meteor meteor, appearance of a small particle flying through space that interacts with the earth's upper atmosphere. While still outside the atmosphere, the particle is known as a meteoroid. Countless meteoroids of varying sizes are moving about the solar system at any time.
..... Click the link for more information. ; see also planetary science planetary science or planetology, study of planets and planetary systems as a whole. Planetary science applies the theories and methods of traditional disciplines such as astronomy, geology, physics, chemistry, and mathematics to the study of
..... Click the link for more information. and planetary system planetary system, a star and all the celestial bodies bound to it by gravity, especially planets and their natural satellites . Until the last decade of the 20th cent.
..... Click the link for more information. , as well as the table entitled Major Planets of the Solar System Major Planets of the Solar System
Planet Distance from the sun

(AU)

Period of revolution Period of rotation Mass

(earth=1)

Diameter

(earth=1)

..... Click the link for more information. ).

Classification of the Major Planets

The major planets are classified either as inferior, with an orbit between the sun and the orbit of Earth (Mercury Mercury, in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible
..... Click the link for more information. and Venus Venus, in astronomy, 2d planet from the sun; it is often called the evening star or morning star and is brighter than any object in the sky except the sun and the moon.
..... Click the link for more information. ), or as superior, with an orbit beyond that of Earth (Mars Mars, in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth.

Physical Characteristics

Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice
..... Click the link for more information. , Jupiter Amalthea (diameter: 117 mi/189 km), a small, elongated satellite discovered (1892) by Edward Barnard , probably results from a coating of sulfur particles ejected from Io.

Metis (diameter: 25 mi/40 km),

Adrastea (diameter: 12 mi/20 km), and

..... Click the link for more information. , Saturn Saturn, in astronomy, 6th planet from the sun.

Astronomical and Physical Characteristics of Saturn

Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.
..... Click the link for more information. , and Uranus Uranus (y

rā`nəs, y
..... Click the link for more information. , Neptune Neptune, in astronomy, 8th planet from the sun at a mean distance of about 2.8 billion mi (4.5 billion km) with an orbit lying between those of Uranus and the dwarf planet Pluto; its period of revolution is about 165 years.
..... Click the link for more information. . Pluto Pluto, in astronomy, a dwarf planet and the first Kuiper belt, or transneptunian, object (see comet ) to be discovered (1930) by astronomers. Pluto has an elliptical orbit usually lying beyond that of Neptune .
..... Click the link for more information. , 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.

On the basis of their physical properties the planets are further classified as terrestrial or Jovian. 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 Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are much larger in size and have thick, gaseous atmospheres and low densities. Their periods of rotation 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.

Recognition of the Planets

Identification of the Solar Planets

The ancient Greeks applied the term planet to the five major planets then known—Mercury, Venus, Mars, Jupiter, and Saturn—as well as to the sun and moon; all these bodies were observed to move back and forth against the background of the apparently fixed stars and to shine with a steady light. In the Ptolemaic system Ptolemaic system (tŏl'əmā`ĭk), historically the most influential of the geocentric cosmological theories, i.e.
..... Click the link for more information. the earth was thought to lie at rest in the center of the universe while the planets moved about it in a complicated scheme of circles. The heliocentric, or sun-centered, Copernican system Copernican system, first modern European theory of planetary motion that was heliocentric, i.e., that placed the sun motionless at the center of the solar system with all the planets, including the earth, revolving around it.
..... Click the link for more information. , introduced in the 16th cent., viewed the planets, including the earth, as revolving about the sun; the moon was viewed as a natural satellite satellite, natural, celestial body orbiting a planet, dwarf planet, asteroid, or star of a larger size. The most familiar natural satellite is the earth's moon ; thus, satellites of other planets are often referred to as moons.
..... Click the link for more information. of the earth. At the start of the 17th cent. Johannes Kepler Kepler, Johannes (yōhä`nəs kĕp`lər), 1571–1630, German astronomer. From his student days at the Univ.
..... Click the link for more information. refined the Copernican model by showing that the orbits of the planets around the sun were elliptical rather than circular.

With the development of the telescope other planets became visible. Uranus, detected in 1781 by Sir William Herschel, was the first planet discovered in modern times. Neptune was discovered in 1846 as the result of a mathematical analysis of the irregularities in the motion of Uranus, and the dwarf planet Pluto, whose existence was predicted from the perturbations of both Uranus and Neptune, was found in 1930. In addition to the major planets, the telescope has revealed thousands of minor planets, or asteroids, which orbit the sun in a bandlike cluster between Mars and Jupiter; the largest of these, the dwarf planet Ceres Ceres (sîr`ēz), in astronomy, a dwarf planet, the first asteroid to be discovered. It was found on Jan. 1, 1801, by G. Piazzi .
..... Click the link for more information. , was also the first discovered (1801), and was regarded as a planet for many years. Additional minor planets have been discovered since 1992 beyond the orbit of Neptune in the Kuiper belt; at least one of these transneptunian objects, Eris, has a diameter (1,500 mi/2,400 km) slightly larger than that of Pluto.

Discovery of the Extrasolar Planets

Although speculation concerning the existence of extrasolar planets and planetary systems dates back to antiquity, it was not until the last decade of the 20th cent. that astronomical tools and techniques made their detection possible. Because stars are so distant and bright and an extrasolar planet, no matter how large, is relatively small and dim, it cannot be seen or photographed directly. Its presence is usually inferred from a periodic wobble in the spectrum of a target star's frequencies. This wobble, produced by gravitational influences, causes tiny shifts in the star's frequencies that are caught by telescopes and analyzed to yield information on the body affecting the star. Another technique that proved fruitful in 1999 is the use of a telescope to record the dimming of light from a star when a planet's orbit carries it between the star and the earth.

Spurred on by the discovery of three bodies orbiting a pulsar by radio astronomers in 1992, the first extrasolar planet orbiting a sunlike star was detected in 1995. Located in the constellation Pegasus Pegasus (pĕg`əsəs), in astronomy, northern constellation lying SW of Andromeda and SE of Cygnus.
..... Click the link for more information. , about 40 light-years from earth, the planet—called 51 Pegasi—has about half the mass of Jupiter and is so close to the star that it has a surface temperature of about 1,000°C; and completes its orbit in only four days. By the end of the decade, more than two dozen extrasolar planets were detected, including three orbiting the star Upsilon Andromedae—the first multiplanet extrasolar planetary system—that were discovered in 1999. Within six years the number of known extrasolar planets had surpassed 150, a number approached by the known planetary systems. The most earthlike of the extrasolar planets discovered to date orbits the star Gliese 876 about 15 light-years from earth. It is about 7.5 times as massive as earth, with a radius twice as large and a circular orbit slightly less than 2 million miles from its star, and is believed to be composed of nickel-iron rock. Like the other two known planets circling Gliese 876, most of the known extrasolar planets are giant gas planets with masses ranging from one half to five times that of Jupiter, the largest of the solar planets. Many have orbits that are highly elliptical rather than only slightly so, are closer to their star than the earth is to the sun, and have orbital periods ranging from three days to more than four years. In addition, the ages of the extrasolar planets differ from one another and from that of the solar planets; the oldest planet, discovered in the globular cluster M4 in 2003, is believed to have been formed 12.7 billion years ago, within a billion years of the origin of the universe and 8 billion years before the earth. Because these data are so different from that of the solar planets, planetary scientists are rethinking the accepted theories of planetary formation.

Bibliography

See P. Halpern, The Quest for Alien Planets: Exploring Worlds Outside the Solar System (1997); J. R. Gribbin and S. Goodwin, Empire of the Sun: Planets and Moons of the Solar System (1998).

planet

Any large natural body that orbits the Sun or another star (see planets of other stars) and that is not radiating energy from internal nuclear fusion; dwarf planets, comets, asteroids, meteoroids (see meteor), and natural satellites are excluded. The word planet comes from the Greek for “wanderer,” because the planets' positions change relative to those of the stars. The eight (formerly nine) recognized planets that orbit the Sun are, in order of increasing distance, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first four are called terrestrial planets and the next four giant, or Jovian, planets. The terrestrial planets (of which Earth is the largest) are rocky with comparatively thin or negligible atmospheres. The Sun's heat is thought to have prevented the abundant volatile substances in the solar nebula from condensing in them as they formed. The giant planets formed farther out, where the gases were cool enough to condense, so the planets grew very massive and accumulated huge atmospheres of light gases, mainly hydrogen and helium. None of the giant planets has an accessible surface; their gaseous atmospheres gradually merge with their liquid interiors. Pluto, although it was recognized as a planet for decades, is distinct from either group, being much smaller than any of the eight planets and resembling a giant comet nucleus. In 2006 the International Astronomical Union demoted Pluto to the category of dwarf planet, which reflected astronomers' conclusion that it is a very large member of the Kuiper belt. Additional bodies rivaling or exceeding Pluto in size (e.g., Eris) exist in the outer region of the solar system. The term minor planet is sometimes used to refer to any of the asteroids that lie mostly between the terrestrial and the giant planets. In astrology great importance is placed on the planets' positions in the 12 constellations of the zodiac. See also planetesimal; solar system.

Planet - ["An Experiment in Language Design for Distributed Systems", D. Crookes et al, Soft Prac & Exp 14(10):957-971 (Oct 1984)].