Dwarf planet

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Image:Pluto artistimpression.gif
Artist's impression of Pluto (background) and Charon (foreground). Pluto, considered a planet for 76 years, was reclassified as a dwarf planet in 2006.

A dwarf planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium (that is, a near-spherical shape), but has not cleared the neighbourhood around its orbit of planetesimals and is not a satellite.[1][2]

The term dwarf planet (not to be confused with minor planets) was adopted in 2006 as part of a three-way classification of bodies orbiting the Sun. Objects that are large enough to have cleared the neighbourhood of their orbit are defined as planets, while those which are too small to be in hydrostatic equilibrium are defined as small solar system bodies. The category dwarf planet is not a subset of the category planet, but a separate category altogether; that is to say, a dwarf planet is not a planet. The term dwarf planet was developed for the Solar system; bodies in other planetary systems with the characteristics of dwarf planets have not been addressed,[3] though if they were detectable they would not be considered planets.[4]

Three dwarf planets are currently recognized: Ceres, Pluto, and Eris, though about seventy objects are suspected to be dwarf planets, and there are estimated to be some 200 out to the distance of the Kuiper Belt.[5]

Contents

[edit] History of the name

Main article: 2006 definition of planet

Before the discoveries of the early 21st century, astronomers hadn't really needed a formal definition for planets. With the discovery of Pluto in 1930, astronomers considered the solar system to have nine planets, along with thousands of smaller bodies such as asteroids and comets. Pluto was thought to be larger than Mercury. However, with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately for the first time. They discovered that its mass was much smaller than expected [6], roughly one twentieth of that of Mercury, making it by far the smallest planet, smaller even than the Moon, although it was still over ten times as massive as the largest asteroid, Ceres. Then, in the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt Objects, or KBOs.[7] Many of these shared some of Pluto's key orbital characteristics, and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, and some astronomers stopped referring to Pluto as a planet. Then, starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna and Eris) all comparable to Pluto in terms of size and orbit, it became clear that they all would either have to be called planets or Pluto would have to be reclassified. Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other solar systems. In 2006, the matter came to a head with the measurement of the size of 2003 UB313. Eris (as it is now known) turned out to be slightly larger than Pluto, and so was thought to be equally deserving of the status of 'planet'. For this reason, the International Astronomical Union, after much heated debated announced a three-category system for celestial bodies within the Solar system. The final definition, as passed on 24 August 2006 is: [8]

“ 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 planet [1] 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 into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects[1].

Footnote:

[1] An IAU process will be established to select a name for this category.

”

[edit] List of dwarf planets

The IAU has officially identified three celestial bodies that have immediately received dwarf planet classification:[9]

Name Ceres Pluto Eris
MPC number 1 134340 136199
Astronomical symbol Image:Ceres symbol.svg Image:Pluto symbol.svg
Region of Solar System Asteroid belt Kuiper belt Scattered disc
Diameter 941±32 km 2306±30 km 2400±100 km
Mean equatorial radius*
in km 		0.0738
471 		0.180
1,148.07 		0.19
~1,200
Mass in kg
compared to Earth 		9.5×1020 kg
0.00016 		1.305×1022 kg
0.0022 		~1.67×1022 kg[10]
0.0028
Volume*
 0.00042
0.005
0.007
Density (in Mg/m³) 2.08 2.0 2.1
Equatorial gravity (in m/s2) 0.27 0.60 ~0.68
Escape velocity (in km/s) 0.51 1.2 ~1.3
Rotation period (d)
(in sidereal days) 		0.3781 -6.38718
(retrograde)
Orbital radius* (AU)
semi-major axis
in km 		2.5-2.9
2.766
413,715,000 		29.66-49.30
39.48168677
5,906,376,200 		37.77-97.56
67.6681
10,210,000,000
Orbital period*(a)
(in sidereal years) 		4.599 248.09 557
Mean orbital speed
(in km/s) 		17.882 4.666 3.437
Orbital eccentricity 0.080 0.24880766 0.44177
Orbital inclination 10.587° 17.14175° 44.187°
Inclination of the equator from the orbit
(see Axial tilt) 		4° 119.61°
Planetary discriminant 0.33 7.7×10−2 0.10
Mean surface temperature (in K) 167 40 30
Atmosphere composition CH4
Number of natural satellites 0 3 1
Date of discovery January 1, 1801 February 18, 1930 October 21, 2003

[edit] Other candidates

Image:TheTransneptunians Size Albedo Color.svg
Illustration of the relative sizes, albedos and colours of the largest Trans-Neptunian Objects.

The smallest icy body known to have achieved hydrostatic equilibrium is Mimas, which averages 400 km in diameter; the largest known irregular icy body is Proteus, which also averages about 400 km in diameter. Mike Brown suggests that the lower limit for an icy dwarf planet is therefore likely to be somewhere under 400 km. Although it is difficult to measure the diameters of TNOs, Brown believes that, as of August 2006, Sedna and 42 bodies in the Kuiper Belt and Scattered Disc, in addition to Pluto and Eris, met this requirement and qualified as dwarf planets. His team is observing another 30 such objects that they have yet to announce, and believe that the total number will eventually prove to be about 200, with many more such as Sedna beyond that.[11] Of the asteroids, 4 Vesta may qualify, as it appears to deviate from hydrostatic equilibrium only because of a large impact which occurred after Vesta solidified; the definition of dwarf planet does not specifically address this issue. It is possible that 2 Pallas and 10 Hygieia also qualify, though current images suggest that they may be irregular.

Among these candidates, current measurements suggest the following may be greater than 700 km in diameter. (An artist's depiction of some of these, compared to the Earth, is on the IAU website.)

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Possible dwarf planets
Name Category Diameter Mass
2005 FY9 ("Easterbunny") Cubewano ~1503 km[12], ~1300-1900 km ~4.0 × 1021 kg
Sedna Scattered-Extended object ~1500 km[12], 1180–1800 km1.7-6.1 × 1021 kg
2003 EL61 ("Santa") Cubewano ~1960×1518×996 km; ~1380 km[12] ~4.2 × 1021 kg
Quaoar Cubewano ~1260 kmkm[12], 755-1050 km[13]; 1070-1450 km 1.0-2.6 × 1021 kg
2002 TC302 Scattered disc object ~1200 km[12], 920-1480[13]~7.8 × 1019 kg
Orcus Plutino ~909 km[12], 875-1020 km[13]6.2 - 7.0 × 1020 kg
(19308) 1996 TO66 Cubewano ~900 km[12]
Varuna Cubewano ~874 km[12] (755-1025 km[14]; >480-800 km[13]~5.9 × 1020 kg
19521 Chaos Cubewano ~745 km[12]
2002 AW197 (other) ~793 km[12], 625-850[13]; 700-800 km (770-1010 km[15])~5.2 × 1020 kg
(24835) 1995 SM55 (other) ~702 km[12]
(42301) 2001 UR163 Scattered-Extended object ~636 km[12]
Ixion Plutino ~570 kmkm[12], 430-710 km[13]; <822 km (900-1230 km[15]) ~5.8 × 1020 kg
2002 UX25 Cubewano ~649 km[12], 570-795 km[13] ~7.9 × 1020 kg
2002 TX300 Cubewano 530 km ? (<709 km); ~709 km[12] 1.6 - 3.7 × 1020 kg
1996 TL66 Scattered disk object ~632 km[12]
(84922) 2003 VS2 Plutino ~725 km[12], 540-925 km[13]
(90568) 2004 GV9 Cubewano ~677 km[12], 610-750 km[13]
(119951) 2002 KX14 380-780 km[13]
2002 MS4 600-850 km[13]
2003 AZ84 590-785 km[13]

The status of Charon, currently regarded as a satellite of Pluto, remains uncertain, as there is presently no clear definition of what distinguishes a satellite system from a binary (double planet) system. The original draft resolution (5)[3] presented to the IAU stated that Charon could be considered a planet because:

  1. Charon independently would satisfy the size and shape criteria for planetary status (and in the terms of the final resolution, for the status of dwarf planet)
  2. Charon, on account of its large mass relative to Pluto, revolves with Pluto around a common barycentre located in space between Pluto and Charon rather than around a point located within Pluto.

This definition, however, was not preserved in the IAU's final resolution. It is unknown if it will be taken up at a future date. If a similar definition were to be adopted, Charon would be added to the list of dwarf planets.

The second, third, and fourth largest objects in the main asteroid belt (Vesta, Pallas and Hygiea) could be classified as dwarf planets if it is shown that their shape is determined by hydrostatic equilibrium. At present this has not been demonstrated conclusively.[16] The Dawn probe, expected to enter orbit around Vesta in 2011, may provide evidence for or against dwarf planet status in that case.

[edit] Size and mass of dwarf planets

The upper and lower limits to the size and mass of dwarf planets are not specified in the IAU resolution. There is strictly no upper limit, and an object larger or more massive than Mercury that is considered not to have cleared the neighborhood around its orbit may still be classified as a dwarf planet.

The lower limit is determined by the concept of hydrostatic equilibrium shape, but the size or mass at which an object attains this shape is undefined, and empirical observations suggest that it may vary according to the composition and history of the object. The original draft of IAU resolution 5 defined hydrostatic equilibrium shape as applying "to objects with mass above 5×1020 kg and diameter greater than 800 km",[3] but this language was not retained in the final resolution 5A that was passed.

According to some astronomers, the new definition could mean the addition of up to 45 new dwarf planets.[17][18]

[edit] Orbital dominance

Main article: Clearing the neighbourhood

Using a parameter developed by S. Alan Stern and Harold F. Levison, Steven Soter and other astronomers have argued for a distinction between dwarf planets and the other eight planets based on their inability to "clear the neighborhood around their orbits", that is, to remove smaller bodies whose orbits bring them nearby by collision, capture, or gravitational disturbance. This concept is combined with a concept of orbital dominance measured in terms of the ratio of the mass of a planetary candidate to the combined mass of all other objects in its vicinity. Dwarf planets are too small in mass to significantly alter their environment in the manner of a planet.

There are several other theories that try to differentiate between planets and dwarf planets, but the current definition of what constitutes a planet uses this concept.

Stern et al. introduce a parameter Λ, expressing the probability of an encounter resulting in a given deflection of orbit. The value of this parameter in Stern’s model is proportional to the square of the mass and inversely proportional to the period. Following the authors, this value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit. Stern and Levison found a gap of five orders of magnitude in Λ between the smallest terrestrial planets and the largest asteroids and KBOs:

Planetary discriminants
Body Mass (ME*)
 Λ/ΛE**
 µ***
Mercury 0.055 0.0126 9.1×104
Venus 0.815 1.08 1.35×106
Earth 1.00 1.00 1.7×106
Mars 0.107 0.0061 1.8×105
Ceres 0.00015 8.7×10−9 0.33
Jupiter 317.7 8510 6.25×105
Saturn 95.2 308 1.9×105
Uranus 14.5 2.51 2.9×104
Neptune 17.1 1.79 2.4×104
Pluto 0.0022 1.95×10−8 0.077
Eris 0.0028 3.5×10−8 0.10

*ME in Earth masses.
**Λ/ΛE = M²/P, in Earth masses squared per year.
***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.

[edit] Contention

A number of scientists expressed their disagreement[19] with the currently adopted IAU definition of dwarf planet by means of car bumper stickers.

While accepting the characterisation of dwarf planet for Ceres and Eris (dwarf planet in this case meaning just a small planet), Stern 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.[20] Thus, he and his team will still refer to Pluto as the ninth planet. One should also note, that it will be in pages hosted by NASA and controlled by Stern's team, that the upcoming information and the first photographs of Pluto will be unveiled to the world. However, NASA has announced that it will use the new guidelines established by the IAU.[21]

Prior to the 2006 IAU reclassification, several terms were suggested for bodies which are now formally cited as a dwarf planet, including minor planet, subplanet and planetoid.

[edit] Types of dwarf planets

The IAU's Resolution 6a[9] 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. The former name was generally deprecated[22] and was abandoned in the final draft resolution (6b)[23]; the latter name failed to win majority approval on a 180–186 vote in the IAU General Assembly on August 24 2006. The category, while established, remains nameless.

At an earlier stage in the definition process, the category (then described as "pluton") was defined to be a planet whose orbit took more than 200 Julian years to complete and whose orbit was more highly inclined and elliptical than a traditional planetary orbit.[24]

This category of Pluto-like objects only applies to dwarf planets which 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.

The membership of this class, other than Pluto itself, remains obscure. Eris and the objects listed in the table "Possible dwarf planets" (above) also qualify in terms of the minimum period, and most exhibit orbital eccentricity and inclination that are significant, though not always equal to or greater than Pluto's. Quaoar, however, has a much smaller eccentricity and inclination, and so possibly does not qualify as a Pluto-like object.

[edit] See also

 v • d • e The Solar System
<imagemap>

Image:Solar System XXVII.png

  1. The Sun

circle 0 0 90 35 The Sun

  1. Mercury

circle 112 18 6 Mercury

  1. Venus

circle 153 18 8 Venus

  1. Earth and the Moon

circle 203 8 4 The Moon circle 194 18 8 Earth

  1. Mars and satellites

circle 239 13 3 Phobos and Deimos circle 233 18 8 Mars

  1. Ceres and the asteroid belt
  2. - by placing the rectangle code for the asteroid belt AFTER Ceres, Ceres is "on top" (and can co-exist)

circle 271 18 8 Ceres rect 256 0 288 35 The asteroid belt

  1. Jupiter and satellites

circle 316 18 15 Jupiter circle 329 5 6 Moons of Jupiter

  1. Saturn and satellites

circle 372 18 10 Saturn circle 381 7 6 Moons of Saturn

  1. Uranus and satellites

circle 418 18 9 Uranus circle 427 10 6 Moons of Uranus

  1. Neptune and satellites

circle 471 10 3 Moons of Neptune circle 462 18 12 Neptune

  1. Pluto, satellites, and the Kuiper belt
  2. - by placing the rectangle code for the Kuiper belt AFTER Pluto, Pluto is "on top" (and can co-exist)

circle 508 13 3 Moons of Pluto circle 504 18 8 Pluto rect 492 0 527 35 The Kuiper Belt

  1. Eris, Dysnomia, and the Scattered disc
  2. - by placing the rectangle code for the Scattered disc AFTER Eris, Eris is "on top" (and can co-exist)

circle 544 14 3 Dysnomia circle 540 18 8 Eris rect 528 0 567 35 The Scattered Disc rect 568 0 597 35 The Oort Cloud

desc none

  1. - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
  1. Notes:
  2. Details on the new coding for clickable images is here: [3]
  3. The smaller planets have a bit of an overlap just to ensure they're locatable, especially in the belts.
  4. While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
  5. - I've placed moons on "top" so that their smaller circles won't disappear "under" their respective planets or dwarf planets.
  6. The "poly" code would be more appropriate for the moons of Jupiter, Saturn, and Uranus. However, there appears to be a bug with that aspect of the code.
  7. - I've compensated by using oversized circles for those moon groups, and tucking them UNDER their planets for now.
  8. The Sun is a rectangle as that approximates the edge closely enough for the purposes of this template.
  9. I've guessed as to the boundaries for the KB, SD, and OC - if they need adjustment, load the image into Paint and use the pencil tool to find the appropriate coordinates.

</imagemap>

The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris
Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies:   Meteoroids · Asteroids/Asteroid moons (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Solar System Portal

[edit] References

  1. ^ IAU 2006 General Assembly: Result of the IAU Resolution votes
  2. ^ Dwarf Planet: What Defines a Planet?
  3. ^ a b c Draft Resolution 5 for GA-XXVI: Definition of a Planet.
  4. ^ Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union. IAU (2001). Retrieved on 2006-05-25.
  5. ^ Mike Brown page on dwarf planets.
  6. ^ Orbits and photometry of Pluto’s satellites: Charon, S/2005 P1 and S/2005 P2 Eliot F. Young, Leslie A. Young, and S. Alan Stern, Southwest Research Institute, Boulder, CO
  7. ^ Much Ado about Pluto plutopetition.com
  8. ^ "IAU 2006 General Assembly: Resolutions 5 and 6", IAU, 24 August 2006. 
  9. ^ a b IAU 2006 General Assembly: Result of the IAU Resolution votes.
  10. ^ Brown, M.E. et al. 2006. Satellites of the Largest Kuiper Belt Objects. Astrophysical Journal, 639:L43-L46 More accurate work based on Dysnomia's orbit in preparation.
  11. ^ [1]
  12. ^ a b c d e f g h i j k l m n o p q r [2]
  13. ^ a b c d e f g h i j k l http://arxiv.org/PS_cache/astro-ph/pdf/0702/0702538v1.pdf
  14. ^ http://www.ifa.hawaii.edu/faculty/jewitt/varuna.html
  15. ^ a b http://www.ifa.hawaii.edu/faculty/jewitt/kb.html
  16. ^ Three new planets may join solar system. New Scientist. Retrieved on 2006-08-16.
  17. ^ Nine Planets Become 12 with Controversial New Definition. Space.com. Retrieved on 2006-08-16.
  18. ^ What makes a planet?. Michael E. Brown. Retrieved on 2006-08-16.
  19. ^ http://news.bbc.co.uk/2/hi/science/nature/5283956.stm
  20. ^ Unabashedly Onward to the Ninth Planet.
  21. ^ Hotly-Debated Solar System Object Gets a Name, NASA press release.
  22. ^ Astronomers divided over "planet" definition.
  23. ^ The Final IAU Resolution on the definition of "planet" ready for voting.
  24. ^ Draft definition, IAU press release.

[edit] External links

als:Zwergplanet ar:كوكب قزم ast:Planeta nanu bn:বামন গ্রহ zh-min-nan:É-he̍k-chheⁿ be-x-old:Карлікавая плянэта bs:Patuljasta planeta br:Planedenn-gorr bg:Планета-джудже ca:Planeta nan cs:Trpasličí planeta cy:Planed gorrach da:Dværgplanet de:Zwergplanet et:Kääbusplaneet es:Planeta enano eo:Nanplanedo eu:Planeta ipotx fa:سیاره کوتوله fr:Planète naine ga:Abhacphláinéad gl:Planeta anano ko:왜행성 id:Planet katai ia:Planeta nano is:Dvergreikistjarna it:Pianeta nano he:כוכב לכת ננסי jv:Planet cilik ka:ჯუჯა პლანეტა sw:Sayari kibete la:Planetula lb:Zwergplanéit lt:Nykštukinė planeta jbo:cmaplini hu:Törpebolygó nl:Dwergplaneet ja:準惑星 no:Dvergplanet nn:Dvergplanet nrm:Ragot d'plianète nds:Dwargplanet pl:Planeta karłowata pt:Planeta anão ksh:Zwershplanett ro:Planetă pitică qu:Tuna puriq quyllur ru:Карликовая планета scn:Pianeta nanu simple:Dwarf planet sk:Trpasličia planéta sl:Pritlikavi planet sr:Патуљасте планете fi:Kääpiöplaneetta sv:Dvärgplanet th:ดาวเคราะห์แคระ vi:Hành tinh lùn tr:Cüce gezegen uk:Карликова планета ur:بونا سیارہ vec:Pianeta nano zh:矮行星

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