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The issue of a correct definition for "planet" has come to a head in recent years and the International Astronomical Union , or IAU , which is the body responsible for resolving issues of astronomical namenclature, has stated that it intends to release its final decision on the matter in September , 2006 . http://iau.org/TRANS-NEPTUNIAN_OBJECT_2003_UB.324.0.html HISTORY AND ETYMOLOGY planet model as understood by the ancients]] There has never been a single, precise definition for the word "planet." When originally coined by the Ancient Greeks , a planet was any object that appeared to wander against the field of fixed Star s that made up the night sky (''asteres planetai'' "wandering stars"). This included not only the five "classical" planets, that is, Mercury , Venus , Mars , Jupiter and Saturn , but also the Sun and the Moon (the " Seven Heavenly Objects "). However, a distinction was occasionally made in terminology, so that if one said the "five planets", it would mean all of the "naked eye" ones except for the sun and moon. There was no need for an exact classification of a planet and so the sun and moon could be included or excluded as planets in discussions. Eventually, when the Heliocentric Model was accepted over the Geocentric , Earth was placed among their number and the Sun was dropped, and after Galileo discovered his four Satellites of Jupiter, the Moon was also eventually reclassified. However, the Galilean Satellites of Jupiter (in 1610 ), Saturn's satellite Titan in 1659 , and Iapetus and Rhea in 1673 were initially described as "planets", not "moons"; the word "moon" at that time only referred to Earth's Moon . Over the following centuries, the parameters of planethood have changed as new observations and theories have refined our understanding of the solar system. In the late 20th and early 21st Centuries a new wave of observations rendered many once-clear demarcations indistinct. ISSUES AND CONTROVERSIES While there is much disagreement between current definitions of "planet", most focus on three general criteria: that it must Orbit a star, be '''above a certain size''' (usually large enough to be rounded by its own gravity), and yet '''not be large enough to commence Nuclear Fusion '''. Each of these criteria has been challenged by various discoveries, outlined below. Minor planets Within our Planetary System , the nine objects currently accepted as planets orbit the Sun with other objects as diverse as Asteroid s, Meteoroid s, Comet s, and Cosmic Dust . The term " Minor Planet " is often used to describe those objects that, while they orbit the Sun, are not seen to fulfill certain criteria common to the "major" planets. What these criteria are, or even if they should exist at all, is the subject of some debate. Shared orbit One possible criterion to distinguish a major planet from a minor planet is whether its orbit is unique, or shared by other objects of similar size. The discovery of Uranus in 1781 seemed to validate Bode's Law , a mathematical function which generates the size of the Semimajor Axis of planetary Orbit s. Astronomers had considered the Law a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's Law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801 , Ceres , a world so small it was barely visible through a telescope of the time, was found to lie at just the correct point in space. The object was hailed as a new planet. Then in 1802 , Heinrich Olbers discovered Pallas , a second "planet" at roughly the same distance from the Sun as Ceres. The idea that two planets could occupy the same orbit was an affront to centuries of thinking. Some years later, another world, Juno , was discovered in a similar orbit with discoveries accelerating thereafter. William Herschel suggested that these worlds be given their own separate classification, Asteroid s, meaning "starlike" since they were so small they resembled stars through a telescope, though most astronomers preferred to refer to them as planets. Herschel determined that Ceres was 260 km in diameter; today we know that Ceres is about four times larger. He also determined that Pallas was 237 km in diameter; we now know it to be twice as large. Science textbooks in 1828 , after Herschel's death, still numbered the asteroids among the planets. By 1851 , the number of asteroids had increased to 15, and a new method of classifying them, by adding a number before their names, was adopted, inadvertently placing them in their own distinct category. By the 1860s , observatories in Europe and America began referring to them as " Minor Planets ", or "small planets", though it took the first four asteroids longer to be grouped as such. http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html (on the left) with (from top to bottom) the Moon , Pluto and its moon Charon , Sedna , Quaoar , and Ceres on the right.]] The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of s with an orbit that often sent it flying off the plane of the ecliptic or even inside that of Neptune . However, it was, as far as anyone could tell, unique. Then, beginning in 1992 , astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-hypothesized Kuiper Belt (sometimes called the Edgeworth-Kuiper Belt ), a band of icy debris that is the source for "short-period" comets—those, like Halley , with orbital periods of up to 200 years. Pluto's orbit was found to lie right in the middle of this band and thus its planetary status was thrown into question; the precedent set by officially voted in 1999 to retain Pluto's classification as a planet.http://www.iau.org/STATUS_OF_PLUTO.238.0.html http://www.asterism.org/newsletter/l9904-3.htm Throughout the late , it is officially designated as a minor planet—the Provisional Designation referring to its official listing in the minor-planet archive as the 7827th object first identified in observations made in the second half of October 2003. Sphericity 's large moon Proteus is not spherical]] A number of astronomers, such as , and that the criterion at issue is size. Objects in orbit round the Sun range in size from Jupiter to dust particles, so obviously there would need to be a lower limit. The most oft-mooted potential limit is when an object becomes spherical under its own gravity. Many astronomers favour this definition, because it would allow Pluto to retain its status as a planet. However, deciding which objects in the Solar system are spherical or spheroid is more complicated than it seems. In mathematical terms, Spheroid s consist of an ellipse rotated around one axis. Consequently they have two axes of equal length and one that is either longer or shorter; they resemble spheres that have been deformed (by stretching or squashing) in one dimension. A section through one axis will produce a circle, and a section through the other two axes will produce an ellipse. Ellipsoid is a general term for bodies including spheres and spheroids, but is used here for ''scalene ellipsoids'', bodies in which all three axes differ in length. Every section through a scalene ellipsoid produces an ellipse. All ellipsoids, however, have the points on their surfaces joined by smooth curves (which form the elliptical or circular sections). Obviously, on a topographically irregular body this can only be an approximation; however, we can contrast bodies that are, allowing for topographic variation, generally ellipsoidal with ''irregular'' bodies whose Limb s do not show smooth curvature, such as Neptune 's moon Proteus . If we use this mathematical basis to define a spheroid, then the dividing line between spheroidal and irreguar objects within our Solar system frays noticably, as this table illustrates: An object's density is a rough guide to its composition: the lower the density, the higher the fraction of ices, and the lower the fraction of rock. The most dense of these objects, Vesta and Juno, are composed entirely of rock with very little ice, and have a density close to the Moon's, while the less dense, such as Proteus and Enceladus, are composed mainly of ice. Note that objects made of ices, such as Enceladus and Miranda, assume a spherical shape more easily than those made of rock, such as Vesta and Pallas. Heat energy, from Gravitational Collapse , Impact s, Tidal Force s, or Radioactive Decay also factor into whether an object will be spherical or not, as does temperature; Saturn's icy moon Mimas is spherical, but Neptune 's larger moon Proteus , which is similarly composed but colder because of its greater distance from the Sun, is not. Note also that Ceres is spherical, but the Kuiper Belt Object , which is several times more massive, (included for comparison, as it is the largest known non-spherical object in the solar system) has been elongated into an ellipsoid by its faster rotation. Jupiter and Saturn have also been rendered highly oblate by their rapid rotations. Mimas, Enceladus, and Miranda have been stretched into prolate spheroids by tidal forces. Other astronomers have suggested that, to overcome this ambiguity, the diameter limit for planethood should be arbitrarily pinned at that of Pluto, thus preserving the traditional nine planets while allowing the possibility of future additions, http://www.gps.caltech.edu/~mbrown/planetlila/#name while others have suggested that it be fixed at 1000 km, which would potentially define at least three smaller KBOs as planets alongside Pluto. http://www.dfw.com/mld/dfw/news/nation/13515853.htm Double planets image of Pluto and Charon .]] The in tandem. Charon is not importantly affected by its orbit —unlike the synchronous orbital radius, for instance, which does have profound consequences for the orbiting body (see Phobos , for example). Even our own and Callisto or Saturn and Iapetus form double planets. Also, many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. Jupiter 's moon Ganymede and Saturn 's moon Titan are both larger in terms of diameter (though not mass) than Mercury , and Titan even has a substantial atmosphere, thicker than the Earth 's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede has a Magnetic Field . Rogue planets and sub-stars The dividing line between "star" and "planet" has been blurred considerably since have always existed at the boundary of that distinction. Too small to commence sustained hydrogen fusion, they have been granted star status on their ability to fuse Deuterium . However, due to the relative rarity of that isotope, this process lasts only a tiny fraction of the star's lifetime, and hence most brown dwarfs would have ceased fusion long before their discovery. http://astron.berkeley.edu/~basri/defineplanet/whatsaplanet.htm Since Binary Star s or other multiple-star formations are common, many brown dwarfs are likely to orbit other stars, and since they would not be producing energy through fusion, they could be described as "planets". Similarly, an orbiting White Dwarf , such as Sirius B , since it too has ceased fusion, could be considered a planet. But the confusion does not end with brown dwarfs. Zapatario Osorio et al. have discovered many objects in young star clusters of masses below that required to sustain fusion of any sort (currently calculated to be roughly 13 Jupiter masses). http://www.sciencemag.org/cgi/content/short/290/5489/103 These have been described as " Free Floating Planets " because current theories of solar system formation suggest that planets may be ejected from solar systems altogether if their orbits become unstable; indeed the Hubble Telescope may have imaged just such an ejection of a body larger than Jupiter in 1998. http://hubblesite.org/newscenter/newsdesk/archive/releases/1998/19/image/a] This would suggest that the original criterion that a planet must orbit a star should instead be amended to indicate that it must have originated in orbit around a star. However, it is also possible that these "free floating planets" could have formed in the same manner as stars; thus their discoverers also term them "grey dwarfs" or and Helium , while the planets are said to have formed from the "bottom up"; from the accretion of dust and gas in orbit around the young star, and thus should have cores of Silicate s or ices. As yet it is uncertain whether gas giants possess such cores. If it is indeed possible that a gas giant could form as a star does, then it raises the question of whether such an object, even one as familiar as Jupiter or Saturn , should be referred to as an orbiting low-mass star rather than a planet. has blurred the distinction between Star s and Planets .]] The IAU has officially released a statementhttp://www.dtm.ciw.edu/boss/IAU/div3/wgesp/definition.html]to define what constitutes an extrasolar planet and what constitutes an orbiting star: #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. #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. #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 creates ambiguity by making location, rather than formation or composition, the determining characteristic for planethood. A free-floating object with a mass below 13 Jupiter masses is a brown dwarf, whereas such an object in orbit round a fusing star is a planet, even if, in all other respects, the two objects may be identical. This ambiguity was highlighted in December, 2005, when the Spitzer Space Telescope observed the smallest Brown Dwarf yet found, only eight times Jupiter's mass with what appears to be the beginnings of its own Star System . Were this object found in orbit round another Star , it would have been termed a Planet .http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html THE IAU DEBATE For most astronomers, the issue of what constitutes a planet will be decided by the , a group of 19 IAU members, which had already been working on a definition since the discovery of Sedna in 2003 , narrowed their choices to a shortlist of three, allowing each member to vote for more than one. The definitions were:
The first would be an essentially cultural/historical definition, recognising Pluto's historical identity as a planet by setting an arbitrary limit immediately below its diameter. Under this definition, the only known planets in our solar system would be the current nine, plus . The second provides a more scientific basis for a limit, and also avoids the "roundness" cutoff muddied by objects such as , but still discounts many irregular objects, such as Pallas , that are larger than many regular objects. By its criterion, dozens of objects in our Solar system could be considered planets. The final definition would leave only eight planets in our Solar System , relegating Pluto to the status of Minor Planet . Perhaps for this reason, it proved the least popular. Since no overall consensus could be reached, the committee decided to put these three definitions to a wider vote, most likely at the IAU General Assembly meeting in Prague in August, 2006 http://www.newscientistspace.com/article.ns?id=dn8681 http://kencroswell.com/TenthPlanetFirstAnniversary.html; the IAU has stated that it will publish a definition early in the following month. SEE ALSO PRIMARY SOURCES SECONDARY SOURCES
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