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.
The General Assembly of the IAU further resolved:
Pluto is [...] recognized as the prototype of a new category of Trans-Neptunian Objects.
 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.
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.
|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/m³)||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)|
39.481 686 77
5 906 376 200
6 484 000 000
6 850 000 000
10 210 000 000
| Orbital period*(a)|
(in sidereal years)
| Mean orbital speed|
|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|