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     The International Astronomical Union defines "planet" as a celestial body that, within the Solar System,
   (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 neighbourhood around its orbit;
or within another system,
   (i) is in orbit around a star or stellar remnants;
   (ii) has a mass below the limiting mass for thermonuclear fusion of deuterium; and
   (iii) is above the minimum mass/size requirement for planetary status in the Solar          System.

     Our solar system is thus considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Under a separate resolution, it is also considered to have three dwarf planets: Ceres, Pluto, and Eris. There have been more than two hundred planets discovered orbiting other stars to date.

     Historically, there had been no formal scientific definition of "planet" and without one, the Solar System had been considered to have various planets over the years. This changed when a resolution covering planets within our solar system was formally adopted by the IAU in 2006, limiting the number to eight. However, the IAU's position on those in other systems remains only a working definition in place since 2003, and as such, is easily subject to change. The IAU has not yet taken a position on free-floating objects of planetary mass outside star systems, other than to exclude those in young star clusters.


Etymology

     In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered to be stationary. The "wandering" lights were first called "p?a??t??" (planetes), by the Ancient Greeks, a Greek term meaning "wanderer", and it is from this that the word "planet" was derived.

     In near-universal practice in the Western world, the planets in the Solar System are named after Graeco-Roman gods, as, in Europe, it was the Greeks who named them, although the practice of naming planets after gods originated in the West with the Sumerians, who lived in modern-day Iraq in about 3000 BC. Successive Mesopotamian civilisations, such as the Babylonians, retained the Sumerian naming convention but adapted it to their own very different pantheons. The Greeks borrowed much of their astronomy, including constellations and the zodiac, from the Babylonians, and by 600 BC had already begun using Babylonian concepts in their calculations. The Greeks grafted the names of their own gods onto the Babylonian planet list, although there was some confusion in translation: for instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestillence.

     Because of the influence of the Roman Empire and, later, the Catholic Church, in most countries in the West the planets are known by their Roman (or Latin) names rather than the Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names.

     In ancient times, there were seven known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth: the Moon (called Luna by the Romans, and Selene by the Greeks), Mercury (called Hermes by the Greeks), Venus (Aphrodite), the Sun (called Sol by the Romans, Helios by the Greeks), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Eventually, the Sun and Moon were removed from the list of planets in accordance with the heliocentric model. However, when subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.

     Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth, in Ptolemaic orbit order listed inwards. As a result, a list of which god has charge of the first hour in each day came out as Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order. Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, Wóden, Thunor, and Fríge, Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

     Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, the Greeks retain their original name, G? (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.

     Some non-European cultures use their own planetary naming systems. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.


History

     As scientific knowledge progressed, understanding of the term "planet" changed from something that moved across the sky (in relation to the starfield), to a body that orbited the Earth (or that were believed to do so at the time). When the heliocentric model gained sway in the 16th century, it became accepted that a planet was actually something that orbited the Sun, and the Earth was itself a planet, and the Sun and Moon were not. Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period.

    During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846.

     However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years.

     In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the tenth planet, brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).


Definition and disputes

    With the discovery during the latter half of the twentieth century of more objects within the Solar System and large objects around other stars, dispute arose over what should constitute a planet. There was particular disagreement over whether round objects that existed in belts, and large deuterium fusing objects should qualify.

     In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:
   1) Objects with true masses below the limiting mass for thermonuclear fusion of         deuterium (currently calculated to be 13 Jupiter masses for objects of solar         metallicity) that orbit stars or stellar remnants are "planets" (no matter how they         formed). The minimum mass/size required for an extrasolar object to be         considered a planet should be the same as that used in our Solar System.
   2) Substellar objects with true masses above the limiting mass for thermonuclear         fusion of deuterium are "brown dwarfs", no matter how they formed nor where         they are located.
   3) Free-floating objects in young star clusters with masses below the limiting mass         for thermonuclear fusion of deuterium are not "planets", but are "sub-brown         dwarfs" (or whatever name is most appropriate).

     This definition has been widely used by astronomers when publishing discoveries in journals since this time, although it remains a temporary, working definition until a more permanent one is formally adopted. It also did not address the controversy over the lower mass limit.

     However, in 2006, the general assembly of the IAU voted to pass a resolution that redefined planets within the Solar System as:
A celestial body that is (a) 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 neighbourhood around its orbit.

     Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto and Eris) are classified as dwarf planets, providing they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.

     This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:
The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets.

     In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,[citation needed] and some astronomers have even stated that they will not use it. Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU conference is not until 2009, when modifications could be made to the definition, also possibly including extrasolar planets.

     Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status during most of the 20th century, in a similar way to Ceres and its kin in the 1800s. More recently, the discovery of Eris was widely reported in the media as the "tenth planet". The reclassification of all three objects as dwarf planets has attracted much media and public attention.


Formation

     It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that do not condense under gravity to form a protostar. Instead, these remnants become a thin, protoplanetary disk of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc known as planetesimals. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets. After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere. This serves to increase the capture rate of the planetesimals by a factor of ten.

     When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small solar system bodies.

     The energetic impacts of the smaller planetesimals will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from thesubsequent impact of comets. (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)

     With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity is now believed to determine the likelihood that a star will have planets. Hence it is thought less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star.