Dwarf planet

From Wikipedia, the free encyclopedia

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.[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.

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

Contents

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. 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.[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.[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.[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.[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. 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.[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.[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. 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.[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:

“ 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3, 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 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

Planetary discriminants[23]
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.

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.[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).[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.[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).[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.[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. 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).[4]

Current members

Ceres as seen by the Hubble Space Telescope
Ceres as seen by the Hubble Space Telescope
Pluto in approximately true colours
Pluto in approximately true colours
Eris as seen by the Hubble Space Telescope
Eris as seen by the Hubble Space Telescope

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

  1. Ceres Ceres – discovered on January 1, 1801 (16 years before Neptune), considered a planet for half a century before reclassification as an asteroid;
  2. Pluto 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.

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

Orbital attributes of dwarf planets[27]
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 0.080 0.33
Pluto Kuiper belt 39.48 248.09 4.666 17.14 0.249 0.077
Eris Scattered disc 67.67 557 3.436 44.19 0.442 0.10
Physical attributes of dwarf planets
Name Relative
equatorial
diameter[E]		 Equatorial
diameter
(km)		 Relative
mass[E]		 Mass
(×1022 kg)		 Density
(×103g/m³)		 Surface
gravity
(m/s2)		 Escape
velocity
(km/s)		 Axial
inclination		 Rotation
period
(days)		 Moons Surface
temp.
(K)		 Atmosphere
Ceres[28][29] 0.074 974.6±3.2 0.0002 0.095 2.08 0.27 0.51 ~3° 0.38 0 167 none
Pluto[30][31] 0.19 2306±30 0.0021 1.305 2.0 0.58 1.2 119.59° -6.39 3 44 temporary
Eris[32][33] 0.19 2400±100 0.0025 1.67 2.3 ~0.8 1.3 ~0.3 1 42 temporary
E  Measured relative to the Earth.

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. This will presumably change if NASA's spacecrafts Dawn and New Horizons will reach Ceres and Pluto, respectively, in 2015.[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.[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.[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. 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)
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.[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.[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):

TNOs that are larger or comparable in diameter to Ceres
Name Category Estimated diameter (km) Mass
(×1020 kg)		 Orbital
radius
(AU)
by [42] by [43] by [44]
2003 EL61 cubewano 2000 1380 1350 1200 ~42 43.31
Sedna extended-SDO 1800 1500 <1800 <1500 17–61 486.0
2005 FY9 cubewano 1600 1500 1500 1250 ~40 45.66
Quaoar cubewano 1290 1260 1260 1200 10–26 43.58
Orcus plutino 1100 909 946 1500 6.2–7.0 39.34
Ixion plutino 980 570 650 1065 ~5.8 39.65
Ceres asteroid
975
 9.5 2.77
2002 AW197 cubewano 940 793 977 890 ~5.2 47.30
Varuna cubewano 780 874 1016 900 ~5.9 42.90
2002 TC302 SDO 710 1200 1150 — 0.78 55.02

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. 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.[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.[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.[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.[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

References

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  2. ^ Dwarf Planets. NASA. Retrieved on 2008-01-22.
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  5. ^ a b c d The IAU draft definition of "planet" and "plutons". International Astronomical Union (August 16, 2006).
  6. ^ Working Group on Extrasolar Planets of the International Astronomical Union. International Astronomical Union (2001). Retrieved on 2008-01-26.
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  16. ^ Brown, Michael E. (2004). What is the definition of a planet?. California Institute of Technology, Department of Geological Sciences. Retrieved on 2008-01-26.
  17. ^ Brown, Mike. "War of the Worlds", New York Times, 2006-08-16. Retrieved on 2008-02-20. 
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  19. ^ Brown, Michael E.. What makes a planet?. California Institute of Technology, Department of Geological Sciences. Retrieved on 2008-01-26.
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  22. ^ 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 (Spanish). Science and Research Institute, Mercedes, Uruguay. Retrieved on 2008-02-11.
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  24. ^ a b c Stern, S. Alan; Levison, Harold F. (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy 12: 205–13, as presented at the XXIVth General Assembly of the IAU–2000 Manchester, UK, 7–18 August 2000. 
  25. ^ In theory it is possible to have bodies as massive as Jupiter be classified as dwarf planets if they "did not clear their path", but in practice it is unlikely that bodies at least as large as Mercury will be discovered in the Solar system
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  29. ^ Calculated based on the known parameters. APmag and AngSize generated with Horizons (Ephemeris: Observer Table: Quantities = 9,13,20,29)
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  32. ^ Stansberry, John; Grundy, Will; Brown, Mike; Spencer, John; Trilling, David; Cruikshank, Dale; Margot, Jean-Luc (2007). Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope (PDF). University of Arizona, Lowell Observatory, California Institute of Technology, NASA Ames Research Center, Southwest Research Institute, Cornell University. Retrieved on 2007-05-18.
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  39. ^ 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.
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  41. ^ Britt, Robert Roy (2006-08-16). Nine Planets Become 12 with Controversial New Definition. Space.com. Retrieved on 2008-01-26.
  42. ^ Johnston, Robert (2007-11-24). List of Known Trans-Neptunian Objects. Johnston's Archive.net. Retrieved on 2008-01-26.
  43. ^ Barucci, M.A.; Stansberry, John; Grundy, Will; Brown, Mike; Cruikshank, Dale; Spencer, John; Trilling, David; Margot, Jean-Luc (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". The Solar System beyond Neptune. University of Arizona Press. 
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  48. ^ Compared with Mercury, Quaoar has a similar inclination and a much smaller eccentricity; the eccentricity is in fact only slightly larger than that of Earth
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External links

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