Planet

Official definition of planet

History of planetary science

Planets of the solar system

Planetary astronomy

A planet is generally defined as any major celestial object that orbits a star and does not emit visible light by internal means (but only shines by reflected light from the parent star). The study of planets, specifically the evolution, structure, and composition of planets and planetary systems, is called planetary science. Planetary scientists usually compare Earth with other planets in order to learn more about the properties contained in and about Earth. These scientists use physics, mathematics, chemistry, geology, astronomy, atmospheric science, biology, and many other sciences in their work with planets. Although the word planet is used frequently in science, it was not officially defined in astronomy until 2006.

Official definition of planet

Before the 1990s, only nine planets were known: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The planets varied in size and characteristics. Mercury, Venus, Earth, and Mars were classified as terrestrial planets while Jupiter, Saturn, Uranus, and Neptune were considered gas planets. Pluto was not included in either group due to its small size and location within the Kuiper Belt (a ring of millions of frozen, rocky objects between the orbit of Neptune and reaching out past Pluto’s orbit).

For the most part, the differences did not necessitate a formal definition. However, early in the 1990s, many tiny bodies beyond the orbit of Neptune (what are called trans-Neptunian objects) were discovered. These icy bodies were similar in composition and size to Pluto (what was then the ninth planet in the solar system and furthest from the sun). In addition, hundreds of exoplanets (planets orbiting stars other than the sun) were found to exist. These discoveries added a wide variety of sizes and characteristics when describing planets. Some bodies were as large as stars, while others were as small as the moon. Some small stars, called brown dwarfs and looking like planets, were discovered orbiting larger stars. Finally, in 2005, a body called 2003UB₃₁₃—which was larger than Pluto—was found outside Neptune’s orbit. Astronomers decided it was time to define the word planet.

Consequently, on August 24, 2006, members of the International Astronomical Union (IAU) at its General Assembly in Prague, Czech Republic, passed Resolution 5A. (The IAU is an organization whose mission is to promote astronomy through international cooperation. It also officially names celestial bodies) According to the IAU, a planet is any “celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”

History of planetary science

In the sixteenth and seventeenth centuries Polish astronomer Nicolaus Copernicus (1473–1543), astronomer and physicist Galileo Galilei (1564–1642), and others introduced and developed the concept that Earth (a planet) revolves around the sun (a star) rather than the opposite, traditionally held view, which positioned Earth at the center of the universe. Over the next three centuries, astronomical discoveries have shown that the sun is an average-sized star in the universe that is filled with billions and billions of stars. Since the 1990s, it has been shown that some of these stars also are encircled with one or more planets. The use of telescopes (such as radio and optical ones), spectroscopy (the study of the electromagnetic spectrum), charge-coupled devices (which records light particles when they hit its surface), unmanned spacecraft (which carry a host of scientific instruments), and other technologically advanced instruments have helped to develop the concepts of planetary science.

Planets are generally thought to have formed from the same gases and dust that condensed to make the parent star. Materials were expelled from giant stars as they ended their lives. These explosions, called super-novae, send shock waves through clouds of interstellar gases and dust. Such actions allowed gravitational forces (gravity) to slowly form these clouds into stars and planets. The resulting planets are, today, seen with the naked eye, ground-based telescopes, and orbiting telescopes. Many of these planets themselves often contain orbiting moons and dust rings.

Planets of the solar system

The eight major planets in the solar system, which are in elliptical orbits near the ecliptic plane, are divided into two classes: the inner and outer planets. The inner planets (Mercury, Venus, Earth, and Mars) are made of rocky material surrounding an iron-nickel metallic core. Earth and Venus have substantial cloud-forming atmospheres, and Mars has a thin atmosphere similar in composition to the of Venus.

The outer planets (Jupiter, Saturn, Uranus, and Neptune) are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock plus metal cores. Pluto, which has been demoted to a dwarf planet as of 2006, is made of ice and rock. It is probably an escaped moon of Neptune.

Specifically, Pluto has been disqualified from being a planet due to its highly elliptical orbit that overlapped Neptune’s orbit. Instead, Pluto is recognized by the IAU as a dwarf planet—“a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.” The largest dwarf planet in the solar system is 2003UB₃₁₃, as of September 2006 it has been officially named Eris.

Planetary astronomy

Other stars have planets orbiting about them. Astronomers have discovered that the star- and planet-formation mechanisms seen in the solar system are similar throughout the universe. When stars form, leftover gases and dust accumulate into planetesimals by mutual gravitational attraction. Observations of disk-shaped dust clouds around newly formed stars are a clear indication of planet formation in progress.

Planetary astronomy is a very active field, thanks to space probes like the Galileo unmanned spacecraft. In 1995, scientists found evidence that Jupiter’s moon Europa has a liquid ocean and, perhaps, the right conditions for life. The Mars Pathfinder mission landed a small roving vehicle on the planet in 1997, providing up-close pictures suggesting that liquid water had once scoured the surface. Pathfinder’s roving vehicle Sojourner also performed soil-chemistry analysis, and other probes like National Aeronautics and Space Administration’s (NASA’s) Mars Reconnaissance Orbiter (MRO) will continue to provide new information about planetary surfaces. In fact, MRO, which was launched on August 12, 2005, began to begin its primary mission in November 2006. It is conducting exploration and reconnaissance of Mars’ landforms, minerals, water/ice, stratigraphy (rocks) while in orbit about the planet. Its explorations will help to pave the way for future manned and unmanned missions to the planet.

Astronomers have found planets circling stars other than the sun. The first planet found was in 1995 when Michel Mayor and Didier Queloz found a planet around star 51 Pegasi, an almost perfect twin of the sun. Since October 2006 (according to the IAU), thanks to new, indirect observational techniques, 208 extrasolar planets have been discovered, with masses ranging from that of Jupiter to the upper size limit for a planet (about 15 Jupiter masses). Since 2002, more than 20 extrasolar planets have been discovered on average each year.

These new planets cannot be seen directly, but are inferred from the perturbative motions (wobble) or brightness seen on some stars, as observed from large telescopes on Earth. The wobble is caused by the gravitational pull of large planets near the star. Because these planets are big, gassy, and close to their star, they are not likely to contain any life, but their existence shows that there is nothing special about the fact that planets circle the sun.

The two primary methods to detect new planets are: the radial-velocity technique (which uses spectroscopy—the analysis of the electromagnetic spectra emitted by stars—to detect perturbations of stars orbited by planets) and the transit method (which uses the concept of the transit—the passage of a planet directly between its star and the Earth, and can occur only when the planet’s orbit happens to be oriented edge-on to the Earth-sun—to show the star’s apparent brightness that dims for several minutes.

Evidence that planets exist on other stars come from the Hubble Space Telescope. It has captured, for example, images of a dust ring around the star HR 4796A, 220 light-years from Earth. The ring roughly resembles that of Saturn, but on a vastly larger scale. Some objects in the rings could be planets, or the slender shape of the ring may be influenced by nearby planets.

One extrasolar planet has been found only 15 light-years from Earth, circling the star Gliese 876. This distance is much closer than other extrasolar planets. Gliese 876 is a small star, less than one-third the mass of the sun, suggesting that extrasolar planets are anything but rare.

In 1999, astronomers announced the first-ever detection of an entire solar system around a star. Only 44 light-years from Earth, three large planets were found circling the star Upsilon Andromedae, a sunlike star visible to the naked eye on Earth. Again, the presence of the planets was inferred from gravitational wobbling. Astronomers suspect the planets are similar to Jupiter and Saturn—huge spheres of gas without a solid surface. One of them completely circles its star in only 4.6 Earth-days. Such discoveries show that planetary science will likely be a fruitful and surprising field for years to come.

In November 2008, the Kepler space observatory, a space telescope especially designed to scan large areas of the sky for transits by planets as small as Earth, is scheduled to be launched by NASA. By 2012 or 2013, Kepler should have gathered enough data to pinpoint hundreds of extrasolar planets and to determine how typical Earth’s solar system is in the universe. As of 2006, astronomers estimate that at least 10% of all stars similar in size and characteristics to the sun have one or more planets orbiting them.

KEY TERMS

Ecliptic plane— The plane of Earth’s orbit around the sun. The other planets of the solar system also have their orbits near this plane and in the same direction of rotation as Earth.

Planetesmals— Small clumps of matter held together by electromagnetic forces that, when gathered together, form the planets.

The study of planets, especially Earth, has produced much information about their origins, evolution, and processes. Information learned about other planets—whether within the solar system or outside of it—helps scientists learn more about life and processes on Earth. So much information is being learned in outer space that many countries, including the United States, Japan, Russia, China, India, and the countries of the European Space Agency, are actively developing manned spacecraft to explore the nearby planets and unmanned probes to investigate the more distant planets of the solar system. Here on Earth and in orbiting satellites, telescopes are discovering previously unknown planets that someday will add to the scientific knowledge of how they were formed and how they are developing in their own sections of the universe.

See also Mercury (planet); Neptune; Planetary atmospheres; Planetary nebulae; Planetary ring systems.

James O’Connell

Planet

A planet is a relatively cold body that orbits a star . Planets are thought to have formed from the same gas and dust that condensed to make the parent star. They can be seen by eye and telescope because of the light they reflect from their star. The planets themselves often have orbiting moons and dust rings.

The nine planets in our solar system that are in elliptical orbits near the ecliptic plane are divided into two classes: the inner and outer planets. The inner planets (Mercury, Venus , Earth , and Mars ) are made of rocky material surrounding an iron-nickel metallic core. Earth and Venus have substantial cloud-forming atmospheres, and Mars has a thin atmosphere similar in composition to the of Venus.

The outer planets (Jupiter , Saturn , Uranus , Neptune , and Pluto ) are, with the exception of Pluto, large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock plus metal cores. Pluto, made of ice and rock, is probably an escaped moon of Neptune.

It is likely that other stars have planets orbiting them since the star- and planet-formation mechanisms are similar throughout the universe. When stars form the leftover gas and dust accumulate by mutual gravitational attraction into planetesmals. Observation of disk-shaped dust clouds around newly formed stars are an indication of planet formation in progress.

Planetary astronomy is a very active field, thanks to new space probes like the Galileo unmanned spacecraft. In 1995 scientists found evidence that Jupiter's moon Europa has a liquid ocean and, perhaps, the right conditions for life. The Mars Pathfinder mission landed a small roving vehicle on the planet in 1997, providing up-close pictures suggesting that liquid water had once scoured the surface. Pathfinder's roving vehicle Sojourner also performed soil chemistry analysis, and other probes like the Mars Polar Lander will continue to provide new information about planetary surfaces.

Astronomers have also found planets circling stars other than our own. The first was in 1995, when Michel Mayor and Didier Queloz found a planet around star 51 Pegasi, an almost perfect twin of the Sun . Since then nearly two dozen "extrasolar" planets had been discovered by 1999. These new planets are usually large, like Jupiter. They cannot be seen directly, but are inferred from the wobble seen on some stars, as observed from large telescopes on Earth. The wobble is caused by the gravitational pull of large planets near the star. Because these planets are big, gassy, and close to their star, they are not likely to contain any life, but their existence shows that there is nothing special about the fact that planets circle our Sun.

Other special arrangements have been found in the 1990s. The Hubble Space Telescope captured an image of a dust ring around the star HR 4796A, 220 light-years from Earth. The ring roughly resembles that of Saturn, but on a vastly larger scale. Some objects in the rings could be planets, or the slender shape of the ring may be influenced by nearby planets.

One extrasolar planet has been found only 15 light-years from Earth, circling the star Gliese 876. This is much closer than other extrasolar planets , which mostly lie at a distance of 40 to 80 light-years. Gliese 876 is a small star, less than 1/3 the mass of the Sun, suggesting that extrasolar planets are anything but rare.

In 1999 astronomers announced the first-ever detection of an entire solar system around a star. Only 44 light-years from Earth, three large planets were found circling the star Upsilon Andromedae, a sun-like star visible to the naked eye on Earth. Again the presence of the planets was inferred from gravitational wobbling. Astronomers suspect the planets are similar to Jupiter and Saturn—huge spheres of gas without a solid surface. One of them completely circles its star in only 4.6 Earth days. Such discoveries show that planetary science will likely be a fruitful and surprising field for years to come.

See also Mercury (planet); Neptune; Planetary atmospheres; Planetary nebulae; Planetary ring systems.

James O'Connell

KEY TERMS

Ecliptic plane

—The plane of Earth's orbit around the Sun. The other planets of the solar system also have their orbits near this plane and in the same direction of rotation as Earth.

Planetesmals

—Small clumps of matter held together by electromagnetic forces that, when gathered together, form the planets.

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 (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

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’.