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#REDIRECT ] |
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{{Distinguish|plutino|pluton}} |
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{{TNO}} |
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{{TNO imagemap}} |
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{{Redirect category shell| |
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A '''plutoid''' or '''ice dwarf''' is a ] ], i.e. a body orbiting beyond ] that is large enough to be ] in shape. The term ''plutoid'' was adopted by the ] (IAU) working group ''Committee on Small Bodies Nomenclature'', but was rejected by the IAU working group ''Planetary System Nomenclature''.<ref>{{cite journal |author=Karel A. van der Hucht, {{dabbr|ed.|editor}} | title=Reports on Astronomy 2006–2009 | date=2009 | journal=Transactions IAU | volume=XXVIIA | doi=10.1017/S1743921308025398|bibcode = 2009IAUTA..27..149B |pages=149 }}</ref> The term ''plutoid'' is not widely used by astronomers, though ''ice dwarf'' is not uncommon. |
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{{R from alt name}} |
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There are thought to be thousands of plutoids in the Solar System, although only four have been formally designated as such by the IAU. |
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{{R to section}} |
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}} |
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The IAU developed this category of ]s as a consequence of its ]. The IAU's formal definition of "plutoid", announced 11 June 2008, is: |
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:Plutoids are celestial bodies in orbit around the ] at a ] greater than that of ] that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a ] (near-spherical) shape, and that have not ] around their orbit. Satellites of plutoids are not plutoids themselves.<ref name="IAU0804">{{cite web |date=June 11, 2008 |location=Paris |title=Plutoid chosen as name for Solar System objects like Pluto |publisher=] (news release – IAU0804) |url=http://www.iau.org/news/pressreleases/detail/iau0804/ |accessdate=2008-06-11}}</ref> |
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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}}, ], {{dp|Eris}}, {{dp|Haumea}}, and {{dp|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.<ref name=Brown>{{cite web|authorlink=Michael E. Brown|title=The Dwarf Planets|first=Michael E.|last=Brown |publisher=California Institute of Technology, Department of Geological Sciences|url=http://web.gps.caltech.edu/~mbrown/dwarfplanets/|accessdate=2008-01-26}}</ref> |
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] of the ] believes the outer planets show signs of collisions with plutoids 1,000 to 2,000 kilometers in diameter: ] could have been tipped off its axis by a plutoid, and ], the largest ] of ], is probably a captured plutoid from the Kuiper belt.<ref name="Agnor06">{{cite journal |
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| author = Craig B Agnor |
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| author2 = Douglas P Hamilton |
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| title = Neptune's capture of its moon Triton in a binary–planet gravitational encounter |
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| journal = Nature |
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| date = May 2006 |
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| volume = 441 |
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| issue = 7090 |
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| pages = 192–194 |
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| doi = 10.1038/nature04792 |
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| url = http://www.nature.com/nature/journal/v441/n7090/abs/nature04792.html |
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| accessdate = 2006-05-10 |
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| pmid = 16688170 |
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| bibcode=2006Natur.441..192A |
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}}</ref> |
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== History of the term == |
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{{See also|IAU definition of planet}} |
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{{multiple image |
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| align = right |
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| width = 350 |
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| direction = vertical |
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| image1 = Masses of the plutoids.png |
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| caption1 = Comparative masses of the largest (likely) plutoids, plus ]. ] (pink) and Pluto (red) dominate.<br />Data as of 2011; values of ], ], and ] are approximate. |
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| image2 = Plutoids compared to Luna.png |
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| caption2 = The masses of the same bodies compared to the ] (lavender).}} |
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On August 24, 2006, the IAU decided to reclassify ] as a dwarf planet, requiring that a ] must "clear the neighbourhood around its orbit". |
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The General Assembly of the IAU further resolved: |
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{{quote|Pluto is recognized as the prototype of a new category of Trans-Neptunian Objects.<ref group="note">An IAU process will be established to select a name for this category.</ref>}} |
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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 "]" is actually a pre-existing ] term, and many geological experts wrote in complaints pointing this out. "Pluton" was dropped midway through the Assembly<ref>{{cite news|title=Astronomers divided over "planet" definition|work=Deutsche Presse Agentur|publisher= The Raw Story|url= http://rawstory.com/news/2006/Astronomers_divided_over_planet_def_08222006.html |date=2006-08-22|accessdate=2008-01-26 |archiveurl = http://web.archive.org/web/20071111050127/http://rawstory.com/news/2006/Astronomers_divided_over_planet_def_08222006.html <!-- Bot retrieved archive --> |archivedate = 2007-11-11}}</ref> and was abandoned in the final draft resolution (6b);<ref>{{cite web|title=The Final IAU Resolution on the definition of "planet" ready for voting|url=http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html|publisher=International Astronomical Union|date=2006-08-24|accessdate=2008-01-26 |archiveurl = http://web.archive.org/web/20070927034056/http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html <!-- Bot retrieved archive --> |archivedate = 2007-09-27}}</ref> "Plutonian object" failed to win majority approval on a 183–186 vote in the IAU General Assembly on August 24, 2006.<ref name=iau>{{cite news|publisher=International Astronomical Union|title=IAU 2006 General Assembly: Result of the IAU Resolution votes|url=http://www.iau.org/iau0603.414.0.html|date=2006|accessdate=2008-01-26}}</ref> |
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The definition of the category also fluctuated during its early stages. When first proposed, the category (then named "pluton") defined members as planets whose orbital period around the Sun was more than 200 ], and whose orbit was more highly inclined and more elliptical than a traditional planetary orbit.<ref>{{cite web|date=2006-08-16|title=Draft definition, IAU press release|publisher=International Astronomical Union|url=http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html|accessdate=2008-01-26 |archiveurl = http://web.archive.org/web/20071030012908/http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html <!-- Bot retrieved archive --> |archivedate = 2007-10-30}}</ref> 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.<ref name=res5and6>{{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=Definition of a Planet in the Solar System: Resolutions 5 and 6|publisher=International Astronomical Union|date=2006-08-24|work=IAU 2006 General Assembly|accessdate=2008-01-26}}</ref> |
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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, and approved by the IAU Executive Committee at its meeting in Oslo, Norway, on 11 June 2008.<ref name="nasa">{{cite web|url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Dwarf&Display=OverviewLong|title=Dwarf Planets|publisher=NASA|accessdate=2008-01-22}}</ref> The term was announced after the Executive Committee meeting, along with a greatly simplified definition: all trans-Neptunian dwarf planets are plutoids.<ref name="IAU0804" /> However, due to a communication error, the IAU Working Group for Planetary System Nomenclature (WG-PSN) was not consulted, and they subsequently rejected the term.<ref>IAU (2009). . ''Transactions IAU, Volume XXVIIA''.</ref> |
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As of 2009, the term had not been widely accepted by the scientific community, with the term ''ice dwarf (planet)'' sometimes used instead.<ref>Grundy et al., 2009. .</ref> |
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== Analogous terminology == |
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Prior to the emergence of the term "plutoid" as an IAU-sanctioned categorization, there had been some attempts at applying a title for this class of object. |
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The term ''"ice dwarf"'' saw some adoption as a near-synonym to the eventual term "plutoid". "Ice dwarf", however, also saw some use as an umbrella term for all so-called "distant ]s" (trans-Neptunian objects plus ]s) or other, broad applications; one attempted definition was that an ice dwarf "is larger than the ] of a normal ] and icier than a typical ]".<ref name=darling>{{cite web |
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|title=Ice dwarf |
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|publisher=Encyclopedia of Astrobiology, Astronomy and Spaceflight |
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|author=David Darling |
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|url=http://www.daviddarling.info/encyclopedia/I/icedwarf.html |
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|accessdate=2008-06-22}}</ref> There are large numbers of such objects in the ] and the ]. However, it is not clear whether all so-categorized ice dwarfs are actually icier than icy asteroids such as ] (now considered a dwarf planet). Nonetheless, Ceres is sometimes called a ''terrestrial dwarf'' to distinguish it from Pluto and Eris. |
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"Ice dwarf" as a term for an icy ] that generally orbits beyond ] was coined as part of a conception of a threefold division of the ] into inner '']s'', central '']s'', and outer ''ice dwarfs'', of which Pluto was the principal member. This conception foreshadowed the reclassification of Pluto to dwarf planet and plutoid after the discovery of Eris. |
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== Naming process for plutoids == |
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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 ] and the ] 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 ]. |
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=== Complications related to "dwarf planet" definition === |
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When the definition of "dwarf planet" was instated at the ], 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. |
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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 ] brighter than H = +1 magnitude''.<ref name="IAU0804"/> |
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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.<ref name=bruton>{{cite web |
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|title=Conversion of Absolute Magnitude to Diameter for Minor Planets |
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|publisher=Department of Physics & Astronomy (Stephen F. Austin State University) |
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|author=Dan Bruton |
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|url=http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html |
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|accessdate=2008-06-13}}</ref><ref name=h>{{cite web |
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|title=Conversion of Absolute Magnitude to Diameter |
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|publisher=]: ] |
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|url=http://www.minorplanetcenter.org/iau/lists/Sizes.html |
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|accessdate=2010-10-27}}</ref> 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. However, 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.<ref name="IAU0804"/> |
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This decision allowed for the naming of Makemake and Haumea, and their formal recognition as plutoids and dwarf planets, bringing the number of IAU-accepted plutoids to four. However, the (co)discoverer of Eris, Makemake, and Haumea, Mike Brown, estimates that between 100 and 400 ''known'' bodies are plutoids, and that there are many more that have yet to be discovered. See ]. |
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== References == |
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;Notes |
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{{Reflist|group="note"}} |
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;References |
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{{Reflist}} |
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== External links == |
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{{Misplaced Pages books|Solar System}} |
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*{{Commons category-inline|Plutoids}} |
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{{Trans-Neptunian dwarf planets}} |
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{{Solar System}} |
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{{Good article}} |
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] |
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] |
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] |
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