A plutoid is a trans-Neptunian dwarf planet or an object that is likely to be such a body and that has received a permanent name as a plutoid. 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.

Accordingly, plutoids can be thought of as the intersection of the set of dwarf planets and the set of trans-Neptunian objects. As of 2008, Pluto, , , and are the only objects classified as plutoids. As of April 2007, upwards of seventy more bodies may yet be determined to meet the definition.

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].


[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 and was abandoned in the final draft resolution (6b); "Plutonian object" failed to win majority approval on a 183–186 vote in the IAU General Assembly on August 24 2006.

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. 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.

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. The term was announced after the Executive Committee meeting, along with a greatly-simplified definition: all trans-Neptunian dwarf planets are plutoids.

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.

Complications related to "dwarf planet" definition

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 can be automatically categorized 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.

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. 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.

This decision allowed for the naming of Makemake and Haumea, and their classification as plutoids and dwarf planets, bringing the total number of plutoids from 2 to 4.

Another good demonstration of this concept is that the planet Mercury and Makemake have the same absolute magnitude (H) of -0.4. If Mercury were to be placed where Makemake is, Mercury would have the same apparent magnitude as Makemake. Mercury would have a larger angular diameter with a lower albedo, resulting in the same amount of light being reflected.

Official plutoids

Four trans-Neptunian objects, Pluto, Eris, Haumea, and Makemake, are officially classified as dwarf planets and therefore as plutoids.

Official Plutoids
Name Pluto
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 kg
(4.2±0.1) kg
~4 × 10 kg
(1.67±0.02) kg (est.)
Density (in Mg/) 2.03 ± 0.06 2.6–3.3 ~2 ?
Equatorial gravity (in m/s2) 0.58 ~ 0.44 (varies) ~0.5 ~0.8
Rotation period (d)
(in sidereal days)
-6.387 18
0.163 14 ? > 0.3 ?
Orbital radius* (AU)
semi-major axis
in km
39.481 686 77
5 906 376 200
6 484 000 000
6 850 000 000
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)
Mean surface temperature (in K) 40 30 32±3 ~30
Number of natural satellites 3 2 0 1
Date of discovery February 18, 1930 2004 December 28 March 31, 2005 October 21, 2003

Plutoid candidates

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. 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 dwarf planets.

See also


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