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planet

planet [Gr.,=wanderer], a large nonluminous body of rock or gas that orbits the sun or another star, has a rounded shape due to gravity, and has cleared its orbit of smaller objects. The term, once limited to any of the eight solid, nonluminous bodies (also called major planets) that revolve around the sun, has been extended to include similar bodies discovered revolving around other stars. The term planet sometimes has been used to include dwarf planets and asteroids (or minor planets); it does not include 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 may be 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. By 2014 the number of known exoplanets was roughly 1,700, and it was estimated that 40% of sunlike stars had planetary systems. It is now estimated that planets are more common than stars.

The CoRoT (launched 2006) and Kepler (launched 2009) space telescopes, especially the latter, 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. In 2014, Kepler scientists announced the discovery of a habitable-zone planet (Kepler 186f) with a radius estimated to be 10% larger than the earth's, that orbited a cool dwarf star with four other planets; because of its size, Kepler 186f was believed to be a rocky planet with the potential to have liquid water.

Many 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, and many of the discovered rocky planets are much larger than earth, often up to 10 times more massive (one, Kepler 10c, is 17 times as massive). 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).

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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 earths 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.

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planet

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’.

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planet

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

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planet

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.

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planet

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.

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planet

planetdammit, 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 • Plantagenetbonnet, 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

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Planet

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 Plutos 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 2003UB313which was larger than Plutowas found outside Neptunes 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 (14731543), astronomer and physicist Galileo Galilei (15641642), 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 Neptunes orbit. Instead, Pluto is recognized by the IAU as a dwarf planeta 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 Jupiters 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. Pathfinders roving vehicle Sojourner also performed soil-chemistry analysis, and other probes like National Aeronautics and Space Administrations (NASAs) 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 spectroscopythe analysis of the electromagnetic spectra emitted by starsto detect perturbations of stars orbited by planets) and the transit method (which uses the concept of the transitthe passage of a planet directly between its star and the Earth, and can occur only when the planets orbit happens to be oriented edge-on to the Earth-sunto show the stars 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 Saturnhuge 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 Earths 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 Earths 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 planetswhether within the solar system or outside of ithelps 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 OConnell

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Planet

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.

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