Plutoid

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.

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, Eris and Makemake are the only objects classified as plutoids. Upwards of 41 more bodies known today may yet be determined to meet the definition.

History of the term

See also: 2006 definition of planet

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 (as yet unnamed) 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, an object will be assumed to be a plutoid if it has:

(a) a semimajor axis greater than that of Neptune.

(b) 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.

Official plutoids

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

Official Plutoids

Name	Pluto	Eris
Minor planet number	134340	136199
Absolute magnitude	−0.7	−1.12 ± 0.01
Albedo	0.49–0.66	0.86 ± 0.07
Diameter	2390 km	2400±300 km
Mass in kg compared to Earth	1.305×1022 kg .0022	(1.67±0.02)×1022 kg (est.) .0025
Density (in Mg/m³)	2.03 ± 0.06	?
Equatorial gravity (in m/s2)	0.58	~0.8
Rotation period (d) (in sidereal days)	-6.38718 (retrograde)	> 0.3 ?
Orbital radius* (AU) semi-major axis in km	29.66-49.30 39.48168677 5,906,376,200	37.77-97.56 67.6681 10,210,000,000
Orbital period*(a) (in sidereal years)	248.09	557
Mean orbital speed (in km/s)	4.7490	3.436
Orbital eccentricity	0.24880766	0.44177
Orbital inclination	17.14175°	44.187°
Inclination of the equator from the orbit (see Axial tilt)	119.61°
Mean surface temperature (in K)	40	30
Number of natural satellites	3	1
Date of discovery	February 18, 1930	October 21, 2003

Plutoids for naming process purposes

Two other trans-Neptunian objects have absolute magnitudes (H) less than +1 and therefore proceeded (or will proceed) through the IAU naming procedures under the assumption that they will be confirmed as plutoids. Until hydrostatic equilibrium is defined and established, however, they cannot be officially categorized as dwarf planets or plutoids.

Plutoids for naming process purposes

Name	Makemake
Minor planet number	136472	136108
Provisional Designation	2005 FY9	2003 EL61
Nickname	"Easterbunny"	"Santa"
Absolute magnitude	−0.48	+0.17
Albedo	0.8 ± 0.2	0.7 ± 0.1
Diameter	1,300–1,900 km	~1,960×1,518×996 km
Mass in kg	~ 0.4×1021 kg	(0.42±0.01)×1021 kg
Density (in Mg/m³)	~ 2 ?	2.6–3.3
Equatorial gravity (in m/s2)	~ 0.5	~ 0.44 (varies)
Escape velocity (in km/s)	~ 0.8	~ 0.84 (varies)
Rotation period (d) (in sidereal days)	?	0.16314
Orbital radius* (AU) semi-major axis in km	38.5–53.1 45.8 6,850,000,000	35.2–51.5 43.3 6,484,000,000
Orbital period*(a) (in sidereal years)	309.9	285.4
Mean orbital speed (in km/s)	4.419	4.484
Orbital eccentricity	0.159	0.18874
Orbital inclination	28.963°	28.19°
Mean surface temperature (in K)	~ 30	32±3
Number of natural satellites	0	2
Date of discovery	2005 March 31	2004 December 28

Another good demonstration of this concept is that the planet Mercury and 2005 FY9 have the same absolute magnitude (H) of -0.4. If Mercury were to be placed where 2005 FY9 is, Mercury would have the same apparent magnitude as 2005 FY9. Mercury would have a larger angular diameter with a lower albedo, resulting in the same amount of light being reflected.

Plutoid candidates

Main article: List of 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 August 2006, it was believed that another 42 bodies beyond Neptune (besides Pluto and Eris) were likely dwarf planets.