text: dwarf planet a dwarf planet is a planetary - mass object that is neither a planet nor a natural satellite . that is , it is in direct orbit of the sun , and is massive enough for its gravity to crush itself into a hydrostatic equilibrium shape ( usually a spheroid ) , but has not cleared the neighborhood of other material around its orbit . the term dwarf planet was adopted in 2006 as part of a three - way categorization of bodies orbiting the sun , brought about by an increase in discoveries of objects farther away from the sun than neptune that rivaled pluto in size , and finally precipitated by the discovery of an even more massive object , eris . 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 astronomer mike brown , who discovered eris and other new dwarf planets , but has been rejected by alan stern , who had coined the term dwarf planet in april 1991 . the international astronomical union ( iau ) currently recognizes five dwarf planets : ceres , pluto , haumea , makemake , and eris . brown criticizes this official recognition : " a reasonable person might think that this means that there are five known objects in the solar system which fit the iau definition of dwarf planet , but this reasonable person would be nowhere close to correct . " 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 . individual astronomers recognize several of these , and in august 2011 mike brown published a list of 390 candidate objects , ranging from " nearly certain " to " possible " dwarf planets . brown currently identifies eleven known objects — the five accepted by the iau plus 2007 or10 , quaoar , sedna , orcus , ( ) 2002 and — as " virtually certain " , with another dozen highly likely . stern states that there are more than a dozen known dwarf planets . however , only two of these bodies , ceres and pluto , have been observed in enough detail to demonstrate that they actually fit the iau 's definition . the iau accepted eris as a dwarf planet because it is more massive than pluto . they 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 ) are to be named under the assumption that they are dwarf planets . the only two such objects known at the time , makemake and haumea , went through this naming procedure and were declared to be dwarf planets . the question of whether other likely objects are dwarf planets has never been addressed by the iau . the classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed . history of the concept starting in 1801 , astronomers discovered ceres and other bodies between mars and jupiter which were for some decades considered to be planets . between then and around 1851 , when the number of planets had reached 23 , astronomers started using the word asteroid for the smaller bodies and then stopped naming or classifying them as planets . with the discovery of pluto in 1930 , most astronomers considered the solar system to have nine planets , along with thousands of significantly smaller bodies ( asteroids and comets ) . for almost 50 years pluto was thought to be larger than mercury , but with the discovery in 1978 of pluto 's moon charon , it became possible to measure pluto 's mass accurately and to determine that it was much smaller than in initial estimates . 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 the asteroid belt , 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 . many of these shared several of pluto 's key orbital characteristics , 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 and planetoid , started to be used for the bodies now known as dwarf planets . by 2005 , three trans - neptunian objects comparable in size to pluto ( quaoar , sedna , and eris ) had been reported . 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 . eris ( then known as 2003 ub313 ) was discovered in january 2005 , which was thought to be slightly larger than pluto , and some reports informally referred to it as the tenth planet . as a consequence , the issue became a matter of intense debate during the iau general assembly in august 2006 . the iau 's initial draft proposal included charon , eris , and ceres in the list of planets . 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 of planetesimals . dropping charon from the list , the new proposal also removed pluto , ceres , and eris , because they have not cleared their orbits . the iau 's final resolution 5a preserved this three - category system for the celestial bodies orbiting the sun . it 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 : ( 1 ) a 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 , ( 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 a satellite . ( 3 ) all other objects , 3 except satellites , orbiting the sun shall be referred to collectively as " small solar system bodies . " footnotes : 1 the eight planets are : mercury , venus , earth , mars , jupiter , saturn , uranus , and neptune . 2 an iau process will be established to assign borderline objects either dwarf planet or other status . 3 these currently include most of the solar system asteroids , most trans - neptunian objects ( tnos ) , comets , and other small bodies . although concerns were raised about the classification of planets orbiting other stars , the issue was not resolved ; it was proposed instead to decide this only when such objects start being observed . name the term dwarf planet has itself been somewhat controversial , as it implies that these bodies are planets , much as dwarf stars are stars . this is the conception of the solar system that stern promoted when he coined the phrase . the older word planetoid ( " having the form of a planet " ) has no such connotation , and is also used by astronomers for bodies that fit the iau definition . brown states that planetoid is " a perfectly good word " that has been used for these bodies for years , and that the use of the term dwarf planet for a non - planet is " dumb " , but that it was motivated by an attempt by the iau division iii plenary session to reinstate pluto as a planet in a second resolution . indeed , the draft of resolution 5a had called these median bodies planetoids , but the plenary session voted unanimously to change the name to dwarf planet . the second resolution , 5b , defined dwarf planets as a subtype of planet , as stern had originally intended , distinguished from the other eight that 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 . however , resolution 5b was defeated in the same session that 5a was passed . because of the semantic inconsistency of a dwarf planet not being a planet due to the failure of resolution 5b , alternative terms such as and were discussed , but there was no consensus among the csbn to change it . in most languages equivalent terms have been created by translating dwarf planet more - or - less literally : french , spanish planeta , german , russian planeta ( ) , arabic ( ) , chinese ( ) , korean ( ; ) , but japanese and latin are exceptions : in japanese they are called ( ) meaning " " or " almost - planets " , and the modern latin name , ( or following the greek ) , is a diminutive derivation of planeta , hence also meaning something less than a planet . iau resolution 6a of 2006 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 , perhaps due to objections from geologists that this would create confusion with their pluton . on june 11 , 2008 , the iau executive committee announced a name , plutoid , and a definition : all trans - neptunian dwarf planets are plutoids , 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 . ... in fact , a vote taken by the wg - psn subsequent to the executive committee meeting has rejected the use of that term " , and it has not come into common use among astronomers . characteristics orbital dominance alan stern and harold f. levison introduced a parameter λ ( lambda ) , expressing the likelihood of an encounter resulting in a given deflection of orbit . the value of this parameter in stern 's model is proportional to the square of the mass and inversely proportional to the period . 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 . a gap of five orders of magnitude in λ was found between the smallest terrestrial planets and the largest asteroids and kuiper belt objects . 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 " : planets are able to remove smaller bodies near their orbits by collision , capture , or gravitational disturbance ( or establish orbital resonances that prevent collisions ) , whereas dwarf planets lack the mass to do so . soter went on to propose a parameter he called the planetary discriminant , designated with the symbol µ ( mu ) , 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 . there are several other schemes that try to differentiate between planets and dwarf planets , but the 2006 definition uses this concept . hydrostatic equilibrium 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 have an irregular shape . 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 the pressure is sufficient to overcome its internal compressive strength and it achieves hydrostatic equilibrium . at this point a body is as round as it is possible to be , given its rotation and tidal effects , and is an ellipsoid in shape . this is the defining limit of a dwarf planet . 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 oblate or even scalene it becomes . 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 . if the body has a massive nearby companion , then tidal forces come into effect as well , distorting it into a prolate spheroid . an example of this is jupiter 's moon io , which is the most volcanically active body in the solar system due to effects of tidal heating . tidal forces also cause a body 's rotation to gradually become tidally locked , such that it always presents the same face to its companion . an extreme example of this is the pluto – charon system , where both bodies are tidally locked to each other . earth 's moon is also tidally locked , as are many satellites of the gas giants . the upper and lower size and mass limits of dwarf planets have not been specified by the iau . 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 . 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 . 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 " , but this was not retained in the final draft . empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object . 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 . in the asteroid belt , ceres is the only body that clearly surpasses the silicaceous limit ( though it is actually a rocky – icy body ) , 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 . dwarf planets and possible dwarf planets 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 . as of january 2015 , about 150 known tnos are thought to be probably dwarf planets , although only rough estimates of the diameters of most of these objects are available . a team is investigating thirty of these , and think that the number will eventually prove to be around 200 in the kuiper belt , with thousands more beyond . the iau has recognized five bodies as dwarf planets since 2008 : ceres , pluto , eris , haumea , and makemake . ceres and pluto are known to be dwarf planets through direct observation . eris is recognized as a dwarf planet because it is more massive than pluto ( measurements by new horizons indicate that pluto 's diameter is larger than that of eris ) , whereas haumea and makemake qualify based on their absolute magnitudes . in relative distance from the sun , the five are : 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 . haumea – discovered on december 28 , 2004 . accepted by the iau as a dwarf planet on september 17 , 2008 . 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 . mike brown considers an additional six trans - neptunian objects to be " nearly certainly " dwarf planets with diameters at or above 900 kilometers . these objects are : orcus – discovered on february 17 , 2004 2002 – discovered on 18 june 2002 – discovered on september 22 , 2004 quaoar – discovered on june 5 , 2002 2007 or10 – discovered on july 17 , 2007 sedna – discovered on november 14 , 2003 tancredi et al. advised the iau to officially accept orcus , sedna and quaoar . in addition , gonzalo tancredi considers the five tnos varuna , ixion , 2003 , 2004 , and 2002 to be dwarf planets as well . these objects are also recognized by mike brown and classified as " highly likely " . an extensive table compares the dwarf planet candidates of the two planetary astronomers in detail . vesta , the next - most - massive body in the asteroid belt after ceres , is roughly spherical , deviating mainly because of massive impacts that formed rheasilvia and veneneia crater after it solidified . furthermore , its triaxial dimensions are not consistent with hydrostatic equilibrium . triton is thought to be a captured dwarf planet . phoebe is a captured body that , like vesta , is no longer in hydrostatic equilibrium , but is thought to have been so early in its history . exploration on march 6 , 2015 , the dawn spacecraft began to orbit ceres , becoming the first spacecraft to orbit a dwarf planet . on 14 july 2015 , the new horizons space probe flew by pluto and its five moons . dawn has also explored the former dwarf planet , vesta . phoebe has been explored by cassini ( most recently ) and voyager 2 , which also explored triton . these three are thought to be former dwarf planets and therefore their exploration helps in the study of the evolution of dwarf planets . contention in the immediate aftermath of the iau definition of dwarf planet , a number of scientists expressed their disagreement with the iau resolution . campaigns included car bumper stickers and t - shirts . mike brown ( the discoverer of eris ) agrees with the reduction of the number of planets to eight . nasa has announced that it will use the new guidelines established by the iau . 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 . thus , in 2011 , he still referred to pluto as a planet , and accepted other dwarf planets such as ceres and eris , as well as the larger moons , as additional planets . several years before the iau definition , he used orbital characteristics to separate " " ( the dominant eight ) from " " ( the dwarf planets ) , considering both types " planets " . planetary - mass moons nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity , and seven of them are more massive than either eris or pluto . they are not physically distinct from the dwarf planets , but are not dwarf planets because they do not directly orbit the sun . the seven that are more massive than eris are the moon , the four galilean moons of jupiter ( io , europa , ganymede , and callisto ) , one moon of saturn ( titan ) , and one moon of neptune ( triton ) . the others are six moons of saturn ( mimas , enceladus , tethys , dione , rhea , and iapetus ) , five moons of uranus ( miranda , ariel , umbriel , titania , and oberon ) , and one moon of pluto ( charon ) . there are additional possibilities among tnos , including dysnomia orbiting eris . alan stern calls these moons " satellite planets " , one of three categories of planet together with dwarf planets and classical planets . the term ( " planetary - mass object " ) covers all three . in a draft resolution for the iau definition of planet , both pluto and charon would have been considered dwarf planets in a binary system , given that they both satisfied the mass and shape requirements for dwarf planets and revolved around a common center of mass located between the two bodies ( rather than within one of the bodies ) . the iau currently states that charon is not considered to be a dwarf planet and is just a satellite of pluto , although the idea that charon might qualify to be a dwarf planet in its own right may be considered at a later date . the location of the barycenter depends not only on the relative masses of the bodies , but also on the distance between them ; the barycenter of the sun – jupiter orbit , for example , lies outside the sun .