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Not to be confused with Minor planet.
Ceres as seen with the Hubble Space Telescope. It is the only dwarf planet in the asteroid belt.
Pluto in approximate true colour based on Hubble Space Telescope albedo data
Haumea with its moons, Hiʻiaka and Namaka (artist's conception)
Makemake (artist's conception)
Eris and its moon Dysnomia as seen with the Hubble Space Telescope

A dwarf planet is a planetary-mass object that is neither a planet nor a satellite. More explicitly, the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun[1] that is massive enough for its shape to be controlled by gravitation, but that unlike a planet has not cleared its orbital region of other objects.[2][3] 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 trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris.[4] This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown,[5][6][7] who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern,[8][9] who had coined the term dwarf planet in 1990.[10]

The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris.[11] However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1)[12] are to be named under the assumption that they are dwarf planets.[13] The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.

It is suspected that another hundred or so known objects in the Solar System are dwarf planets.[14] 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.[15] Individual astronomers recognize several of these,[14][15] and in August 2011 Mike Brown published a list of 390 candidate objects, ranging from "nearly certain" to "possible" dwarf planets.[16] Brown identifies nine known objects – the five accepted by the IAU plus 2007 OR10, Sedna, Quaoar, and Orcus – as "virtually certain", with another two dozen highly likely, and there are probably a hundred or so such objects in total.[14]

The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.[17]

Contents

History of the concept [edit]

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.[18]

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,[19][20] 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.[21] 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.[22] 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.[23]

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.[24] Many of these shared some of the key orbital characteristics of Pluto, and Pluto started being seen as the largest member of a new class of objects, plutinos. This led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets.[25][26] By 2005, three trans-Neptunian objects comparable in size to Pluto (Quaoar, Sedna, and Eris) had been reported.[27] It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified.[28] 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.[29]

In 2006, Eris (then known as 2003 UB313) was believed to be slightly larger than Pluto, and some reports unofficially referred to it as the tenth planet.[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, since 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,[17] the issue was not resolved; it was proposed instead to decide this only when such objects start being observed.[32]

Name [edit]

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 which 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 draught 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 which were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. However, Resolution 5B was defeated in the same session that 5A was passed.[35] Because of the grammatical 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 planète naine, Spanish planeta enano, German Zwergplanet, Russian карликовая планета karlikovaya planeta, Arabic كوكب قزم kaukab qazm, Chinese 矮行星 ǎixíngxīng, etc., but Japanese and Latin are exceptions: In Japanese they are called 準惑星 junwakusei '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,[13] though "in part because of an email miscommunication, the WG-PSN [Working Group for Planetary System Nomenclature] was not involved in choosing the word plutoid. ... 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.

Characteristics [edit]

Planetary discriminants[41]
Body Mass (ME*)
 Λ**
 µ***
Mercury 0.055 1.95 × 103 9.1 × 104
Venus 0.815 1.66 × 105 1.35 × 106
Earth 1 1.53 × 105 1.7 × 106
Mars 0.107 9.42 × 102 1.8 × 105
Ceres 0.000 15 8.32 × 10−4 0.33
Jupiter 317.7 1.30 × 109 6.25 × 105
Saturn 95.2 4.68 × 107 1.9 × 105
Uranus 14.5 3.85 × 105 2.9 × 104
Neptune 17.1 2.73 × 105 2.4 × 104
Pluto 0.002 2 2.95 × 10−3 0.077
Haumea 0.000 67 2.68 × 10–4 0.02
Makemake 0.000 67 2.22 × 10–4 0.02[42]
Eris 0.002 8 2.13 × 10−3 0.10

*ME in Earth masses.
**Λ = k M2 a−3/2,
where k = 0.0043 for units of Yg and AU. Λ > 1 for planets.[43]
***µ = M/m, where M is the mass of the body,
and m is the aggregate mass of all the other bodies
that share its orbital zone. µ > 100 for planets.

Orbital dominance [edit]

Main article: Clearing the neighbourhood

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. Following the authors, this value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where Λ > 1 will eventually clear it. 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), while 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,[8] but the 2006 definition uses this concept.[1]

Size and mass [edit]

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 be angular in 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.

The masses of the four largest plutoids, plus Ceres and Charon, relative to the Earth's Moon. The mass of Makemake is a rough estimate. (See plutoid for a graph of several additional likely dwarf planets without Ceres.)

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.[45] 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",[17] but this was not retained in the final draft.[1]

Empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object. 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. [46] 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.

Among icy bodies, the smallest known to be in hydrostatic equilibrium is Mimas, at 396 km and 3.75×1019 kg. The largest irregular body in the outer Solar System is Proteus, nearly-but-not-quite round at 405–435 km and an assumed mass of ≈4.4×1019 kg. Bodies like Mimas may have had a warmer thermal history than Proteus, or their shape may have resolved after a collision.[47] Neither body is pure ice as used to calculate the lowest limit, however, and Mike Brown suggests that the practical lower limit for an icy dwarf planet is likely to be somewhere under 400 km.[48] There are about 100 TNOs currently estimated to be above this size.

Dwarf planets and possible dwarf planets [edit]

Illustration of the relative sizes, albedos, and colours of the largest trans-Neptunian objects
Main article: List of 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.[48] Although only rough estimates of the diameters of these objects are available, about 100 known TNOs are probably dwarf planets.[14] A team is investigating thirty of these, and believe that the number will eventually prove to be about 200 in the Kuiper belt, with thousands more beyond.[48]

The IAU recognizes five bodies as dwarf planets: Ceres, Pluto, Eris, Haumea, and Makemake.[49] Ceres and Pluto are known to be dwarf planets through direct observation.[50] Eris is generally accepted as a dwarf planet because it is more massive than Pluto, while Haumea and Makemake qualified to be assigned names as dwarf planets based on their absolute magnitudes.[11][39] In relative distance from the Sun, the five are:

  1. Ceres 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.
  2. Pluto 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.
  3. Haumea – discovered on December 28, 2004. Accepted by the IAU as a dwarf planet on September 17, 2008.
  4. Makemake – discovered on March 31, 2005. Accepted by the IAU as a dwarf planet on July 11, 2008.
  5. 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 Eris, Pluto, Haumea, Makemake, and four other objects to be "nearly certainly" dwarf planets, as they are massive enough to be in hydrostatic equilibrium even if they are dense (primarily rocky) and at the lower end of their estimated diameters, and an additional twenty-two to be "highly likely".[14] Tancredi et al. found twelve objects to be dwarf planets, and advised the IAU to officially accept three of them.[50] In relative distance from the Sun, the four most likely additional dwarf planets are:

  1. Orcus – discovered on February 17, 2004.
  2. Quaoar – discovered on June 5, 2002.
  3. 2007 OR10 – discovered on July 17, 2007.
  4. Sedna – discovered on November 14, 2003.
Objects recognized by the IAU as dwarf planets
Orbital attributes[51]
Name Region of
Solar System		 Orbital
radius (AU)		 Orbital period
(years)		 Mean orbital
speed (km/s)		 Inclination
to ecliptic		 Orbital
eccentricity		 Planetary
discriminant
Ceres Asteroid belt 2.77 4.60 17.882 10.59° 0.079 0.33
Pluto Kuiper belt (plutino) 39.48 248.09 4.666 17.14° 0.249 0.077
Haumea Kuiper belt (12:7) 43.13 283.28 28.22° 0.195 0.020
Makemake Kuiper belt (cubewano) 45.79 309.9 4.419 28.96° 0.159 0.02
Eris Scattered disc 67.67 557 3.436 44.19° 0.442 0.10
Physical attributes
Name Equatorial
diameter
relative to
the Moon		 Equatorial
diameter
(km)[14]		 Mass
relative to
the Moon		 Mass
(×1021 kg)		 Density
(g/cm3)		 Surface
gravity
(m/s2)		 Escape
velocity
(km/s)		 Axial
inclination		 Rotation
period
(days)		 Moons Surface
temp.
(K)		 Atmosphere
Ceres 28% 974.6±3.2 1.3% 0.94 2.08 0.27 0.51 ≈ 3° 0.38 0 167 none
Pluto 66% 2306±20 17.8% 13.05 2.0 0.58 1.2 119.59° −6.39 5 44 transient
Haumea ≈ 36% 1240 +69
−58		 5.5% 4.01 ± 0.04 2.6–3.3 (?) 0.44 0.84 0.16 2 32 ± 3  ?
Makemake 41% 1478±17 ≈ 4% ? ≈ 3 ? 1.7 ± 0.3[52]  ? 0.32 0 ≈ 30 none[52]
Eris 67% 2326±12 22.7% 16.7 2.5 ≈ 0.8 1.3 ≈ 1 (0.75–1.4) 1 ≈ 42 transient?
Objects recognized by Brown and Tancredi as dwarf planets
Orbital attributes[51]
Name Region of
Solar System		 Orbital
radius (AU)		 Orbital period
(years)		 Mean orbital
speed (km/s)		 Inclination
to ecliptic		 Orbital
eccentricity		 Planetary
discriminant
Orcus Kuiper belt (plutino) 39.17 245.18 20.57° 0.227 0.003
Quaoar Kuiper belt (cubewano) 43.405 285.97 8.00° 0.039 0.007–0.010
2007 OR10 Scattered disc (10:3?) 67.21 550.98 30.70° 0.500  ?
Sedna Detached 518.57 ≈11,400 11.93° 0.853  ?
Physical attributes
Name Equatorial
diameter
relative to
the Moon		 Equatorial
diameter
(km)		 Mass
relative to
the Moon		 Mass
(×1021 kg)		 Density
(g/cm3)		 Surface
gravity
(m/s2)		 Escape
velocity
(km/s)		 Axial
inclination		 Rotation
period
(days)		 Moons Surface
temp.
(K)		 Atmosphere
Orcus ≈ 26% 917±25 0.9% 0.63 ° 0.55 1
Quaoar ≈ 31% 1,070±38 1.8–2.0% 1.4 ± 0.1 ° 0.74 1
2007 OR10 ≈ 37% 1280±210  ?  ?  ? 0
Sedna ≈ 30% 995 ± 80 ≈1.4% ≈1  ? 0.42 0 ≈ 12

No space probes have visited any of these, though this will change in 2015 if NASA's Dawn and New Horizons successfully complete their missions. That year, Dawn is to go into orbit around Ceres, while New Horizons is to fly by Pluto.

After Ceres, the next-most-massive body in the asteroid belt, Vesta, might also be classified as a dwarf planet, as its shape appears to deviate from hydrostatic equilibrium mainly because of massive impacts that occurred after it solidified.[53] The definition of dwarf planet does not address this issue. Data from the Dawn probe, which orbited Vesta in 2011–2012, may help clarify matters.[54]

Planetary-mass moons [edit]

See also: List of moons by diameter

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 members of that class because they do not directly orbit the Sun. The seven which are more massive than Eris are Earth's moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), Titan of Saturn, 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). The term planemo ("planetary-mass object") covers both dwarf planets and such moons, as well as planets.[55] Alan Stern considers them a special category of planets, "satellite planets".[56]

Despite its requirement that dwarf planets orbit the Sun directly, draft resolution (5) presented to the IAU, which considered dwarf planets to be a category of planet, stated Charon could be considered a planet because it revolves with Pluto around a common center of mass located between the two bodies (rather than within one of the bodies).[note 1][17] Note, however, that the Jupiter–Sun centre of mass lies outside the Sun, and so such a definition would imply that Jupiter is not a satellite of the Sun and therefore not a planet. This definition was not preserved in the IAU's final resolution.

Contention [edit]

In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution.[8] Campaigns included car bumper stickers and T-shirts.[57] Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.[58]

NASA has announced that it will use the new guidelines established by the IAU.[59] 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.[60] Thus, as of 2011, he still referred to Pluto as a planet,[61] and accepted other dwarf planets such as Ceres and Eris, as well as the larger moons, as additional planets.[56] 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'.[43]

See also [edit]

Book icon

Notes [edit]

  1. ^ The footnote in the original text reads: For two or more objects comprising a multiple object system.... A secondary object satisfying these conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto–Charon a double planet.

References [edit]

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  2. ^ "IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. Retrieved 2008-01-26. 
  3. ^ "Dwarf Planets". NASA. Retrieved 2008-01-22. 
  4. ^ Brown, Michael E.; Schaller, Emily L. (15 June 2007). "The Mass of Dwarf Planet Eris". Science 316 (5831): 1585. Bibcode:2007Sci...316.1585B. doi:10.1126/science.1139415. PMID 17569855.  edit
  5. ^ Koski, Olivia (2010-12-27). "Q&A: Astronomer Mike Brown on How He Killed Pluto". Wired. Retrieved 2012-02-12. 
  6. ^ Perlman, David (2006-08-25). "Pluto demoted -- from 9th planet to just a dwarf". San Francisco Chronicle. Retrieved 2012-02-12. 
  7. ^ Kennedy, Stephanie (2006-08-25). "Pluto stripped of planet status". "AM", ABC Local Radio. Retrieved 2012-02-12. 
  8. ^ a b c Rincon, Paul (2006-08-25). "Pluto vote 'hijacked' in revolt". BBC News. Retrieved 2008-01-26. 
  9. ^ Jorge Salazar (2009-11-30). "Alan Stern: 'A Chihuahua is still a dog, and Pluto is still a planet'". EarthSky (Earthsky Interviews). Retrieved 2009-12-08. 
  10. ^ S. Alan Stern, "On the number of planets in the outer solar system: Evidence of a substantial population of 1000-km bodies", Icarus 90:2, April 1991
  11. ^ a b "Dwarf Planets and their Systems". Working Group for Planetary System Nomenclature (WGPSN). 2008-07-11. Retrieved 2008-07-13. 
  12. ^ Dan Bruton. "Conversion of Absolute Magnitude to Diameter for Minor Planets". Department of Physics & Astronomy (Stephen F. Austin State University). Retrieved 2008-06-13. 
  13. ^ a b "Plutoid chosen as name for Solar System objects like Pluto" (Press release). 
  14. ^ a b c d e f Mike Brown, 'How many dwarf planets are there in the outer solar system?' Accessed 2011-08-24
  15. ^ a b "Today we know of more than a dozen dwarf planets in the solar system".The PI's Perspective
  16. ^ "Free the Dwarf Planets!". Michael Brown. 2011-08-24. Retrieved 2011-08-24. 
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  18. ^ Mauro Murzi (2007). Changes in a scientific concept: what is a planet?. Retrieved 2013-04-06.  [Preprint]
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  26. ^ "Hubble Observes Planetoid Sedna, Mystery Deepens". NASA's Hubble Space Telescope home site. 2004-04-14. Retrieved 2008-01-26. 
  27. ^ Brown, Michael E. "The Discovery of Eris, the Largest Known Dwarf Planet". California Institute of Technology, Department of Geological Sciences. Retrieved 2008-01-26. 
  28. ^ Brown, Michael E. (2004). "What is the definition of a planet?". California Institute of Technology, Department of Geological Sciences. Retrieved 2008-01-26. 
  29. ^ Brown, Mike (2006-08-16). "War of the Worlds". New York Times. Retrieved 2008-02-20. 
  30. ^ "Astronomers Measure Mass of Largest Dwarf Planet". NASA's Hubble Space Telescope home site. 2007-06-14. Retrieved 2008-01-26. 
  31. ^ Brown, Michael E. "What makes a planet?". California Institute of Technology, Department of Geological Sciences. Retrieved 2008-01-26. 
  32. ^ a b Britt, Robert Roy (2006-08-19). "Details Emerge on Plan to Demote Pluto". Space.com. Retrieved 2006-08-18. 
  33. ^ However, the term is also parallel to minor planet, which is also not a type of planet.
  34. ^ Karttunen et al., ed. (2007). Fundamental Astronomy (5 ed.). Springer. 
  35. ^ a b Brown, Mike (2010). How I Killed Pluto and Why It Had It Coming. Spiegel & Grau. p. 223. 
  36. ^ Bailey, Mark E. "Comments & discussions on Resolution 5: The definition of a planet – Planets Galore". Dissertatio cum Nuncio Sidereo, Series Tertia – official newspaper of the IAU General Assembly 2006. Astronomical Institute Prague. Retrieved 2008-02-09. 
  37. ^ "Dos uruguayos, Julio Fernández y Gonzalo Tancredi en la historia de la astronomía:reducen la cantidad de planetas de 9 a 8 ...&Anotaciones de Tancredi" (in Spanish). Science and Research Institute, Mercedes, Uruguay. Archived from the original on December 20, 2007. Retrieved 2008-02-11. 
  38. ^ IAU (2009). Reports on Astronomy 2006–2009. Transactions of the IAU, vol. XXVII-A
  39. ^ a b "IAU 2006 General Assembly: Result of the IAU Resolution votes". 
  40. ^ IAU (2009). Division III (Planetary Systems Sciences): Triennial Report 2006–2009. Transactions IAU, Volume XXVIIA
  41. ^ a b c Soter, Steven (2006-08-16). "What is a Planet?". The Astronomical Journal 132 (6): 2513–19. arXiv:astro-ph/0608359. Bibcode:2006AJ....132.2513S. doi:10.1086/508861. 
  42. ^ Calculated using the estimate for the mass of the Kuiper belt found in Iorio, 2007 of 0.033 Earth masses
  43. ^ a b c d Stern, S. Alan; Levison; and Levison, Harold F. (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy 12: 205–213, as presented at the XXIVth General Assembly of the IAU–2000 [Manchester, UK, 7–18 August 2000]. Bibcode:2002HiA....12..205S. 
  44. ^ Lineweaver & Marc Norman, 2010, "The Potato Radius: a Lower Minimum Size for Dwarf Planets"
  45. ^ Indeed, Mike Brown has set out to find such an object. ("Julia Sweeney and Michael E. Brown". Hammer Conversations: KCET podcast. 2007. Archived from the original on 2008-06-26. Retrieved 2008-06-28. )
  46. ^ G.H.A. Cole, 2000, Minimum Radius And Mass For A Planetary Body, archived from http://www.hull.ac.uk/php/dmsghc/planets/html/v_minimum_radius_and_mass_for_a_planetary_body.htm
  47. ^ McKinnon, William B. et al. (2008). "Structure and Evolution of Kuiper Belt Objects and Dwarf Planets". In Barucci, M. A. et al. The Solar System Beyond Neptune. Tucson: University of Arizona Press. p. 220. Bibcode:2008ssbn.book..213M. ISBN 978-0-8165-2755-7. 
  48. ^ a b c Brown, Michael E.. "The Dwarf Planets". California Institute of Technology, Department of Geological Sciences. Retrieved 2008-01-26. 
  49. ^ "IAU names fifth dwarf planet Haumea". Paris: International Astronomical Union. 2008-09-17. Retrieved 16 September 2011. 
  50. ^ a b Tancredi, G.; Favre, S. A. (2008). "Which are the dwarfs in the Solar System?". Icarus 195 (2): 851. doi:10.1016/j.icarus.2007.12.020.  edit
  51. ^ a b Bowell, Ted. "The Asteroid Orbital Elements Database". Lowell Observatory. Retrieved 2008-02-12. 
  52. ^ a b European Southern Observatory. "Dwarf Planet Makemake Lacks Atmosphere: Distant Frigid World Reveals Its Secrets for First Time". ScienceDaily. Retrieved 19 November 2012. 
  53. ^ Thomas, Peter C.; Binzelb, Richard P.; Gaffeyc, Michael J.; Zellnerd, Benjamin H.; Storrse, Alex D.; Wells, Eddie (1997). "Vesta: Spin Pole, Size, and Shape from HST Images". Icarus 128 (1): 88–94. Bibcode:1997Icar..128...88T. doi:10.1006/icar.1997.5736. 
  54. ^ Russel, C.T.; Capaccioni, F.; Coradini, A.; et al. (2006). "Dawn Discovery mission to Vesta and Ceres: Present status". Advances in Space Research 38 (9): 2043–48. Bibcode:2006AdSpR..38.2043R. doi:10.1016/j.asr.2004.12.041. 
  55. ^ Basri, G.; Brown, M.E. (2006). "Planetesimals to Brown Dwarfs: What is a Planet?" (PDF). Annual Review of Earth and Planetary Sciences 34: 193–216. arXiv:astro-ph/0608417. Bibcode:2006AREPS..34..193B. doi:10.1146/annurev.earth.34.031405.125058. 
  56. ^ a b "Should Large Moons Be Called 'Satellite Planets'?". News.discovery.com. 2010-05-14. Retrieved 2011-11-04. 
  57. ^ Chang, Alicia (2006-08-25). "Online merchants see green in Pluto news". Associated Press (USA Today). Retrieved 2008-01-25. 
  58. ^ Brown, Michael E. "The Eight Planets". California Institute of Technology, Department of Geological Sciences. Retrieved 2008-01-26. 
  59. ^ "Hotly-Debated Solar System Object Gets a Name". NASA press release. 2006-09-14. Retrieved 2008-01-26. 
  60. ^ Stern, Alan (2006-09-06). "Unabashedly Onward to the Ninth Planet". New Horizons Web Site. Retrieved 2008-01-26. 
  61. ^ Wall, Mike (2011-08-24). "Pluto's Planet Title Defender: Q & A With Planetary Scientist Alan Stern". SPACE.com. Retrieved 2012-12-03. 

External links [edit]

Look up dwarf planet in Wiktionary, the free dictionary.
The Sun Mercury Venus The Moon Earth Mars Phobos and Deimos Ceres The main asteroid belt Jupiter Moons of Jupiter Rings of Jupiter Saturn Moons of Saturn Rings of Saturn Uranus Moons of Uranus Rings of Uranus Neptune Moons of Neptune Rings of Neptune Pluto Moons of Pluto Haumea Moons of Haumea Makemake The Kuiper Belt Eris Dysnomia The Scattered Disc The Hills Cloud The Oort CloudSolar System Template Final.png

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