A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite. s are significant physical entities, associations or structures which current Science has confirmed to exist in Space. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star The Sun (Sol is the Star at the center of the Solar System. Gravitation is a natural Phenomenon by which objects with Mass attract one another Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. ^[1][2] More explicitly, it has to have sufficient mass for its self-gravity to overcome rigid body forces in order to assume a hydrostatic equilibrium and acquire a near-spherical shape. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Physics, a rigid body is an idealization of a solid body of finite size in which Deformation is neglected Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe

The term dwarf planet was adopted in 2006 as part of a three-way classification of bodies orbiting the Sun. ^[3] This classification states that bodies that are large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to have acquired a round shape are defined as small solar system bodies. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Small Solar System Body ( SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that Dwarf planets come in between and have been called by some astronomers as “something that looks like a planet, but is not a planet. ”^[4] Even now there is no uncontested distinction between the term planet and dwarf planet. The definition officially adopted by the IAU in 2006 has been both praised and criticized, and remains disputed by some scientists.

Artist's impression of Pluto (background) and Charon (foreground). Charon (ˈʃærən; also, as in Χάρων) discovered in 1978 is either the largest Moon of Pluto or the smaller member of a double Pluto, considered a planet for 76 years, was reclassified as a dwarf planet in 2006.

The IAU currently recognizes only three dwarf planets – Ceres, Pluto, and Eris – but it is suspected that at least another 42 discovered objects in the Solar System might belong in this category. Ceres (ˈsɪəriːz The Solar System consists of the Sun and those celestial objects bound to it by Gravity. ^[4] There are estimates that the actual number of dwarf planets might increase to 200 when the entire region known as the Kuiper belt is fully explored, and that the number might increase to around 2000 when objects outside this region are accounted for. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending ^[4] The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed,^[5] although if they were detectable they would not be considered planets. ^[6]

History of the name

Main article: 2006 definition of planet

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. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — For almost 50 years Pluto was thought to be larger than Mercury,^[7][8] but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure the mass of Pluto accurately and it was noticed that actual mass was much smaller than the initial estimations. Charon (ˈʃærən; also, as in Χάρων) discovered in 1978 is either the largest Moon of Pluto or the smaller member of a double ^[9] 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. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. ^[10] Furthermore, having some unusual characteristics such as large orbital eccentricity and a high orbital inclination, it became evident that it was a completely different body from any of the other planets. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape Inclination in general is the Angle between a Reference plane and another plane or axis of direction ^[11]

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. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending ^[12] 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. In Astronomy, a plutino is a Trans-Neptunian object in 23 mean motion resonance with Neptune. 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 a dwarf planets. Minor planet is a term used since the 19th century to describe objects such as Asteroids that are in Orbit around the Sun but are not Planets Minor planet is a term used since the 19th century to describe objects such as Asteroids that are in Orbit around the Sun but are not Planets ^[13][14] By 2005, three other bodies (Quaoar, Sedna, and Eris) comparable to Pluto in terms of size and orbit had been reported in the scientific literature. TemplateInfobox Planet.--> 90377 Sedna (ˈsɛdnə) is a Trans-Neptunian ^[15] It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. ^[16] 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. ^[17]

In 2006, Eris (known at that time as 2003 UB₃₁₃) was determined to be slightly larger than Pluto, and some reports unofficially referred to it as the tenth planet. ^[18] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006. ^[19] IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. Charon (ˈʃærən; also, as in Χάρων) discovered in 1978 is either the largest Moon of Pluto or the smaller member of a double 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. Dr Julio Ángel Fernández Alves (born Montevideo, 5 April 1946) is a Uruguayan astronomer and member of the department of astronomy at the Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, since they have not cleared their orbits. ^[20]

The IAU's final resolution preserved this three-category system for the celestial bodies orbiting the Sun. Fernández suggested calling these median objects planetoids,^[21][22] but the IAU's division III plenary session voted unanimously to call them dwarf planets. ^[3] The resolution read, in full:

Although there were concerns about the classification of planets in other solar systems,^[5] this issue was not resolved; it was proposed instead to decide this only when such objects will start being observed. ^[20]

Characteristics

Orbital dominance

Main article: Cleared the neighbourhood

Alan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the probability of an encounter resulting in a given deflection of orbit. S Alan Stern is an American planetary scientist, born 22 November 1957 New Orleans Louisiana married (three children Harold F "Hal" Levison is a Planetary scientist specializing in planetary dynamics Lambda (uppercase Λ, lowercase λ; Λάμβδα or el Λάμδα Lamda is the 11th letter of the Greek alphabet. ^[24] 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. A gap of five orders of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects (third column of the planetary discriminants table to the right). A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our ^[23]

Using this parameter, Steven Soter and other astronomers argued for a distinction between dwarf planets and the other eight planets based on their inability to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance, while dwarf planets lack the mass to do so. Dr Steven Soter, PhD, is an astrophysicist currently holding the positions of scientist-in-residence for New York University 's Environmental Studies Program and of ^[24] In other words, 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). Mu (uppercase Μ, lowercase μ; Μι or el Μυ) is the 12th letter of the Greek alphabet. ^[23] There are several other schemes that try to differentiate between planets and dwarf planets,^[24] but the 2006 definition uses this concept. ^[3]

Size and mass

The upper and lower limits to the size and the mass of dwarf planets have not been specified by the IAU. There is strictly no upper limit, and an object larger or more massive than Mercury that would be considered not to "have cleared the neighbourhood around its orbit" may be classified as a dwarf planet. ^[25] The lower limit is determined by the concept of hydrostatic equilibrium shape, but the size or mass at which an object attains this shape has not been defined. The original draft of the 2006 IAU resolution defined hydrostatic equilibrium shape as applying "to objects with mass above 5×10²⁰ kg [[1. 1×10²¹ lb]] and diameter greater than 800 km [500 mi]",^[5] but this was not retained in the final draft. ^[3]

Empirical observations suggest that the lower limit may vary according to the composition and history of the object. For example, in the asteroid belt, Ceres, with a diameter of 975 km (606 mi), is the only object known to be round. Therefore, it has been suggested that the limit where other rocky bodies (like Ceres) become spherical might be somewhere around 900 km (560 mi). ^[4] Icy bodies like trans-Neptunian objects have less rigid interiors and therefore gravity more easily gives them a spherical shape. A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. ^[4] The smallest icy body known to have achieved hydrostatic equilibrium is Mimas, while the largest irregular one is Proteus; both average slightly more than 400 km (250 mi) in diameter. TemplateInfobox Planet. --> Mimas (ˈmaɪməs, or as Greek There is also an Asteroid called 9313 Protea. Mike Brown (a leading researcher in this field and discoverer of Eris) suggests that the lower limit for an icy dwarf planet is therefore likely to be somewhere under 400 km (250 mi). Education Brown is a Huntsville Alabama native and graduated from Virgil Grissom High School in 1983 Ice dwarf is a term for a small icy Planetary body (or Asteroid or Planetoid) that orbits beyond Neptune, that was coined as part of a conception ^[4]

Current members

Ceres as seen by the Hubble Space Telescope

Pluto in approximately true colours

Eris as seen by the Hubble Space Telescope

The IAU has classified three celestial bodies as dwarf planets:^[26]

1. Ceres – discovered on January 1, 1801 (16 years before Neptune), considered a planet for half a century before reclassification as an asteroid;
2. Pluto – discovered on February 18, 1930, classified as a planet for 76 years;
3. Eris – discovered on October 21, 2003, once referred to as the "tenth planet" in the media. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Ceres (ˈsɪəriːz New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC Year 1801 ( MDCCCI) was a Common year starting on Thursday (see link for calendar of the Gregorian calendar (or a Common year starting on Tuesday Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Events 3102 BC - Epoch (origin of the Kali Yuga. 1229 - The Sixth Crusade: Frederick II Holy Year 1930 ( MCMXXX) was a Common year starting on Wednesday (link will display 1930 calendar of the Gregorian calendar. Events 1512 - Martin Luther joins the theological faculty of the University of Wittenberg. Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar.

Here is a comparison of the main characteristics of these three bodies:

No human space probes have yet visited any of the dwarf planets and therefore these objects are not as studied as any of the other larger bodies in the Solar System. A space probe is a scientific Space exploration mission in which a Robotic spacecraft leaves the Gravity well of Earth and approaches the History First orbital flights The first successful orbital launch was of the Soviet unmanned Sputnik This will presumably change if NASA's spacecrafts Dawn and New Horizons will reach Ceres and Pluto, respectively, in 2015. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program A spacecraft is a Vehicle or machine designed for Spaceflight. Dawn, launched on September 27 2007 is a Robotic spacecraft being sent by NASA on a Space exploration mission to the two most massive members New Horizons is a Robotic spacecraft mission by NASA currently underway ^[34][35]

Candidates

See also: List of dwarf-planet candidates

As with Ceres, the next three largest objects in the main asteroid belt – Vesta, Pallas, and Hygiea^[36] – could also eventually be classified as dwarf planets if it is shown that their shape is determined by hydrostatic equilibrium. TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt TemplateInfobox Planet. --> 2 Pallas (ˈpæləs, or as Παλλάς) is one of the largest ^[37] While uncertain, the present data suggests that it is unlikely for Pallas and Hygiea. Vesta however appears to deviate from hydrostatic equilibrium only because of a large impact that occurred after it solidified;^[38] the definition of dwarf planet does not specifically address this issue. The Dawn probe scheduled to enter orbit around Vesta in 2011 may help clarify matters. Dawn, launched on September 27 2007 is a Robotic spacecraft being sent by NASA on a Space exploration mission to the two most massive members ^[34]

The status of Charon (currently regarded as a satellite of Pluto) remains uncertain, as there is currently no clear definition of what distinguishes a satellite system from a binary (double planet) system. Charon (ˈʃærən; also, as in Χάρων) discovered in 1978 is either the largest Moon of Pluto or the smaller member of a double " Double planet " is an informal term used to describe a Planet with a moon that may be large enough to be considered a planet in its own right a common definition The original draft resolution (5)^[5] presented to the IAU stated that Charon could be considered a planet because:

1. Charon independently would satisfy the size and shape criteria for a dwarf planet status (in the terms of the final resolution);
2. Charon revolves with Pluto around a common barycentre located between the two bodies (rather than within one of the bodies) because Charon's mass is not insignificant relative to that of Pluto. ^[39]

This definition, however, was not preserved in the IAU's final resolution and it is unknown if it will be included in future debates.

TNO candidates

Illustration of the relative sizes, albedos, and colours of the largest TNOs (an artist's depiction of some of these, compared with the Earth, is on the IAU website^[40])

Trans-Neptunian objects (or simply 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. The albedo of an object is the extent to which it diffusely reflects light from the sun A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. ^[4] Although only rough estimates of the diameters of these objects are available, as of August 2006, it was believed that another 42 bodies in the Kuiper belt (besides Pluto and Eris), plus Sedna in the Scattered disc, were likely dwarf planets. TemplateInfobox Planet.--> 90377 Sedna (ˈsɛdnə) is a Trans-Neptunian The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered ^[4][41] A team is investigating another 30 such objects, and believe that the total number will eventually prove to be about 200 in the Kuiper belt, and many more beyond it. ^[4]

Although the status of many of these objects may be debatable, TNOs with estimated diameters approaching that of Ceres are likely to be dwarf planets regardless of measurement errors. These include cubewanos, plutinos, and scattered disc objects (SDOs):

Pluto prototype

Main article: Pluto prototype

The 2006 IAU's Resolution 6a^[26] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category are not specified, but in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects. A pluton in Geology is an Intrusive Igneous rock body that crystallized from a Magma slowly cooling below the surface of the Earth The former name was generally deprecated^[45] and was abandoned in the final draft resolution (6b);^[46] eventually even the latter name failed to win majority approval on a 183–186 vote in the IAU General Assembly on August 24, 2006. Events 49 BC - Julius Caesar 's General Gaius Scribonius Curio is defeated in the Second Battle of the Bagradas River Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. ^[1] Nevertheless, Pluto was recognized as the prototype of a new category of TNOs and the name of this category was announced by IAU to be established at a later date. ^[3]

At an earlier stage in the definition process, the category (then named "pluton") defined members as planets whose rotation period around the Sun was more than 200 Julian years, and whose orbit was more highly inclined and more elliptical than a traditional planetary orbit. In Astronomy, a Julian year (symbol a) is a unit of measurement of Time defined ^[47] This category of Pluto-like objects now only applies to dwarf planets that meet the conditions of being trans-Neptunian and "like Pluto" in terms of period, inclination, and eccentricity. A dwarf planet may or may not be a member of this category, but all members of the category must be dwarf planets. ^[3]

The membership of this class, other than Pluto itself, remains obscure. Eris and the objects listed in the table above qualify in terms of the minimum period, and most exhibit significant orbital eccentricity and inclination (although not always equal to or greater than Pluto's). However, since Quaoar has a much smaller eccentricity and inclination,^[48][27] it is unclear whether or not it would qualify as a member of this category.

Contention

In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution. ^[49] Campaigns included car bumper stickers and T-shirts. ^[50] Mike Brown (the discoverer of the would-have-been-the-10th-planet Eris) agrees with the reduction of the number of planets to eight. Education Brown is a Huntsville Alabama native and graduated from Virgil Grissom High School in 1983 ^[51]

NASA has announced that it will use the new guidelines established by the IAU. ^[52] However, Alan Stern, the director of the 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. S Alan Stern is an American planetary scientist, born 22 November 1957 New Orleans Louisiana married (three children New Horizons is a Robotic spacecraft mission by NASA currently underway ^[53] Thus, he and his team still refer to Pluto as the ninth planet, while accepting the characterisation of dwarf planet for Ceres and Eris (dwarf planet in this case meaning just a small planet). ^[54]

See also

• List of Solar System bodies formerly regarded as planets
• Table of planets and dwarf planets in the Solar System
• Ice dwarf
• Mesoplanet

References

External links

• NPR: Dwarf Planets May Finally Get Respect (David Kestenbaum)
• BBC News: Q&A New planets proposal, 16 August 2006
• Ottawa Citizen: The case against Pluto (P. Events 1384 - The Hongwu Emperor of Ming China, Emperor Dong hears a case of a couple who tore paper money bills while fighting Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. The Ottawa Citizen is an English -language daily newspaper owned by CanWest Global in Ottawa, Canada. Surdas Mohit) August 24, 2006
• James L. Hilton, When Did the asteroids Become Minor Planets?