List of possible dwarf planets

 The number of dwarf planets in the Solar System is unknown. Estimates have run as high as 200 in the Kuiper belt and over 10,000 in the region beyond. However, consideration of the surprisingly low densities of many dwarf-planet candidates suggests that the numbers may be much lower (e.g. at most 10 among bodies known so far). The International Astronomical Union (IAU) notes five in particular: Ceres in the inner Solar System and four in the trans-Neptunian region: Pluto, Eris, Haumea, and Makemake, the last two of which were accepted as dwarf planets for naming purposes. Only Pluto is confirmed as a dwarf planet, and it has also been declared one by the IAU independently of whether it meets the IAU definition of a dwarf planet.

IAU naming procedures

See also: IAU definition of planet

In 2008, the IAU modified its naming procedures such that objects considered most likely to be dwarf planets receive differing treatment than others. Objects that have an absolute magnitude (H) less than +1, and hence a minimum diameter of 838 kilometres (521 mi) if the albedo is below 100%, are overseen by two naming committees, one for minor planets and one for planets. Once named, the objects are declared to be dwarf planets. Makemake and Haumea are the only objects to have proceeded through the naming process as presumed dwarf planets; currently there are no other bodies that meet this criterion. All other bodies are named by the minor-planet naming committee alone, and the IAU has not stated how or if they will be accepted as dwarf planets.

Limiting values

Calculation of the diameter of Ixion depends on the albedo (the fraction of light that it reflects). Current estimates are that the albedo is 13–15%, a bit under the midpoint of the range shown here and corresponding to a diameter of 620 km.

Beside directly orbiting the Sun, the qualifying feature of a dwarf planet is that it have "sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape". Current observations are generally insufficient for a direct determination as to whether a body meets this definition. Often the only clues for trans-Neptunian objects is a crude estimate of their diameters and albedos. Icy satellites as large as 1500 km in diameter have proven to not be in equilibrium, whereas dark objects in the outer solar system often have low densities that imply they are not even solid bodies, much less gravitationally controlled dwarf planets.

Ceres, which has a significant amount of ice in its composition, is the only confirmed dwarf planet in the asteroid belt although Hygeia may possibly also be one. 4 Vesta, the second-most-massive asteroid and one that is basaltic in composition, appears to have a fully differentiated interior and was therefore in equilibrium at some point in its history, but no longer is today. The third-most massive object, 2 Pallas, has a somewhat irregular surface and is thought to have only a partially differentiated interior; it is also less icy than Ceres. Michael Brown has estimated that, because rocky objects such as Vesta are more rigid than icy objects, rocky objects below 900 kilometres (560 mi) in diameter may not be in hydrostatic equilibrium and thus not dwarf planets.

Based on a comparison with the icy moons that have been visited by spacecraft, such as Mimas (round at 400 km in diameter) and Proteus (irregular at 410–440 km in diameter), Brown estimated that an icy body relaxes into hydrostatic equilibrium at a diameter somewhere between 200 and 400 km. However, after Brown and Tancredi made their calculations, better determination of their shapes showed that Mimas and the other mid-sized ellipsoidal moons of Saturn up to at least Iapetus (which is of the approximate size of Haumea and Makemake) are no longer in hydrostatic equilibrium; they are also icier than TNOs are likely to be. They have equilibrium shapes that froze in place some time ago, and do not match the shapes that equilibrium bodies would have at their current rotation rates. Thus Ceres, at 950 km in diameter, is the smallest body for which gravitational measurements indicate current hydrostatic equilibrium. Much larger objects, such as Earth's moon, are not near hydrostatic equilibrium today, though the Moon is composed primarily of silicate rock (in contrast to most dwarf planet candidates, which are ice and rock). Saturn's moons may have been subject to a thermal history that would have produced equilibrium-like shapes in bodies too small for gravity alone to do so. Thus, at present it is unknown whether any trans-Neptunian objects smaller than Pluto and Eris are in hydrostatic equilibrium.

The majority of mid-sized TNOs up to about 900–1000 km in diameter have significantly lower densities (~ 1.0–1.2 g/ml) than larger bodies such as Pluto (1.86 g/ml). Brown had speculated that this was due to their composition, that they were almost entirely icy. However, Grundy et al. point out that there is no known mechanism or evolutionary pathway for mid-sized bodies to be icy while both larger and smaller objects are partially rocky. They demonstrated that at the prevailing temperatures of the Kuiper Belt, water ice is strong enough to support open interior spaces (interstices) in objects of this size; they concluded that mid-size TNOs have low densities for the same reason that smaller objects do—because they have not compacted under self-gravity into fully solid objects, and thus the typical TNO smaller than 900–1000 km in diameter is (pending some other formative mechanism) unlikely to be a dwarf planet.

Tancredi's assessment

In 2010, Gonzalo Tancredi presented a report to the IAU evaluating a list of 46 candidates for dwarf planet status based on light-curve-amplitude analysis and a calculation that the object was more than 450 kilometres (280 mi) in diameter. Some diameters were measured, some were best-fit estimates, and others used an assumed albedo of 0.10 to calculate the diameter. Of these, he identified 15 as dwarf planets by his criteria (including the 4 accepted by the IAU), with another 9 being considered possible. To be cautious, he advised the IAU to "officially" accept as dwarf planets the top three not yet accepted: Sedna, Orcus, and Quaoar. Although the IAU had anticipated Tancredi's recommendations, a decade later the IAU had never responded.

Brown's assessment

Artistic comparison of Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus, Salacia, 2002 MS₄, and Earth along with the Moon

• v
• t
• e

Brown's categories	Min. ⌀	Number of objects
nearly certainly	>900 km	10
highly likely	600–900 km	17 (27 total)
likely	500–600 km	41 (68 total)
probably	400–500 km	62 (130 total)
possibly	200–400 km	611 (741 total)
Source: Mike Brown, as of 2020 October 22

Mike Brown considers 130 trans-Neptunian bodies to be "probably" dwarf planets, ranked them by estimated size. He does not consider asteroids, stating "in the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round."

The terms for varying degrees of likelihood he split these into:

• Near certainty: diameter estimated/measured to be over 900 kilometres (560 mi). Sufficient confidence to say these must be in hydrostatic equilibrium, even if predominantly rocky. 10 objects as of 2020.
• Highly likely: diameter estimated/measured to be over 600 kilometres (370 mi). The size would have to be "grossly in error" or they would have to be primarily rocky to not be dwarf planets. 17 objects as of 2020.
• Likely: diameter estimated/measured to be over 500 kilometres (310 mi). Uncertainties in measurement mean that some of these will be significantly smaller and thus doubtful. 41 objects as of 2020.
• Probably: diameter estimated/measured to be over 400 kilometres (250 mi). Expected to be dwarf planets, if they are icy, and that figure is correct. 62 objects as of 2020.
• Possibly: diameter estimated/measured to be over 200 kilometres (120 mi). Icy moons transition from a round to irregular shape in the 200–400 km range, suggesting that the same figure holds true for KBOs. Thus, some of these objects could be dwarf planets. 611 objects as of 2020.
• Probably not: diameter estimated/measured to be under 200 km. No icy moon under 200 km is round, and the same may be true of KBOs. The estimated size of these objects would have to be in error for them to be dwarf planets.

Beside the five accepted by the IAU, the 'nearly certain' category includes Gonggong, Quaoar, Sedna, Orcus, 2002 MS4 and Salacia.

Grundy et al.’s assessment

Grundy et al. propose that dark, low-density TNOs in the size range of approximately 400–1000 km are transitional between smaller, porous (and thus low-density) bodies and larger, denser, brighter and geologically differentiated planetary bodies (such as dwarf planets). Bodies in this size range should have begun to collapse the interstitial spaces left over from their formation, but not fully, leaving some residual porosity.

Many TNOs in the size range of about 400–1000 km have oddly low densities, in the range of about 1.0–1.2 g/cm³, that are substantially less than dwarf planets such as Pluto, Eris and Ceres, which have densities closer to 2. Brown has suggested that large low-density bodies must be composed almost entirely of water ice, since he presumed that bodies of this size would necessarily be solid. However, this leaves unexplained why TNOs both larger than 1000 km and smaller than 400 km, and indeed comets, are composed of a substantial fraction of rock, leaving only this size range to be primarily icy. Experiments with water ice at the relevant pressures and temperatures suggest that substantial porosity could remain in this size range, and it is possible that adding rock to the mix would further increase resistance to collapsing into a solid body. Bodies with internal porosity remaining from their formation could be at best only partially differentiated, in their deep interiors. (If a body had begun to collapse into a solid body, there should be evidence in the form of fault systems from when its surface contracted.) The higher albedos of larger bodies is also evidence of full differentiation, as such bodies were presumably resurfaced with ice from their interiors. Grundy et al. propose therefore that mid-size (< 1000 km), low-density (< 1.4 g/ml) and low-albedo (< ~0.2) bodies such as Salacia, Varda, Gǃkúnǁʼhòmdímà and (55637) 2002 UX25 are not differentiated planetary bodies like Orcus, Quaoar and Charon. The boundary between the two populations would appear to be in the range of about 900–1000 km.

If Grundy et al. are correct, then among known bodies in the outer Solar System only Pluto–Charon, Eris, Haumea, Gonggong, Makemake, Quaoar, Orcus, Sedna and perhaps Salacia (which if it were spherical and had the same albedo as its moon would have a density of between 1.4 and 1.6 g/cm³, calculated a few months after Grundy et al's initial assessment, though still an albedo of only 0.04) are likely to have compacted into fully solid bodies, and thus to possibly have become dwarf planets at some point in their past or to still be dwarf planets at present.

Likeliest dwarf planets

The assessments of the IAU, Tancredi et al., Brown and Grundy et al. for the dozen largest potential dwarf planets are as follows. For the IAU, the acceptance criteria were for naming purposes. Several of these objects had not yet been discovered when Tancredi et al. did their analysis. Brown's sole criterion is diameter; he accepts a great many more as highly likely to be dwarf planets (see below). Grundy et al. did not determine which bodies were dwarf planets, but rather which could not be. A red  marks objects too dark or not dense enough to be solid bodies, a question mark the smaller bodies consistent with being differentiated (the question of current equilibrium was not addressed).

Iapetus, Earth's moon and Phoebe are included for comparison, as none of these objects are in equilibrium today. Triton (which formed as a TNO and is likely still in equilibrium) and Charon are included as well.

Designation	Measured mean diameter (km)	Density (g/cm3)	Albedo	Per IAU	Per Tancredi et al.	Per Brown	Per Grundy et al.	Category
The Moon	3475	3.344	0.136	(no longer in equilibrium)	(moon of Earth)
N I Triton	2707±2	2.06	0.76	(likely in equilibrium)	(moon of Neptune)
134340 Pluto	2376±3	1.854±0.006	0.49 to 0.66	2:3 resonant
136199 Eris	2326±12	2.43±0.05	0.96	SDO
136108 Haumea	≈ 1560	≈ 2.018	0.51	(naming rules)	cubewano
S VIII Iapetus	1469±6	1.09±0.01	0.05 to 0.5	(no longer in equilibrium)	(moon of Saturn)
136472 Makemake	1430+38 −22	1.9±0.2	0.81	(naming rules)	cubewano
225088 Gonggong	1230±50	1.74±0.16	0.14	NA	3:10 resonant
P I Charon	1212±1	1.70±0.02	0.2 to 0.5	(possibly in equilibrium)	(moon of Pluto)
50000 Quaoar	1110±5	2.0±0.5	0.11	cubewano
90377 Sedna	995±80	?	0.32	detached
1 Ceres	946±2	2.16±0.01	0.09	(close to equilibrium)	asteroid
90482 Orcus	910+50 −40	1.53±0.14	0.23	2:3 resonant
120347 Salacia	846±21	1.5±0.12	0.04	cubewano
(307261) 2002 MS4	778±11	?	0.10	NA	cubewano
(55565) 2002 AW197	768±39	?	0.11	cubewano
174567 Varda	749±18	1.27±0.06	0.10	4:7 resonant
(532037) 2013 FY27	742+78 −83	?	0.17	NA	SDO
(208996) 2003 AZ84	707±24	0.87±0.01?	0.10	2:3 resonant
S IX Phoebe	213±2	1.64±0.03	0.06	(no longer in equilibrium)	(moon of Saturn)

Largest candidates

The following trans-Neptunian objects have estimated diameters at least 400 kilometres (250 mi) and so are considered "probable" dwarf planets by Brown's assessment. Not all bodies estimated to be this size are included. The list is complicated by bodies such as 47171 Lempo that were at first assumed to be large single objects but later discovered to be binary or triple systems of smaller bodies. The dwarf planet Ceres is added for comparison. Explanations and sources for the measured masses and diameters can be found in the corresponding articles linked in column "Designation" of the table.

The Best diameter column uses a measured diameter if one exists, otherwise it uses Brown's assumed-albedo diameter. If Brown does not list the body, the size is calculated from an assumed-albedo of 9% per Johnston.

Designation	Best diameter km	Measured	per measured	Per Brown	Notes on shape	Result per Tancredi	Category
Mass (1018 kg)	H	Diameter (km)	Method	Geometric albedo (%)	H	Diameter (km)	Geometric albedo (%)
134340 Pluto	2377	13030	−0.76	2377±3	direct	63	−0.7	2329	64	spherical	accepted (measured)	2:3 resonant
136199 Eris	2326	16466	−1.17	2326±12	occultation	96	−1.1	2330	99	spherical	accepted (measured)	SDO
136108 Haumea	1559	4006	0.43	1559	occultation	49	0.4	1252	80	Jacobi ellipsoid	accepted	cubewano
136472 Makemake	1429	3100	0.05	1429+38 −20	occultation	83	0.1	1426	81	slightly oblate	accepted	cubewano
225088 Gonggong	1230	1750	2.34	1230±50	thermal	14	2	1290	19	3:10 resonant
50000 Quaoar	1103	1400	2.74	1103+47 −33	occultation	11	2.7	1092	13	Maclaurin spheroid	accepted (and recommended)	cubewano
1 Ceres	939	939	3.36	939±2	direct	9	Maclaurin spheroid	asteroid belt
90482 Orcus	910	641	2.31	910+50 −40	thermal	25	2.3	983	23	accepted (and recommended)	2:3 resonant
90377 Sedna	906	1.83	906+314 −258	thermal	40	1.8	1041	32	accepted (and recommended)	detached
120347 Salacia	846	492	4.27	846±21	thermal	5	4.2	921	4	possible	cubewano
(307261) 2002 MS4	787	3.5	787±13	occultation	11	4	960	5	Maclaurin spheroid	cubewano
(55565) 2002 AW197	768	3.57	768+39 −38	thermal	11	3.6	754	12	accepted	cubewano
174567 Varda	749	245	3.61	749±18	occultation	11	3.7	689	13	Maclaurin spheroid	possible	cubewano
(532037) 2013 FY27	742	3.15	742+78 −83	thermal	18	3.5	721	14	SDO
28978 Ixion	710	3.83	710±0.2	occultation	10	3.8	674	12	Maclaurin spheroid	accepted	2:3 resonant
(208996) 2003 AZ84	707	3.74	707±24	occultation	11	3.9	747	11	Jacobi ellipsoid	accepted	2:3 resonant
(90568) 2004 GV9	680	4.23	680±34	thermal	8	4.2	703	8	accepted	cubewano
(145452) 2005 RN43	679	3.89	679+55 −73	thermal	11	3.9	697	11	possible	cubewano
(55637) 2002 UX25	659	125	3.87	659±38	thermal	12	3.9	704	11	cubewano
2018 VG18	656	3.6	3.9	656	12	SDO
229762 Gǃkúnǁʼhòmdímà	655	136	3.69	655+14 −13	occultation	14	3.7	612	17	Maclaurin spheroid	SDO
20000 Varuna	654	3.76	654+154 −102	thermal	12	3.9	756	9	Jacobi ellipsoid	accepted	cubewano
2018 AG37	645	4.19	SDO
2014 UZ224	635	3.4	635+65 −72	thermal	14	3.7	688	11	SDO
(523794) 2015 RR245	626	3.8	4.1	626	11	SDO
(523692) 2014 EZ51	626	3.8	4.1	626	11	detached
2010 RF43	611	3.9	4.2	611	10	SDO
19521 Chaos	600	4.8	600+140 −130	thermal	5	5	612	5	cubewano
2010 JO179	597	4	4.3	597	10	SDO
2012 VP113	597	4	4.3	597	10	detached
2010 KZ39	597	4	4.3	597	10	detached
(303775) 2005 QU182	584	3.8	584+155 −144	thermal	13	3.8	415	33	cubewano
(543354) 2014 AN55	583	4.1	4.4	583	10	SDO
2015 KH162	583	4.1	4.4	583	10	detached
(78799) 2002 XW93	565	5.5	565+71 −73	thermal	4	5.4	584	4	SDO
2006 QH181	556	4.3	4.6	556	9	SDO
2002 XV93	549	5.42	549+22 −23	thermal	4	5.4	564	4	2:3 resonant
(84922) 2003 VS2	548	4.1	548+30 −45	occultation	15	4.1	537	15	triaxial ellipsoid	not accepted	2:3 resonant
(523639) 2010 RE64	543	4.4	4.7	543	8	SDO
(523759) 2014 WK509	543	4.4	4.7	543	8	detached
(528381) 2008 ST291	543	4.4	4.7	543	8	detached
(470443) 2007 XV50	543	4.4	4.7	543	8	cubewano
(482824) 2013 XC26	543	4.4	4.7	543	8	cubewano
(523671) 2013 FZ27	543	4.4	4.7	543	8	1:2 resonant
2004 XR190	538	4.3	538	occultation	12	4.6	556	9	oblate	detached
2015 BP519	530	4.5	4.8	530	8	SDO
(278361) 2007 JJ43	530	4.5	4.8	530	8	cubewano
(470308) 2007 JH43	530	4.5	4.8	530	8	2:3 resonant
2014 WP509	530	4.5	4.8	530	8	cubewano
(145451) 2005 RM43	524	4.4	524+96 −103	thermal	11	4.7	543	8	possible	SDO
2013 AT183	518	4.6	4.9	518	8	SDO
2014 FC69	518	4.6	4.9	518	8	detached
(499514) 2010 OO127	518	4.6	4.9	518	8	cubewano
2014 YA50	518	4.6	4.8	518	8	cubewano
2017 OF69	518	4.6	4.9	518	8	2:3 resonant
2020 FY30	517	4.67	SDO
(84522) 2002 TC302	514	3.9	514±15	occultation	14	4.2	591	12	oblate	2:5 resonant
(120348) 2004 TY364	512	4.52	512+37 −40	thermal	10	4.7	536	8	not accepted	2:3 resonant
(145480) 2005 TB190	507	4.4	507+127 −116	thermal	15	4.4	469	15	detached
(470599) 2008 OG19	506	4.7	5	506	7	elongated	SDO
2014 FC72	506	4.7	5	506	7	detached
2014 HA200	506	4.7	5	506	7	SDO
(315530) 2008 AP129	506	4.7	5	506	7	cubewano
(472271) 2014 UM33	506	4.7	5	506	7	cubewano
(523681) 2014 BV64	506	4.7	5	506	7	cubewano
2010 FX86	506	4.7	5	506	7	cubewano
2015 BZ518	506	4.7	5	506	7	cubewano
(202421) 2005 UQ513	498	3.6	498+63 −75	thermal	26	3.8	643	11	cubewano
(523742) 2014 TZ85	494	4.8	5.1	494	7	4:7 resonant
(523635) 2010 DN93	490	4.8	5.1	490	7	detached
2003 QX113	490	5.1	5.1	490	7	SDO
2003 UA414	490	5	5.1	490	7	SDO
(523693) 2014 FT71	490	5	5.1	490	7	4:7 resonant
2014 HZ199	479	5	5.2	479	7	cubewano
2014 BZ57	479	5	5.2	479	7	cubewano
(523752) 2014 VU37	479	5.1	5.2	479	7	cubewano
(495603) 2015 AM281	479	4.8	5.2	479	7	detached
(455502) 2003 UZ413	472	4.38	472+122 −25	thermal	15	4.7	536	8	2:3 resonant
(523645) 2010 VK201	471	5	5.3	471	7	cubewano
2015 AJ281	468	5	5.3	468	7	4:7 resonant
(523757) 2014 WH509	468	5.2	5.3	468	7	cubewano
2014 JP80	468	5	5.3	468	7	2:3 resonant
2014 JR80	468	5.1	5.3	468	7	2:3 resonant
(523750) 2014 US224	468	5	5.3	468	7	cubewano
2013 FS28	468	4.9	5.3	468	7	SDO
2010 RF188	468	5.2	5.3	468	7	SDO
2011 WJ157	468	5	5.3	468	7	SDO
(120132) 2003 FY128	460	4.6	460±21	thermal	12	5.1	467	8	SDO
2010 ER65	457	5.2	5.4	457	6	detached
(445473) 2010 VZ98	457	4.8	5.4	457	6	SDO
2010 RF64	457	5.7	5.4	457	6	cubewano
(523640) 2010 RO64	457	5.2	5.4	457	6	cubewano
2010 TJ	457	5.7	5.4	457	6	SDO
2014 OJ394	457	5.1	5.4	457	6	detached
2014 QW441	457	5.2	5.4	457	6	cubewano
2014 AM55	457	5.2	5.4	457	6	cubewano
(523772) 2014 XR40	457	5.2	5.4	457	6	cubewano
(523653) 2011 OA60	457	5.1	5.4	457	6	cubewano
(26181) 1996 GQ21	456	4.9	456+89 −105	thermal	6	5.3	468	7	SDO
(84719) 2002 VR128	449	5.58	449+42 −43	thermal	5	5.6	459	5	2:3 resonant
2013 SF106	451	4.96	SDO
2012 VB116	449	5.2	5.4	449	6	cubewano
(471137) 2010 ET65	447	5.1	5.5	447	6	SDO
(471165) 2010 HE79	447	5.1	5.5	447	6	2:3 resonant
2010 EL139	447	5.6	5.5	447	6	2:3 resonant
(523773) 2014 XS40	447	5.4	5.5	447	6	cubewano
2014 XY40	447	5.1	5.5	447	6	cubewano
2015 AH281	447	5.1	5.5	447	6	cubewano
2014 CO23	447	5.3	5.5	447	6	cubewano
(523690) 2014 DN143	447	5.3	5.5	447	6	cubewano
(523738) 2014 SH349	447	5.4	5.5	447	6	cubewano
2014 FY71	447	5.4	5.5	447	6	4:7 resonant
(471288) 2011 GM27	447	5.1	5.5	447	6	cubewano
(532093) 2013 HV156	447	5.2	5.5	447	6	1:2 resonant
471143 Dziewanna	433	3.8	433+63 −64	thermal	30	3.8	475	25	SDO
(444030) 2004 NT33	423	4.8	423+87 −80	thermal	12	5.1	490	7	4:7 resonant
(182934) 2002 GJ32	416	6.16	416+81 −73	thermal	3	6.1	235	12	SDO
(469372) 2001 QF298	408	5.43	408+40 −45	thermal	7	5.4	421	7	2:3 resonant
(175113) 2004 PF115	406	4.54	406+98 −85	thermal	12	4.5	482	12	2:3 resonant
38628 Huya	406	5.04	406±16	thermal	10	5	466	8	Maclaurin spheroid	accepted	2:3 resonant
(307616) 2003 QW90	401	5	401+63 −48	thermal	8	5.4	457	6	cubewano
(469615) 2004 PT107	400	6.33	400+45 −51	thermal	3	6	302	8	cubewano

1. ^ The measured diameter, else Brown's estimated diameter, else the diameter calculated from H using an assumed albedo of 9%.
2. ^ This is the total system mass (including moons), except for Pluto and Ceres.
3. ^ The geometric albedo  is calculated from the measured absolute magnitude  and measured diameter  via the formula: 
4. ^ Diameters with the text in red indicate that Brown's bot derived them from heuristically expected albedo.

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.