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 dwarf planets and asteroids (or minor planets) but excludes the other members of the solar system : comets and meteoroids (see meteor ; see also planetary science and planetary system , as well as the table entitled Major Planets of the Solar System ).

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 and Venus ), or as superior, with an orbit beyond that of Earth ( Mars , Jupiter , Saturn , and Uranus , Neptune . Pluto , 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. Any dwarf planet beyond the orbit of Neptune is now classified as a plutoid.

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 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 , introduced in the 16th cent., viewed the planets, including the earth, as revolving about the sun; the moon was viewed as a natural satellite of the earth. At the start of the 17th cent. Johannes Kepler 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 , 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 (or exoplanets) 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 , 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. A decade later the number of known exoplanets had surpassed 400, and it was estimated that 40% of sunlike stars had planetary systems . The launches of the CoRoT (2006) and Kepler (2009) space telescopes, especially the latter, has significantly increased the number of known possible exoplanets. Kepler had by early 2011 identified more than 50 near-earth-sized planets that were located in the habitable zone, and the discovery of an earth-sized rocky planet (not, however, located in the habitable zone) was announced late in 2011.

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 exoplanets 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).

318. Planets

See also 24. ASTROLOGY ; 25. ASTRONOMY ; 100. COSMOLOGY ; 133. EARTH ; 259. MARS ; 280. MOON .

aphelion

the point in the orbit of a heavenly body where it is farthest from the sun. See also perihelion .

apocynthion

apolune.

apogee

the farthest point in an orbit from the body being orbited.

apolune

the farthest point from the moon in a lunar orbit, as that of a spacecraft. Also called apocynthion .

areography

the study of the physical features of the planet Mars.

celidography

Archaic. a description of the surface markings of the sun or a planet. —celidographer, n.

exobiology

the branch of biology that studies life beyond the earth’s atmosphere, as on other planets. —exobiologist, n.

meridian

the highest point a planet or other orbiting heavenly body reaches in its orbit. —meridian, meridional, adj.

occultation

the process of one heavenly body disappearing behind another as viewed by an observer.

pericynthion

perilune.

perigee

the closest point in an orbit to the body being orbited.

perihelion

the point in the orbit of a heavenly body where it is nearest the sun. Also called perihelium . See also aphelion .

perihelium

perihelion.

perilune

the closest point to the moon in a lunar orbit, as that of a spacecraft. Also called pericynthion .

planetarium

1. a representation of the planetary system, particularly one in which the movements of the planets are simulated by projectors.

2. a room or building housing such an apparatus.

planetoid

Astronomy. any of thousands of small celestial bodies that revolve about the sun in orbits chiefly between those of Mars and Jupiter ranging in diameter from one mile to 480 miles. Also called asteroids, minor planets . —planetoidal, adj.

zenography

the study and description of Jupiter. —zenographical, adj.

planet a celestial body moving in an elliptical orbit round a star; originally, each of the seven major celestial bodies visible from the earth which move independently of the fixed stars and were believed to revolve the earth in concentric spheres centred on the earth (in order of their supposed distance from the earth in the Ptolemaic system, the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn). In astrology, a celestial body distinguished from the fixed stars by having an apparent motion of its own (including the moon and sun), especially with reference to its supposed influence on people and events.

The eight planets of the solar system are either gas giants—Jupiter, Saturn, Uranus, and Neptune—or smaller rocky bodies—Mercury, Venus, Earth, and Mars.

Recorded from Middle English, the word comes via Old French and late Latin from Greek planētēs ‘wanderer, planet’, from planan ‘wander’.

planet A non‐luminous body in orbit around the Sun, or another star, which has sufficient mass to have become rounded by its own gravity and which has significantly cleared its orbital neighbourhood of smaller objects. Planets can consist of rock and metal, as do the inner planets of the Solar System, or predominantly of liquid and gas, as do the giant outer planets. The term does not include comets or other small objects such as meteoroids. Asteroids, however, are sometimes referred to as minor planets. A planet can have a mass up to about 10 times that of Jupiter, above which it would become a brown dwarf. In 2006 the International Astronomical Union introduced the term dwarf planet to describe objects that are rounded in shape but which have not cleared the neighbourhood around their orbit; this category includes the largest members of the asteroid belt and the largest trans‐Neptunian objects.

planet (Gk. ‘wandering star’) Large, non-stellar body in orbit around a star, shining only by reflecting the star's light. Planets may be either rocky in composition, such as Mercury, Venus, Earth and Mars, or mainly gaseous, such as Jupiter, Saturn, Uranus and Neptune. In our Solar System there are nine major planets, together with thousands of asteroids or minor planets. In the 1990s astronomers confirmed the first extra-solar (outside our Solar System) planets. In 1995 scientists discovered the first planet (51 Pegasi b) orbiting a main-sequence star, 51 Pegasi, similar to our Sun. By 2002 astronomers had discovered 101 extra-solar planets. See also Pluto

plan·et / ˈplanit/ • n. a celestial body moving in an elliptical orbit around a star. ∎  (the planet) the earth: no generation has the right to pollute the planet. ∎  chiefly Astrol., hist. a celestial body distinguished from the fixed stars by having an apparent motion of its own (including the moon and sun), esp. with reference to its supposed influence on people and events. DERIVATIVES: plan·e·tol·o·gy / ˌplaniˈtäləjē/ n.

planet †(old astron.) heavenly body having an apparent motion among the fixed stars XII; (mod. astron.) heavenly body revolving round the sun XVII. — (O)F. planète — late L. planēta, planētēs (only in pl. planētæ, for older L. stellæ errantes) — Gr. planḗtēs wanderer (pl. astéres planâtai wandering stars), f. planán lead astray, wander.
So planetary XVII. — late L.

planets see giant planets

planet •dammit, Hammett, Mamet •emmet, semmit •helmet, pelmet •remit • limit • kismet • climate •comet, grommet, vomit •Goldschmidt •plummet, summit •Hindemith •hermit, Kermit, permit •gannet, granite, Janet, planet •magnet • Hamnett • pomegranate •Barnet, garnet •Bennett, genet, jennet, rennet, senate, sennet, sennit, tenet •innit, linnet, minute, sinnet •cygnet, signet •cabinet • definite • Plantagenet •bonnet, sonnet •cornet, hornet •unit •punnet, whodunnit (US whodunit) •bayonet • dragonet • falconet •baronet • coronet •alternate, burnet •sandpit • carpet • armpit • decrepit •cesspit • bear pit • fleapit •pipit, sippet, skippet, snippet, tippet, Tippett, whippet •limpet • incipit • limepit •moppet, poppet •cockpit • cuckoo-spit • pulpit • puppet •crumpet, strumpet, trumpet •parapet • turnspit