Monday, November 26, 2012

Dwarf planetDwarf planet,, planet, as defined by the International Astronomical Union (IAU), is a celestial body in direct orbit of the Sun[1]that is massive enough that its shape is controlled by gravitational forces rather than mechanical forces (and is thus anellipsoid), but has not cleared the neighboring region of other objects. Sony VAIO PCG-3B1M Battery More explicitly, it is a planetary-mass object—it has sufficient mass to overcome its internal compressive strength and achieve hydrostatic equilibrium—but is neither a planet nor a satellite. The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, Sony VAIO PCG-3C1T Battery brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris.[4] This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Sony VAIO PCG-3D1M Battery Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown,[5][6][7] who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern,[8][9] who had coined the term dwarf planet in 1990. Sony VAIO PCG-3G2M Battery The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris.[11] However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. Sony VAIO PCG-5R1M Battery The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1)[12] are to be named under the assumption that they are dwarf planets.[13] The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets. Sony VAIO PCG-7141M Battery Individual astronomers have accepted several additional bodies as dwarf planets. It is suspected that another hundred or so known objects in the Solar System are dwarf planets.Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered. Sony VAIO PCG-7143M Battery Mike Brown published in August 2011 a list of 390 candidate objects, organized in categories from "nearly certainly" to "possibly" meeting the IAU's criteria, along with his classification methodology.[16] Brown identifies nine known objects – the five accepted by the IAU plus 2007 OR10, Sony VAIO PCG-7151M Battery Sedna,Quaoar, and Orcus – as "virtually certain", with another two dozen highly likely, and there are probably a hundred or so such objects in total.[14] The classification of bodies in other planetary systems with the characteristics of dwarf planets has not yet been addressed. Sony VAIO PCG-7154M Battery Before the discoveries of the early 21st century, astronomers had no strong need for a formal definition of a planet. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies such as asteroids and comets. For almost 50 years Pluto was thought to be larger thanMercury, Sony VAIO PCG-7162M Battery but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and determine that it is much smaller than the initial estimates.[20] It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in theasteroid belt, Sony VAIO PCG-7181M Battery Ceres, it was one-fifth that of Earth's Moon.Furthermore, having some unusual characteristics such as large orbital eccentricity and a high orbital inclination, it became evident it was a completely different kind of body from any of the other planets. In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away. Sony VAIO PCG-41112M Battery Many of these shared some of the key orbital characteristics of Pluto, and Pluto started being seen as the largest member of a new class of objects, plutinos. This led some astronomers to stop referring to Pluto as a planet. Several terms including minor planet, subplanet, and planetoid started to be used for the bodies now known as dwarf planets. Sony VAIO PCG-7153M Battery By 2005, three other bodies comparable to Pluto in terms of size and orbit (Quaoar, Sedna, and Eris) had been reported in the scientific literature.It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified.Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain a planet. Sony VAIO PCG-71312M Battery In 2006, Eris (then known as 2003 UB313) was believed to be slightly larger than Pluto, and some reports unofficially referred to it as the tenth planet.[29] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006.[30] The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. Sony VAIO PCG-7144M Battery After many astronomers objected to this proposal, an alternative was drawn up by Uruguayan astronomer Julio Ángel Fernández, in which he created a median classification for objects large enough to be round but that had not cleared their orbits ofplanetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, since they have not cleared their orbits. Sony VAIO PCG-7191L Battery The IAU's final resolution preserved this three-category system for the celestial bodies orbiting the Sun. Fernández suggested calling these median objectsplanetoids,[32][33] but the IAU's division III plenary session voted unanimously to call them dwarf planets.[1] The resolution, #5A, reads: The IAU ... resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way: Sony VAIO PCG-3C1M Battery (1) A planet1 is 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, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, Sony VAIO PCG-3F1M Battery (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,2 (c) has not cleared the neighbourhood around its orbit, and (d) is not asatellite.The term dwarf planet has itself been somewhat controversial, as a grammatical reading suggests these bodies are planets. Sony VAIO PCG-3H1M Battery The IAU's Resolution 5A was accompanied by a second resolution, 5B, which defined dwarf planets as a subtype of planet, distinguished from the other eight which were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight "classical planets" and four "dwarf planets". Sony VAIO PCG-3J1M Battery However, resolution 5B was defeated in the same session that 5A was passed, so that only the dwarf planet half of the proposal was made official.[34] Because of the grammatical inconsistency of a dwarf planet not being a planet, while a dwarf star is a star, alternative proposals such asnanoplanet and subplanet were suggested. Sony VAIO PCG-8141M Battery However, it was judged that the term dwarf planet had already entered general use and that it was too late to change it.[35] The term is parallel to minor planet, which is also not a type of planet. In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine, German Zwergplanet, Russianкарликовая планета karlikovaya planeta, Sony VAIO PCG-8161M Battery Arabic كوكب قزم kaukab qazm, Chinese 矮行星 ǎixíngxīng, etc., but Japanese is an exception: In Japanese these bodies are called junwakusei 準惑星, where wakusei 惑星 is 'planet' and jun- 準 is a prefix corresponding to English quasi-, pene- (almost), and sub-. Thus in Japanese they are called 'subplanets' or 'almost-planets'. Sony VAIO PCG-3C2M Battery The modern Latin name for this class of objects, planetulus, or planetion following a Greek-styled name, is also a diminutive form of planeta, hence meaning "small planet". Although concerns were raised about the classification of planets orbiting other stars,[17] the issue was not resolved; it was proposed instead to decide this only when such objects start being observed.[31] Sony VAIO PCG-5N2M Battery The 2006 IAU's Resolution 6a[36] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects but neither name was carried forward. Sony VAIO PCG-5P1M Battery On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids,[13] though "in part because of an email miscommunication, the WG-PSN [Working Group for Planetary System Nomenclature] was not involved in choosing the word plutoid. ... Sony VAIO PCG-5S1M Battery In fact, a vote taken by the WG-PSN subsequent to the Executive Committee meeting has rejected the use of that specific term."[37] On July 18, 2008, the WG-PSN classified the object then known as (136472) 2005 FY9 as a dwarf planet, and named it Makemake; this was followed in September by Haumea. Sony VAIO PCG-9Z1M Battery Alan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the likelihood of an encounter resulting in a given deflection of orbit.[41] The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. Following the authors, this value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where Λ > 1 will eventually clear it. Sony VAIO PCG-7171M Battery A gap of five orders of magnitude in Λ was found between the smallestterrestrial planets and the largest asteroids and Kuiper belt objects.[39] Using this parameter, Steven Soter and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": Sony VAIO PCG-7186M Battery planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), while dwarf planets lack the mass to do so.[41] Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol µ (mu), Sony VAIO PCG-81112M Battery that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where µ > 100 is deemed to be cleared.[39] There are several other schemes that try to differentiate between planets and dwarf planets,[8] but the 2006 definition uses this concept. Sony VAIO PCG-31311M Battery Sufficient internal pressure, caused by the body's gravitation, will turn a body plastic, and sufficient plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to be angular in shape. Sony VAIO PCG-8152M Battery Larger objects, where gravitation is significant but not dominant, are "potato" shaped; the more massive the body is, the higher its internal pressure and the more rounded its shape, until it achieves maximum rounding at hydrostatic equilibrium. This is the defining limit of a dwarf planet.[42] Sony VAIO PCG-31111M Battery When an object is in hydrostatic equilibrium, a global layer of liquid covering its surface would form a liquid surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. If the body does not rotate, it will be a sphere, but the faster it does rotate, the more oblateor even scalene it becomes. Sony VAIO PCG-61111M Battery However, if such a rotating body were to be heated until it melted, its overall shape would not change when liquid. The extreme example of a non-spherical body in hydrostatic equilibrium is Haumea, which is twice as long along its major axis as it is at the poles. The upper and lower size and mass limits of dwarf planets have not been specified by the IAU. Sony VAIO PCG-51112M Battery There is no defined upper limit, and an object larger or more massive than Mercury that has not "cleared the neighbourhood around its orbit" would be classified as a dwarf planet.[43] The lower limit is determined by the requirements of achieving a hydrostatic equilibrium shape, but the size or mass at which an object attains this shape depends on its composition and thermal history. Sony VAIO PCG-51211M Battery The original draft of the 2006 IAU resolution redefined hydrostatic equilibrium shape as applying "to objects with mass above 5×1020 kg and diameter greater than 800 km",[17] but this was not retained in the final draft.[1] Empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object. Sony VAIO PCG-51212M Battery For a body made of rigid silicates, such as the stony asteroids, the transition to hydrostatic equilibrium should occur at a diameter of approximately 600 km and a mass of some 3.4×1020 kg. For a body made of less rigid water ice, the limit should be about 320 km and ×1019 kg.[44] In the asteroid belt, Ceres is the only body that clearly surpasses the silicaceous limit (though it is actually a rocky–icy body), Sony VAIO PCG-41111M Battery and its shape is an equilibrium spheroid. 2 Pallas and 4 Vesta, however, are rocky and are just below the limit. Pallas, at 525–560 km and 1.85–2.4×1020 kg, is "nearly round" but still somewhat irregular. Vesta, at 530 km and 2.6×1020 kg, deviates from an ellipsoid shape primarily due to a large impact basin at its pole. Sony VAIO PCG-41111V Battery Among icy bodies, the smallest known to be in hydrostatic equilibrium is Mimas, at 396 km and 3.75×1019 kg. The largest irregular body in the outer Solar System is Proteus, nearly-but-not-quite round at 405–435 km and an assumed mass of ≈4.4×1019 kg. Bodies like Mimas may have had a warmer thermal history than Proteus, or their shape may have resolved after a collision. Sony VAIO PCG-61412V Battery Neither body is pure ice as used to calculate the lowest limit, however, and Mike Brown suggests that the practical lower limit for an icy dwarf planet is likely to be somewhere under 400 km.[46] There are about 100 TNOs currently estimated to be above this size. As of 2011, five objects are recognized as dwarf planets by the IAU. Sony VAIO PCG-71112M Battery Mike Brown considers these and four others to be "nearly certain".[14] For Ceres and Pluto, this is known through direct observation.Brown thinks the other seven are massive enough to be in hydrostatic equilibrium even if they are dense (primarily rocky) and at the lower end of their estimated diameters. Sony VAIO PCG-81111V Battery Eris is more massive than Pluto; Haumea and Makemake were assigned names as dwarf planets based on their absolute magnitudes.[11][36] In relative distance from the Sun, the five "official" dwarf planets are: Sony VAIO PCG-81111V Battery
  1. Ceres   – discovered on January 1, 1801, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Accepted as a dwarf planet by the IAU on September 13, 2006.
Pluto   – discovered on February 18, 1930. Classified as a planet for 76 years. Reclassified as a dwarf planet by the IAU on August 24, 2006. Sony VAIO PCG-81212M Battery
  1. Haumea – discovered on December 28, 2004. Accepted by the IAU as a dwarf planet on September 17, 2008.
  2. Makemake – discovered on March 31, 2005. Accepted by the IAU as a dwarf planet on July 11, 2008.
Eris – discovered on January 5, 2005. Called the "tenth planet" in media reports. Accepted by the IAU as a dwarf planet on September 13, 2006. Sony VAIO PCG-81212V Battery The other four "nearly certain" objects per Brown, Tancredi, et al. are:
  1. Orcus – discovered on February 17, 2004.
  2. Quaoar – discovered on June 5, 2002.
  3. 2007 OR10 – discovered on July 17, 2007.
  4. Sedna – discovered on November 14, 2003.
No space probes have visited any of these. Sony VAIO PCG-51111M Battery This will change if NASA's Dawn and New Horizons missions reach Ceres and Pluto, respectively. The plans are for Dawnto orbit Ceres, and New Horizons to fly by Pluto, both in 2015. Dawn entered orbit around the potential dwarf planet Vesta on 16 July 2011.[49] On 5 September 2012, Dawn left Vesta in order to orbit Ceres, which it is scheduled to reach in February 2015. Sony VAIO VPCS13X9E/B battery After Ceres, the next-most-massive body in the asteroid belt, Vesta, might also be classified as a dwarf planet, as its shape appears to deviate from hydrostatic equilibrium mainly because of massive impacts that occurred after it solidified.[55] The definition of dwarf planet does not address this issue. Data from the Dawn probe, which orbited Vesta in 2011–2012, may help clarify matters. Sony VAIO VPCS12V9E/B battery Many trans-Neptunian objects (TNOs) are thought to have icy cores and therefore would require a diameter of perhaps 400 km (250 mi)—only about 3% of that of Earth—to relax into gravitational equilibrium, making them dwarf planets of the plutoid class. Sony VAIO VPCS12V9E/B battery Although only rough estimates of the diameters of these objects are available, as of 2011 it was believed that a hundred of the known bodies beyond Neptune were probable dwarf planets.[14] A team is investigating thirty of these, and believe that the number will eventually prove to be about 200 in the Kuiper belt and many more beyond. Sony VAIO VPCF13M8E/B battery Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity. These bodies have no significant physical differences from the dwarf planets, but are not members of that class under the IAU definition because they do not directly orbit the Sun. They are Earth's moon, Sony VAIO VPCF13Z0E/B battery the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), seven moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, Titan, andIapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), one moon of Neptune (Triton), and one moon of Pluto (Charon). Sony VAIO VPCM13M1E/L battery The term planemo("planetary-mass object") covers both dwarf planets and such moons, as well as planets.[57] Alan Stern considers them a special category of planets, "satellite planets".[58] Despite its requirement that dwarf planets orbit the Sun directly, draft resolution (5) presented to the IAU, Sony VAIO VPCM13M1E/W battery which considered dwarf planets to be a category of planet, stated Charon could be considered a planet because it revolves with Pluto around a common center of mass located between the two bodies (rather than within one of the bodies).Note, however, that the Jupiter–Sun centre of mass lies outside the Sun, and so such a definition would imply that Jupiter is not a satellite of the Sun and therefore not a planet. Sony VAIO VPCF22M1E battery This definition was not preserved in the IAU's final resolution. In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution.[8] Campaigns included car bumper stickers and T-shirts.[59] Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.[60] Sony VAIO VPCF11M1E/H battery NASA has announced that it will use the new guidelines established by the IAU.[61] However, Alan Stern, the director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets. Sony VAIO VPCF13M0E/B battery Thus, as of January 2008, he and his team still referred to Pluto as the ninth planet,[63] and accept other dwarf planets such as Ceres and Eris as additional planets. Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both 'planets'. Sony VAIO VPCYB2M1E battery Ceres, formally 1 Ceres, is the only dwarf planet in the inner Solar System, and the largest asteroid. It is a rock–ice body 950 km (590 mi) in diameter, and though the smallest identified dwarf planet, it constitutes a third of the mass of the asteroid belt.[22][23] Discovered on 1 January 1801 by Giuseppe Piazzi,[24] it was the first asteroid to be identified, though it was classified as a planet at the time. Sony VAIO VPCYB3V1E battery It is named after Ceres, the Roman goddess of growing plants, the harvest, and motherly love. The Cererian surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clays.[14]It appears to be differentiated into a rocky core and icy mantle,[8] and may harbour an ocean of liquid water under its surface. Sony VAIO VPCY11M1E battery From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, and hence even at its brightest it is still too dim to be seen with the naked eye except under extremely dark skies.[15] The unmanned Dawn spacecraft, launched on 27 September 2007 by NASA, is expected to be the first to explore Ceres after its scheduled arrival there in 2015. Sony VAIO VPCS12L9E/B battery The spacecraft left asteroid 4 Vesta about 5 September 2012,[29] which it had been orbiting since July 2011.The idea that an undiscovered planet could exist between the orbits of Mars and Jupiter was suggested by Johann Elert Bode in 1772.[24] Previously, in 1596, Kepler had already noticed the gap between Mars and Jupiter. Sony VAIO VPCF11S1E/B battery Bode's considerations were based on the Titius–Bode law, a now discredited hypothesis which had been first proposed by Johann Daniel Titius in 1766, observing that there was a regular pattern in the semi-major axes of the known planets marred only by the large gap between Mars and Jupiter.The pattern predicted that the missing planet ought to have a semi-major axis near 2.8 AU. Sony VAIO VPCYB3V1E/R Battery William Herschel's discovery of Uranus in 1781[24] near the predicted distance for the next body beyond Saturn increased faith in the law of Titius and Bode, and in 1800, they sent requests to twenty-four experienced astronomers, asking that they combine their efforts and begin a methodical search for the expected planet. Sony VAIO VPCF23P1E Battery The group was headed by Franz Xaver von Zach, editor of the Monatliche Correspondenz. While they did not discover Ceres, they later found several large asteroids. One of the astronomers selected for the search was Giuseppe Piazzi at the Academy ofPalermo, Sicily. Sony VAIO VPCF23N1E Battery Before receiving his invitation to join the group, Giuseppe Piazzi discovered Ceres on 1 January 1801.[31] He was searching for "the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another".[24] Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet. Sony VAIO VPCY21S1E/L Battery Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his observations. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode of Berlin. He reported it as a comet but "since its movement is so slow and rather uniform, Sony VAIO VPCY21S1E/G battery it has occurred to me several times that it might be something better than a comet".[24] In April, Piazzi sent his complete observations to Oriani, Bode, and Jérôme Lalande in Paris. The information was published in the September 1801 issue of the Monatliche Correspondenz. By this time, the apparent position of Ceres had changed (mostly due to the Earth's orbital motion), Sony VAIO VPCF24M1E battery and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. Sony VAIO VGN-AW110J/H Battery In only a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach andHeinrich W. M. Olbers found Ceres near the predicted position and thus recovered it.[32] The early observers were only able to calculate the size of Ceres to within about an order of magnitude. Sony VAIO VGN-AW11M/H Battery Herschel underestimated its size as 260 km in 1802, while in 1811 Johann Hieronymus Schröter overestimated it as 2,613 km.Piazzi originally suggested the name Cerere Ferdinandea for his discovery, after both the mythological figure Ceres(Roman goddess of agriculture, Italian Cerere) and King Ferdinand III of Sicily. Sony VAIO VGN-AW11S/B Battery "Ferdinandea" was not acceptable to other nations of the world and was thus dropped. Ceres was also called Hera for a short time in Germany.[36] In Greece, it is called Demeter (Δήμητρα), after the Greek equivalent of the Roman goddess Cerēs;[37] in English, that name is used for the asteroid 1108 Demeter. The adjectival form of the name is Cererian,derived from the Latingenitive Cerēris. Sony VAIO VGN-AW11XU/Q Battery The old astronomical symbol of Ceres is a sickle, ⚳ ( ),[39] similar to Venus's symbol ♀, but with a gap in the upper circle (and with a variant   under the influence of the initial 'C'); this was later replaced with the numbered disk ①. The element cerium, discovered in 1803, was named after the asteroid. Sony VAIO VGN-AW11Z/B Battery In the same year, another element was also initially named after Ceres, but its discoverer changed its name to palladium (after the second asteroid, 2 Pallas) when cerium was named. The classification of Ceres has changed more than once and has been the subject of some disagreement. Sony VAIO VGN-AW120J/H Battery Johann Elert Bode believed Ceres to be the "missing planet" he had proposed to exist between Mars and Jupiter, at a distance of 419 million km (2.8 AU) from the Sun.[24] Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century. Sony VAIO VGN-AW150Y/H Battery As other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies.[24] In 1802 Sir William Herschel coined the term asteroid ("star-like") for such bodies, writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".Sony VAIO VGN-AW160J/Q Battery As the first such body to be discovered, it was given the designation 1 Ceres under the modern system of asteroid numbering. The 2006 debate surrounding Pluto and what constitutes a 'planet' led to Ceres being considered for reclassification as a planet. Sony VAIO VGN-AW170Y/Q Battery A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet".Sony VAIO VGN-AW180Y/Q Battery Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun.[48] It was not accepted, and in its place an alternate definition came into effect as of 24 August 2006, carrying the additional requirement that a "planet" must have "cleared the neighborhood around its orbit".Sony VAIO VGN-AW190JAH Battery By this definition, Ceres is not a planet because it does not dominate its orbit, sharing it with the thousands of other asteroids in the asteroid belt and constituting only about a third of the total mass. It is instead now classified as adwarf planet. It is sometimes assumed that Ceres has been reclassified as a dwarf planet, and that it is therefore no longer considered an asteroid. Sony VAIO VGN-AW190NAB Battery For example, a news update at spoke of "Pallas, the largest asteroid, and Ceres, the dwarf planet formerly classified as an asteroid",[49] while an IAU question-and-answer posting states, "Ceres is (or now we can say it was) the largest asteroid", though it then speaks of "other asteroids" crossing Ceres's path and otherwise implies that Ceres is still one of the asteroids. Sony VAIO VGN-AW190NBB Battery The Minor Planet Center notes that such bodies may have dual designations.[51] The 2006 IAU decision that classified Ceres as a dwarf planet never addressed whether it is or is not an asteroid, as indeed the IAU has never defined the word 'asteroid' at all, preferring the term 'minor planet' until 2006, and 'small Solar System body' and 'dwarf planet' after 2006. Sony VAIO VGN-AW190NCB Battery Lang (2011) comments, "The [IAU has] added a new designation to Ceres, classifying it as a dwarf planet. [...] By [its] definition, Eris, Haumea, Makemake and Pluto, as well as the largest asteroid, 1 Ceres, are all dwarf planets", and describes it elsewhere as "the dwarf planet–asteroid 1 Ceres". NASA continues to refer to Ceres as an asteroid, saying in a 2011 press announcement that "Dawn will orbit two of the largest asteroids in the Main Belt",[53] as do various academic textbooks. Sony VAIO VGN-AW190YAB Battery Ceres is the largest object in the asteroid belt between Mars and Jupiter.[14] The mass of Ceres has been determined by analysis of the influence it exerts on smaller asteroids. Results differ slightly between researchers.[57] The average of the three most precise values as of 2008 is 9.4×1020 kg. Sony VAIO VGN-AW190YBB Battery With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2×1021 kg mass of the asteroid belt,[22] which is in turn about 4% of the mass of the Moon. The surface area is approximately equal to the land area of India or Argentina.[58] The mass of Ceres is sufficient to give it a nearly spherical shape in hydrostatic equilibrium.[8] Sony VAIO VGN-AW190YCB Battery In contrast, other large asteroids such as2 Pallas,[59] 3 Juno,[60] and in particular 10 Hygiea[61] are known to be somewhat irregular in shape. Ceres's oblateness is inconsistent with an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[8] Sony VAIO VGN-AW190YDB Battery

No comments:

Post a Comment