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71-404: The Hillary Montes / ˈ h ɪ l ə r i ˈ m ɒ n t iː z / or / ˈ m ɒ n t eɪ z / (less officially, Hillary Mountains ) are a mountain range that reach 3.5 km (2.2 mi; 11,000 ft) above the surface of the dwarf planet Pluto . They are located northwest of Tenzing Montes in the southwest border area of Sputnik Planitia in the south of Tombaugh Regio (or

142-900: A dwarf planet not being a planet due to the failure of Resolution 5B, alternative terms such as nanoplanet and subplanet 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 planète naine , Spanish planeta enano , German Zwergplanet , Russian karlikovaya planeta ( карликовая планета ), Arabic kaukab qazm ( كوكب قزم ), Chinese ǎixíngxīng ( 矮 行星 ), Korean waesohangseong ( 왜소행성 / 矮小行星 ) or waehangseong ( 왜행성 / 矮行星 ), but in Japanese they are called junwakusei ( 準惑星 ), meaning "quasi-planets" or "peneplanets" ( pene- meaning "almost"). IAU Resolution 6a of 2006 recognizes Pluto as "the prototype of

213-423: A body that may be scalene due to rapid rotation is Haumea , which is twice as long on its major axis as it is at the poles. If the body has a massive nearby companion, then tidal forces gradually slow its rotation until it is tidally locked; that is, it always presents the same face to its companion. Tidally locked bodies are also scalene, though sometimes only slightly so. Earth's Moon is tidally locked, as are all

284-565: A body would fit the definition of dwarf planet rather than planet. Indeed, Mike Brown set out to find such an object. The lower limit is determined by the requirements of achieving and retaining hydrostatic equilibrium, but the size or mass at which an object attains and retains equilibrium depends on its composition and thermal history, not simply its mass. An IAU 2006 press release question-and-answer section estimated that objects with mass above 0.5 × 10  kg and radius greater than 400 km would "normally" be in hydrostatic equilibrium (

355-510: A class of planets. The IAU decided that dwarf planets are not to be considered planets, but kept Stern's term for them. Other terms for the IAU definition of the largest subplanetary bodies that do not have such conflicting connotations or usage include quasi-planet and the older term planetoid ("having the form of a planet"). Michael E. Brown stated that planetoid is "a perfectly good word" that has been used for these bodies for years, and that

426-399: 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

497-549: A dwarf planet in a 2022–2023 annual report. More bodies have been proposed, such as Salacia and (307261) 2002 MS 4 by Brown; Varuna and Ixion by Tancredi et al., and (532037) 2013 FY 27 by Sheppard et al. Most of the larger bodies have moons, which enables a determination of their mass and thus their density, which inform estimates of whether they could be dwarf planets. The largest TNOs that are not known to have moons are Sedna, (307261) 2002 MS 4 , (55565) 2002 AW 197 and Ixion. In particular, Salacia has

568-404: A dwarf planet; it is a borderline body by many criteria, and is therefore italicized. Charon, a moon of Pluto that was proposed as a dwarf planet by the IAU in 2006, is included for comparison. Those objects that have absolute magnitude greater than +1, and so meet the threshold of the joint planet–minor planet naming committee of the IAU, are highlighted, as is Ceres, which the IAU has assumed

639-402: A known mass and diameter, putting it as a borderline case by the IAU's 2006 Q&A. At the time Makemake and Haumea were named, it was thought that trans-Neptunian objects (TNOs) with icy cores would require a diameter of only about 400 km (250 mi), or 3% the size of Earth – the size of the moons Mimas , the smallest moon that is round, and Proteus , the largest that

710-516: A minimum diameter of 838 km at the maximum geometric albedo of 1) was to be named by a joint committee consisting of the Minor Planet Center and the planetary working group of the IAU. At the time (and still as of 2023), the only bodies to meet this threshold were Haumea and Makemake . These bodies are generally assumed to be dwarf planets, although they have not yet been demonstrated to be in hydrostatic equilibrium, and there

781-452: 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,

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852-513: A planet, and accepted other likely 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 "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets". Names for large subplanetary bodies include dwarf planet , planetoid (more general term), meso-planet (narrowly used for sizes between Mercury and Ceres), quasi-planet , and (in

923-475: A round shape. Because this requires a much lower mass than gravitationally dominating the region of space near their orbit, there are a population of objects that are massive enough to have a world-like appearance and planetary geology, but not massive enough to clear their neighborhood. Examples are Ceres in the asteroid belt and Pluto in the Kuiper belt. Dynamicists usually prefer using gravitational dominance as

994-417: A similar parameter Π (upper case Pi ). It is based on theory, avoiding the empirical data used by Λ . Π > 1  indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets. There are several other schemes that try to differentiate between planets and dwarf planets, but the 2006 definition uses this concept. Enough internal pressure, caused by

1065-454: A small asteroid that lacks internally driven geology. This necessitated the creation of the category of dwarf planets to describe this intermediate class. Alan Stern and Harold F. Levison introduced a parameter Λ (upper case lambda ) in 2000, 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

1136-472: A software engineer in Massachusetts. NASA has used his Haumea, Eris, and Makemake symbols, as well as the traditional astrological symbol for Pluto [REDACTED] when referring to it as a dwarf planet. Symbols have been proposed for the next-largest named candidates, but do not have consistent usage among astrologers. The Unicode proposal for Quaoar, Orcus, Haumea, Makemake, and Gonggong mentions

1207-410: 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 . Resolution 5B was defeated in the same session that 5A was passed. Because of the semantic inconsistency of

1278-552: A very dark surface. Despite this determination, Grundy et al. call it "dwarf-planet sized", while calling Orcus a dwarf planet. Later studies on Varda suggest that its density may also be high, though a low density could not be excluded. In 2023, Emery et al. wrote that near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 suggests that Sedna, Gonggong, and Quaoar underwent internal melting, differentiation, and chemical evolution, like

1349-725: Is a dwarf planet since they first debated the concept. The masses of given dwarf planets are listed for their systems (if they have satellites) with exceptions for Pluto and Orcus. Ceres [REDACTED] and Pluto [REDACTED] received planetary symbols, as they were considered to be planets when they were discovered. By the time the others were discovered, planetary symbols had mostly fallen out of use among astronomers. Unicode includes symbols for Quaoar [REDACTED] , Sedna [REDACTED] , Orcus [REDACTED] , Haumea [REDACTED] , Eris [REDACTED] , Makemake [REDACTED] , and Gonggong [REDACTED] that are primarily used by astrologers: they were devised by Denis Moskowitz,

1420-414: Is close to equilibrium, but some gravitational anomalies remain unexplained. Eris is generally assumed to be a dwarf planet because it is more massive than Pluto. In order of discovery, these three bodies are: The IAU only established guidelines for which committee would oversee the naming of likely dwarf planets: any unnamed trans-Neptunian object with an absolute magnitude brighter than +1 (and hence

1491-460: Is in hydrostatic equilibrium, a global layer of liquid on its surface would form a surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. The body will have a spherical shape if it does not rotate and an ellipsoidal one if it does. The faster it rotates, the more oblate or even scalene it becomes. If such a rotating body were heated until it melts, its shape would not change. The extreme example of

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1562-477: Is not presently clear whether a lower size bound will be established as part of the definition of small Solar System bodies in the future, or if it will encompass all material down to the level of meteoroids , the smallest macroscopic bodies in orbit around the Sun. (On a microscopic level there are even smaller objects such as interplanetary dust , particles of solar wind and free particles of hydrogen .) Except for

1633-506: Is not – to relax into gravitational equilibrium. Researchers thought that the number of such bodies could prove to be around 200 in the Kuiper belt , with thousands more beyond. This was one of the reasons (keeping the roster of 'planets' to a reasonable number) that Pluto was reclassified in the first place. Research since then has cast doubt on the idea that bodies that small could have achieved or maintained equilibrium under

1704-512: Is some disagreement for Haumea: These five bodies – the three under consideration in 2006 (Pluto, Ceres and Eris) plus the two named in 2008 (Haumea and Makemake) – are commonly presented as the dwarf planets of the Solar System, though the limiting factor (albedo) is not what defines an object as a dwarf planet. The astronomical community commonly refers to other larger TNOs as dwarf planets as well. At least four additional bodies meet

1775-604: The New Horizons space probe flew by Pluto and its five moons. Ceres displays such evidence of an active geology as salt deposits and cryovolcanos , while Pluto has water-ice mountains drifting in nitrogen-ice glaciers, as well as a significant atmosphere. Ceres evidently has brine percolating through its subsurface, while there is evidence that Pluto has an actual subsurface ocean. Small Solar System bodies This encompasses all comets and all minor planets other than those that are dwarf planets . Thus SSSBs are:

1846-639: The nine largest candidates are dwarf planets – in rough order of size, Pluto , Eris , Haumea , Makemake , Gonggong , Quaoar , Ceres , Orcus , and Sedna . Considerable uncertainty remains over the tenth largest candidate Salacia , which may thus be considered a borderline case. Of these ten, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine

1917-419: 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. Jean-Luc Margot refined Stern and Levison's concept to produce

1988-693: The IAU Executive Committee announced a new term, plutoid , and a definition: all trans-Neptunian dwarf planets are plutoids. Other departments of the IAU have rejected the term: ...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

2059-411: The IAU and perhaps the majority of astronomers have excluded them from the roster of planets. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter that for decades were considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid (from Greek, meaning 'star-like' or 'star-shaped') for

2130-945: The IAU plus Gonggong , Quaoar , Sedna , Orcus , (307261) 2002 MS 4 , and Salacia ) as "near certain" to be dwarf planets, and another 16, with diameter greater than 600 km, as "highly likely". Notably, Gonggong may have a larger diameter ( 1230 ± 50 km ) than Pluto's round moon Charon (1212 km). But in 2019 Grundy et al. proposed, based on their studies of Gǃkúnǁʼhòmdímà , that dark, low-density bodies smaller than about 900–1000 km in diameter, such as Salacia and Varda , never fully collapsed into solid planetary bodies and retain internal porosity from their formation (in which case they could not be dwarf planets). They accept that brighter (albedo > ≈0.2) or denser (> ≈1.4 g/cc) Orcus and Quaoar probably were fully solid: Orcus and Charon probably melted and differentiated, considering their higher densities and spectra indicating surfaces made of relatively clean H 2 O ice. But

2201-538: The Rheasilvia crater on Vesta, which is the reason Vesta is not usually considered a dwarf planet today. In 2024, Kiss et al. found that Quaoar has an ellipsoidal shape incompatible with hydrostatic equilibrium for its current spin. They hypothesised that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon Weywot . If so, this would resemble

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2272-710: The Solar System: classical planets, dwarf planets, and satellite planets . Dwarf planets were thus conceived of as a category of planet. In 2006, however, the concept was adopted by the International Astronomical Union (IAU) as a category of sub -planetary objects, part of a three-way recategorization of bodies orbiting the Sun: planets, dwarf planets, and small Solar System bodies . Thus Stern and other planetary geologists consider dwarf planets and large satellites to be planets, but since 2006,

2343-399: The body's gravitation, will turn a body plastic , and enough 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 and may be rubble piles. Larger objects, where gravity is significant but not dominant, are potato-shaped;

2414-421: The classification of planets orbiting other stars, the issue was not resolved; it was proposed instead to decide this only when dwarf-planet-size objects start to be observed. In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution. Campaigns included car bumper stickers and T-shirts. Mike Brown (the discoverer of Eris) agrees with

2485-402: The comets; the classical asteroids , with the exception of the dwarf planet Ceres ; the trojans ; and the centaurs and trans-Neptunian objects , with the exception of the dwarf planets Pluto , Haumea , Makemake , Quaoar , Orcus , Sedna , Gonggong and Eris and others that may turn out to be dwarf planets . The current definition was included in the 2006 IAU resolution that defined

2556-402: The dynamics of the Solar System, the major distinction is between bodies that gravitationally dominate their neighbourhood (Mercury through Neptune) and those that do not (such as the asteroids and Kuiper belt objects). A celestial body may have a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight, which results in the body acquiring

2627-596: The eight classical planets of the Solar System . The prototypical dwarf planet is Pluto , which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006. Dwarf planets are capable of being geologically active, an expectation that was borne out in 2015 by the Dawn mission to Ceres and the New Horizons mission to Pluto. Planetary geologists are therefore particularly interested in them. Astronomers are in general agreement that at least

2698-419: The expected limit. No other body with a measured mass is close to the expected mass limit, though several without a measured mass approach the expected size limit. Though the definition of a dwarf planet is clear, evidence about whether a given trans-Neptunian object is large and malleable enough to be shaped by its own gravitational field is often inconclusive. There are also outstanding questions relating to

2769-421: The extent of their internal collapse. An object with a cold, relatively pristine surface and a partially collapsed interior should exhibit very distinctive surface geology, with abundant thrust faults indicative of the reduction in total surface area as the interior compresses and shrinks. Salacia was later found to have a somewhat higher density, comparable within uncertainties to that of Orcus, though still with

2840-460: The five TNOs Varuna , Ixion , 2003 AZ 84 , 2004 GV 9 , and 2002 AW 197 to most likely be dwarf planets as well. Since 2011, Brown has maintained a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets, based solely on estimated size. As of September 13, 2019, Brown's list identifies ten trans-Neptunian objects with diameters then thought to be greater than 900 km (the four named by

2911-432: The following symbols for named objects over 600 km diameter: Salacia [REDACTED] , Varda [REDACTED] , Ixion [REDACTED] , Gǃkúnǁʼhòmdímà [REDACTED] and Varuna [REDACTED] . As of 2024, only two missions have targeted and explored dwarf planets up close. On March 6, 2015, the Dawn spacecraft entered orbit around Ceres , becoming the first spacecraft to visit a dwarf planet. On July 14, 2015,

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2982-407: The ice asteroids of the outer Solar System; one attempted definition was that an ice dwarf "is larger than the nucleus of a normal comet and icier than a typical asteroid." Since the Dawn mission, it has been recognized that Ceres is a geologically icy body that may have originated from the outer Solar System. Ceres has since been called an ice dwarf as well. Planetary discriminants of

3053-614: The interpretation of the IAU criterion in certain instances. Consequently the number of currently conformed TNOs which meet the hydrostatic equilibrium criterion is uncertain. The three objects under consideration during the debates leading up to the 2006 IAU acceptance of the category of dwarf planet – Ceres, Pluto and Eris – are generally accepted as dwarf planets, including by those astronomers who continue to classify dwarf planets as planets. Only one of them – Pluto – has been observed in enough detail to verify that its current shape fits what would be expected from hydrostatic equilibrium. Ceres

3124-519: The larger dwarf planets Pluto, Eris, Haumea, and Makemake, but unlike "all smaller KBOs". This is because light hydrocarbons are present on their surfaces (e.g. ethane , acetylene , and ethylene ), which implies that methane is continuously being resupplied, and that methane would likely come from internal geochemistry. On the other hand, the surfaces of Sedna, Gonggong, and Quaoar have low abundances of CO and CO 2 , similar to Pluto, Eris, and Makemake, but in contrast to smaller bodies. This suggests that

3195-471: The larger of these bodies would also have to be classified as planets, or Pluto would have to be reclassified, much as Ceres had been reclassified after the discovery of additional asteroids. This led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid , started to be used for the bodies now known as dwarf planets. Astronomers were also confident that more objects as large as Pluto would be discovered, and

3266-574: The largest, which are in hydrostatic equilibrium , natural satellites (moons) differ from small Solar System bodies not in size, but in their orbits. The orbits of natural satellites are not centered on the Sun , but around other Solar System objects such as planets, dwarf planets , and small Solar System bodies. Some of the larger small Solar System bodies may be reclassified in future as dwarf planets, pending further examination to determine whether or not they are in hydrostatic equilibrium . The orbits of

3337-541: The list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi : They proposed an intermediate category for objects large enough to be round but that had not cleared their orbits of planetesimals . Beside dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits. Although concerns were raised about

3408-444: The lower albedos and densities of Gǃkúnǁʼhòmdímà , 55637 , Varda, and Salacia suggest that they never did differentiate, or if they did, it was only in their deep interiors, not a complete melting and overturning that involved the surface. Their surfaces could remain quite cold and uncompressed even as the interior becomes warm and collapses. The liberation of volatiles could further help transport heat out of their interiors, limiting

3479-406: The more massive the body, the higher its internal pressure, the more solid it is and the more rounded its shape, until the pressure is enough to overcome its compressive strength and it achieves hydrostatic equilibrium . Then, 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. If an object

3550-559: The name Hillary Montes was officially approved together with the names of Tombaugh Regio and twelve other nearby surface features. The Hillary Montes rise to 3.5 km (2.2 mi; 11,000 ft) high from base to peak, about half as high as the Tenzing Montes . Dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun , massive enough to be gravitationally rounded , but insufficient to achieve orbital dominance like

3621-406: The nature of these worlds. Only one, Sedna, has neither been visited nor has any known moons, making an accurate estimate of mass difficult. Some astronomers include many smaller bodies as well, but there is no consensus that these are likely to be dwarf planets. The term dwarf planet was coined by planetary scientist Alan Stern as part of a three-way categorization of planetary-mass objects in

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3692-652: The number of planets would start growing quickly if Pluto were to remain classified as a planet. Eris (then known as 2003 UB 313 ) was discovered in January 2005; it 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

3763-426: The part of Tombaugh Regio south of the equator). The Hillary Montes were first viewed by the New Horizons spacecraft on 14 July 2015, and announced by NASA on 24 July 2015. The mountains are named after Sir Edmund Hillary , New Zealand mountaineer , who, along with Nepalese Sherpa mountaineer Tenzing Norgay , were the first climbers to reach the summit of Mount Everest on 29 May 1953. On 7 September 2017,

3834-416: The planets ( white  ), and of the largest known dwarf planet ( light purple  ) in each orbital population ( asteroid belt , Kuiper belt , scattered disc , sednoids ). All other known objects in these populations have smaller discriminants than the one shown. The category dwarf planet arose from a conflict between dynamical and geophysical ideas of what a useful conception of a planet would be. In terms of

3905-582: The preliminary criteria of Brown, of Tancredi et al., of Grundy et al., and of Emery et al. for identifying dwarf planets, and are generally called dwarf planets by astronomers as well: For instance, JPL/NASA called Gonggong a dwarf planet after observations in 2016, and Simon Porter of the Southwest Research Institute spoke of "the big eight [TNO] dwarf planets" in 2018, referring to Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar , Sedna and Orcus . The IAU itself has called Quaoar

3976-488: The reduction of the number of planets to eight. NASA announced in 2006 that it would use the new guidelines established by the IAU. 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

4047-427: The rounded satellites of the gas giants. Pluto and Charon are tidally locked to each other, as are Eris and Dysnomia , and probably also Orcus and Vanth . There are no specific size or mass limits of dwarf planets, as those are not defining features. There is no clear upper limit: an object very far out in the Solar System that is more massive than Mercury might not have had time to clear its neighbourhood, and such

4118-488: The shape ... would normally be determined by self-gravity ), but that all borderline cases would need to be determined by observation . This is close to what as of 2019 is believed to be roughly the limit for objects beyond Neptune that are fully compact, solid bodies, with Salacia ( r = 423 ± 11 km , m = (0.492 ± 0.007) × 10  kg ) being a borderline case both for the 2006 Q&A expectations and in more recent evaluations, and with Orcus being just above

4189-440: The situation of Saturn's moon Iapetus , which is too oblate for its current spin. Iapetus is generally still considered a planetary-mass moon nonetheless, though not always. The trans-Neptunian objects in the following tables, except Salacia, are agreed by Brown, Tancredi et al., Grundy et al., and Emery et al. to be probable dwarf planets, or close to it. Salacia has been included as the largest TNO not generally agreed to be

4260-503: The smaller bodies and began to distinguish them as minor planets rather than major planets . With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine major 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

4331-428: 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

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4402-416: The term planet , demoting the status of Pluto to that of dwarf planet . In the context, it should be interpreted as, "All objects other than planets and dwarf planets orbiting the Sun shall be referred to collectively as 'Small Solar System Bodies'. The definition excludes interstellar objects traveling through the Solar System, such as the interstellar interlopers 1I/ ʻOumuamua and 2I/Borisov . It

4473-405: The threshold for dwarf planethood in the trans-Neptunian region is a diameter of ~900 km (thus including only Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Sedna), and that even Salacia may not be a dwarf planet. A 2023 study of (307261) 2002 MS 4 shows that it probably has an extremely large crater, whose depth takes up 5.7% of its diameter: this is proportionally larger than

4544-400: The threshold for planethood, because from their perspective smaller bodies are better grouped with their neighbours, e.g. Ceres as simply a large asteroid and Pluto as a large Kuiper belt object. Geoscientists usually prefer roundness as the threshold, because from their perspective the internally driven geology of a body like Ceres makes it more similar to a classical planet like Mars, than to

4615-414: The transneptunian region) plutoid . Dwarf planet , however, was originally coined as a term for the smallest planets, not the largest sub-planets, and is still used that way by many planetary astronomers. Alan Stern coined the term dwarf planet , analogous to the term dwarf star , as part of a three-fold classification of planets, and he and many of his colleagues continue to classify dwarf planets as

4686-399: The typical conditions of the Kuiper belt and beyond. Individual astronomers have recognized a number of objects as dwarf planets or as likely to prove to be dwarf planets. In 2008, Tancredi et al. advised the IAU to officially accept Orcus, Sedna and Quaoar as dwarf planets (Gonggong was not yet known), though the IAU did not address the issue then and has not since. Tancredi also considered

4757-472: The use of that specific term..." The category of 'plutoid' captured an earlier distinction between the 'terrestrial dwarf' Ceres and the 'ice dwarfs' of the outer Solar system, part of a conception of a threefold division of the Solar System into inner terrestrial planets , central giant planets , and outer ice dwarfs , of which Pluto was the principal member. 'Ice dwarf' also saw some use as an umbrella term for all trans-Neptunian minor planets , or for

4828-415: 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

4899-424: The vast majority of small Solar System bodies are located in two distinct areas, namely the asteroid belt and the Kuiper belt . These two belts possess some internal structure related to perturbations by the major planets (particularly Jupiter and Neptune , respectively), and have fairly loosely defined boundaries. Other areas of the Solar System also encompass small bodies in smaller concentrations. These include

4970-402: Was a 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, the plutinos . It became clear that either

5041-440: Was much smaller than 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 had only one-fifth the mass of Earth's Moon . Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination , it became evident that it

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