Plutoid
| TNOs and similar bodies |
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*Trans-Neptunian dwarf planets are "plutoids"
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A plutoid is a trans-Neptunian dwarf planet. The International Astronomical Union (IAU) developed this category of astronomical objects as a consequence of its 2006 resolution defining the word "planet". The IAU's formal definition of 'plutoid,' announced 11 June 2008, is:
- Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves.[1]
Accordingly, plutoids can be thought of as the intersection of the set of dwarf planets and the set of trans-Neptunian objects.
In light of the difficulty of remotely ascertaining hydrostasis, the IAU only formally confers "dwarf planet" [and by extension, "plutoid"] status to those bodies whose minimum estimated size is substantially greater than what is generally thought necessary to guarantee hydrostatic equilibrium. As of 2009[update], Pluto, Eris, Haumea, and Makemake are the only objects officially recognized as plutoids, while upwards of seventy more bodies that currently lack formal recognition are thought likely to meet the definition, and can expect formal recognition at some time in the future.[2]
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History of the term
On August 24, 2006, the IAU decided to re-classify Pluto as a dwarf planet, requiring that a planet must "clear the neighbourhood around its orbit."
The General Assembly of the IAU further resolved:
| “ | Pluto 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. |
” |
This new category had been proposed under the name "pluton" or a "plutonian object" earlier in the General Assembly. The former was rejected, in part because "pluton" is actually a pre-existing geological term, and many geological experts wrote in complaints pointing this out. "Pluton" was dropped midway through the Assembly[3] and was abandoned in the final draft resolution (6b);[4] "Plutonian object" failed to win majority approval on a 183–186 vote in the IAU General Assembly on August 24, 2006.[5]
The definition of the category also fluctuated during its early stages. When first proposed, 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.[6] Once it had been counter-proposed to strip Pluto of planet status, this category of Pluto-like objects was then applied to dwarf planets that met the conditions of being trans-Neptunian and "like Pluto" in terms of period, inclination, and eccentricity. Ultimately, the final resolution left the formal definition, like the name, to be established at a later date.[7]
Following the IAU General Assembly, the name "plutoid" was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III, by the IAU Working Group for Planetary System Nomenclature (WGPSN) and approved by the IAU Executive Committee at its meeting in Oslo, Norway, on 11 June 2008.[8] The term was announced after the Executive Committee meeting, along with a greatly-simplified definition: all trans-Neptunian dwarf planets are plutoids.[1]
Naming process for plutoids
With the creation of the term "dwarf planet," some ambiguity was created as to which of two IAU bodies would be responsible for naming dwarf planets. Eris had been named through the IAU Committee on Small Body Nomenclature and the IAU Working Group for Planetary System Nomenclature working in cooperation with one another. Along with announcing the name "plutoid", the IAU decision of 11 June 2008 institutionalized this cooperative process involving the two bodies in the naming of new plutoids. In keeping with minor planet naming guidelines, priority will be given to names proposed by the discovery teams, and plutoids may not share a name with a small solar system body.
When the definition of "dwarf planet" was instated at the IAU General Assembly of 2006, Ceres, Pluto and Eris were identified by name as the initial members of the dwarf planet class. However, precise regulations as to how hydrostatic equilibrium would be measured were left undefined for the time being. Without an official procedure for calculating the lower bound of size to be a "dwarf planet," no further bodies could be formally recognized as either dwarf planets or plutoids.
It was noted that the naming process would remain stalled without such rules, and that even with them, few of these bodies can be imaged with sufficient resolution to determine their shapes. Therefore, the IAU announced that for naming purposes, a trans-Neptunian object will be assumed to be a plutoid if it has an absolute magnitude brighter than H = +1 magnitude.[1]
Mathematically, the smallest possible object that could possess an absolute magnitude of +1 (a perfectly reflective one with an albedo of 1) would be 838 km in diameter.[9][10] It is highly unlikely that any body of this size or larger, regardless of composition, would not also surpass whatever threshold is ultimately adopted as proof of hydrostatic equilibrium. That said, if it turns out upon further investigation that an object named as if it were a plutoid has not achieved hydrostatic equilibrium, the IAU has stated it will be reclassified, but keep its name.[1]
This decision allowed for the naming of Makemake and Haumea, and their formal recognition as plutoids and dwarf planets, bringing the total number of official plutoids from 2 to 4.
Officially-recognized plutoids
Four trans-Neptunian objects, Pluto, Eris, Haumea, and Makemake, are formally recognized as dwarf planets and therefore as plutoids.
| Name | Pluto | Haumea | Makemake | Eris |
|---|---|---|---|---|
| Minor planet number | 134340 | 136108 | 136472 | 136199 |
| Absolute magnitude | −0.7 | +0.17 | -0.48 | −1.12 ± 0.01 |
| Albedo | 0.49–0.66 | 0.7 ± 0.1 | 0.8 ± 0.2 | 0.86 ± 0.07 |
| Diameter | 2390 km | ~1960×1518×996 km | 1300–1900 km | 2400±300 km |
| Mass in kg compared to Earth |
1.305 × 1022 kg .0022 |
(4.2 ± 0.1) × 1021 kg .0007 |
~4 × 1021 kg .0007 (est.) |
(1.67 ± 0.02) × 1022 kg .0025 |
| Density (in Mg/m³) | 2.03 ± 0.06 | 2.6–3.3 | ~2 | 2.3 ± 0.3 |
| Equatorial gravity (in m/s2) | 0.58 | ~ 0.44 (varies) | ~0.5 | ~0.8 |
| Rotation period (d) (in sidereal days) |
-6.387 18 (retrograde) |
0.163 14 | ? | > 0.3 ? |
| Orbital radius* (AU) semi-major axis in km |
29.66-49.30 39.481 686 77 5 906 376 200 |
35.2–51.5 43.3 6 484 000 000 |
38.5-53.1 45.8 6 850 000 000 |
37.77-97.56 67.668 1 10 210 000 000 |
| Orbital period*(a) (in sidereal years) |
248.09 | 285.4 | 309.88 | 557 |
| Mean orbital speed (in km/s) |
4.7490 | 4.484 | 4.419 | 3.436 |
| Orbital eccentricity | 0.248 807 66 | 0.188 74 | 0.159 | 0.441 77 |
| Orbital inclination | 17.141 75° | 28.19° | 28.96° | 44.187° |
| Inclination of the equator from the orbit (see Axial tilt) |
119.61° | |||
| Mean surface temperature (in K) | 40 | 30 | 32±3 | ~30 |
| Number of natural satellites | 3 | 2 | 0 | 1 |
| Date of discovery | February 18, 1930 | December 28, 2004 | March 31, 2005[11] | October 21, 2003 |
Officially-unrecognized, but likely, plutoids
Trans-Neptunian objects 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, making them dwarf planets of the plutoid class.[2] Although only rough estimates of the diameters of these objects are available, as of April 2007 it was believed that another seventy Trans-Neptunian objects were likely to be plutoids.[2]
| Name | Category | Estimated diameter (km) | Absolute Magnitude (H) |
Mass ( × 1020 kg) |
Orbital radius (AU) |
|||
|---|---|---|---|---|---|---|---|---|
| by [2] | by [13] | by [14] | by [15] | |||||
| Orcus | plutino (1 moon) |
1,100 | 909 | 946 | 1,500 | 2.3 | 6.32 ± 0.05 | 39.2 |
| Huya | plutino | 480 | 480 | — | — | 4.7 | 0.6–1.8? | 39.4 |
| Pluto | plutoid | 2,306 | –0.7 | 130 | 39.4 | |||
| Ixion | plutino | 980 | 570 | 650 | 1,065 | 3.2 | ~3? | 39.6 |
| Varuna | cubewano | 780 | 874 | 500 | 900 | 3.7 | ~3.7? | 42.9 |
| Haumea | plutoid | 1,436 | 0.17 | 40 | 43.3 | |||
| Quaoar | cubewano (1 moon) |
1,290 | 1,260 | 844 | 1,200 | 2.7 | 21–29 | 43.5 |
| Makemake | plutoid | 1,500 | –0.45 | 30 | 45.3 | |||
| (55565) 2002 AW197 | cubewano | 940 | 793 | 735 | 890 | 3.2 | ~4.1? | 47.0 |
| (84522) 2002 TC302 | 5:2 SDO | 710 | 1,200 | 1,150 | — | 3.8 | 15? | 55.4 |
| (225088) 2007 OR10 | 10:3? SDO | 1200? | — | 1.9 | ? | 67.3 | ||
| Eris | plutoid | 2,600 | –1.12 | 167 | 68.0 | |||
| (15874) 1996 TL66 | SDO | — | 632 | 460–690 | — | 5.4 | 2? | 83.9 |
| Sedna | Detached object | 1,800 | 1,500 | < 1,600 | < 1,500 | 1.5 | 8–70? | 509 |
See also
References
- ↑ 1.0 1.1 1.2 1.3 "Plutoid chosen as name for Solar System objects like Pluto". International Astronomical Union (News Release - IAU0804). June 11, 2008, Paris. http://www.iau.org/public_press/news/release/iau0804. Retrieved 2008-06-11.
- ↑ 2.0 2.1 2.2 2.3 Brown, Michael E.. "The Dwarf Planets". California Institute of Technology, Department of Geological Sciences. http://web.gps.caltech.edu/~mbrown/dwarfplanets/. Retrieved 2008-01-26.
- ↑ "Astronomers divided over "planet" definition". Deutsche Presse Agentur (Rawstory.com). 2006-08-22. http://rawstory.com/news/2006/Astronomers_divided_over_planet_def_08222006.html. Retrieved 2008-01-26.
- ↑ "The Final IAU Resolution on the definition of "planet" ready for voting". International Astronomical Union. 2006-08-24. http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html. Retrieved 2008-01-26.
- ↑ "IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. http://www.iau.org/iau0603.414.0.html. Retrieved 2008-01-26.
- ↑ "Draft definition, IAU press release". International Astronomical Union. 2006-08-16. http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html. Retrieved 2008-01-26.
- ↑ "Definition of a Planet in the Solar System: Resolutions 5 and 6" (PDF). IAU 2006 General Assembly (International Astronomical Union). 2006-08-24. http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf. Retrieved 2008-01-26.
- ↑ "Dwarf Planets". NASA. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Dwarf&Display=OverviewLong. Retrieved 2008-01-22.
- ↑ Dan Bruton. "Conversion of Absolute Magnitude to Diameter for Minor Planets". Department of Physics & Astronomy (Stephen F. Austin State University). http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html. Retrieved 2008-06-13.
- ↑ "Conversion of Absolute Magnitude to Diameter". IAU: Minor Planet Center. http://www.cfa.harvard.edu/iau/lists/Sizes.html. Retrieved 2008-06-12.
- ↑ Discovery Circumstances: Numbered Minor Planets
- ↑ All bodies with estimated diameters of 900 km or more, with 3 additional prime suspects (Huya, 2007 OR10, 1996 TL66) cited in Tancredi & Favre. "Which are the dwarfs in the Solar system?". Asteroids, Comets, Meteors. http://www.lpi.usra.edu/meetings/acm2008/pdf/8261.pdf. Retrieved 2008-09-20.
- ↑ Johnston, Robert (2007-11-24). "List of Known Trans-Neptunian Objects". Johnston's Archive.net. http://www.johnstonsarchive.net/astro/tnoslist.html. Retrieved 2008-01-26.
- ↑ 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). http://arxiv.org/pdf/astro-ph/0702538. Retrieved 2008-01-26.
- ↑ David C. Jewitt. "Kuiper Belt: The 1000 km Scale KBOs". University of Hawaii, Institute for Astronomy. http://www2.ess.ucla.edu/~jewitt/kb.html. Retrieved 2008-02-10.
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