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Date Posted: 06:35:39 02/10/16 Wed
Author: robin hasan
Subject: 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.[1][2]

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,[1] 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.[3] 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,[4][5][6] who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern,[7][8] who had coined the term dwarf planet in 1990.[9]

The International Astronomical Union (IAU) currently recognizes five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.[10] 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."[11]

It is suspected that another hundred or so known objects in the Solar System are dwarf planets.[12] 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.[13] Individual astronomers recognize several of these,[12] and in August 2011 Mike Brown published a list of 390 candidate objects, ranging from "nearly certain" to "possible" dwarf planets.[11] Brown currently identifies eleven known objects—the five accepted by the IAU plus 2007 OR10, Quaoar, Sedna, Orcus, (307261) 2002 MS4 and Salacia—as "virtually certain", with another dozen highly likely.[12] Stern states that there are more than a dozen known dwarf planets.[13]

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)[14] are to be named under the assumption that they are dwarf planets.[15] 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
Main article: IAU definition of planet

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.[17]

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,[18][19] 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.[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 the asteroid belt, Ceres, it was one-fifth that of Earth's Moon.[21] 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.[22]

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.[23] 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 subplanet and planetoid, started to be used for the bodies now known as dwarf planets.[24][25] By 2005, three trans-Neptunian objects comparable in size to Pluto (Quaoar, Sedna, and Eris) had been reported.[26] It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified.[27] 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.[28]

Eris (then known as 2003 UB313) was discovered in January 2005,[29] which was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet.[30] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006.[31] 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.[32]

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 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, (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,[16] the issue was not resolved; it was proposed instead to decide this only when such objects start being observed.[32]
Name

The term dwarf planet has itself been somewhat controversial, as it implies that these bodies are planets, much as dwarf stars are stars.[33] 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.[34] 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.[35] Indeed, the draft of Resolution 5A had called these median bodies planetoids,[36][37] but the plenary session voted unanimously to change the name to dwarf planet.[1] 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.[35] Because of the semantic inconsistency of 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.[38]

In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French plančte 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), but Japanese and Latin are exceptions: In Japanese they are called junwakusei (準惑星) meaning "subplanets" or "almost-planets", and the modern Latin name, planetula (or planetion following the Greek), is a diminutive derivation of planeta, hence also meaning something less than a planet.

IAU Resolution 6a of 2006[39] 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.[1] On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids,[15] 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 specific term",[40] and it has not come into common use among astronomers.

Alan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the likelihood of an encounter resulting in a given deflection of orbit.[43] 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.[41]

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.[43] 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.[41] There are several other schemes that try to differentiate between planets and dwarf planets,[7] but the 2006 definition uses this concept.[1]
Hydrostatic equilibrium
Main article: 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.[44]

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.

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.[47] 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.[12] 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.[47]

The IAU has recognized five bodies as dwarf planets since 2008: Ceres, Pluto, Eris, Haumea, and Makemake.[48] Ceres and Pluto are known to be dwarf planets through direct observation.[49] 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.[10][39] 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"[12] dwarf planets with diameters at or above 900 kilometers. These objects are:

Orcus – discovered on February 17, 2004
2002 MS4 – discovered on 18 June 2002
Salacia – 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 AZ84, 2004 GV9, and 2002 AW197 to be dwarf planets as well.[49] 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.

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