List of possible dwarf planets
The number of dwarf planets in the Solar System is unknown. Estimates 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 9 among bodies known so far). The International Astronomical Union (IAU) has accepted five: Ceres in the inner Solar System and four in the trans-Neptunian region: Pluto, Eris, Haumea, and Makemake, the last two accepted for naming purposes.
IAU naming proceduresEdit
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.
Beside directly orbiting the Sun, the qualifying feature of a dwarf planet is that it "has 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 objects as large as 1500 km in diameter have proven to not be in equilibrium, whereas dark objects are unlikely to have been resurfaced and thus not have the active geology expected of a dwarf planet.
Ceres is thought to be the only dwarf planet in the asteroid belt. 4 Vesta, the second-most-massive asteroid, appears to have a fully differentiated interior and was therefore in equilibrium at some point in its history, but it is not today. The third-most massive object, 2 Pallas, has a somewhat irregular surface and is thought to have only a partially differentiated interior. Michael Brown has estimated that, because rocky objects such as Vesta and Pallas 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 moons of Saturn up to Iapetus are no longer in hydrostatic equilibrium. They have an equilibrium shape that froze in some time ago, and do not match the shape equilibrium bodies would have at their current rotation rates. Thus Ceres, at 950 km in diameter, is the smallest body for which detailed measurements are consistent with hydrostatic equilibrium, and Iapetus, at 1,470 km, is the largest body known to not be in equilibrium. It is not clear whether trans-Neptunian objects would behave more like Ceres or Iapetus; thus, some or all trans-Neptunian dwarf planets smaller than Pluto and Eris might not actually be in equilibrium. The IAU has not addressed the issue since these findings.
Furthermore, mid-sized TNOs up to about 900 or 1000 km in diameter have lower densities than larger bodies. 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 temperatures of the Kuiper Belt, water ice is strong enough to support open interior spaces (interstices) in objects of this size, and conclude that they 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 object smaller than 900 or 1000 km in diameter is (pending some other formative mechanism) unlikely to be a dwarf planet.
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 the assumption that the object was more than 450 kilometres (280 mi) in diameter. Some diameters are measured, some are best-fit estimates, and others use an assumed albedo of 0.10. Of these, he identified 15 as dwarf planets by his criteria (including the four accepted by the IAU), with another nine 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, as of 2013, they have not responded.
|Brown's categories||Min. ⌀||Number of objects|
|nearly certainly||>900 km||10|
|highly likely||600–900 km||16|
|Source: Mike Brown, as of Jun 21, 2018. (Summary figures differ on M. Brown's website using a cumulative count).|
Mike Brown considers a large number of trans-Neptunian bodies, ranked by estimated size, to be "probably" dwarf planets. He did 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.
- 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.
- 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.
- 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.
- 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.
- Probably not: diameter estimated/measured to be under 200 km. No icy moon under 200 km is round, suggesting that the same is true for KBOs. The estimated size of these objects would have to be in error for them to be dwarf planets.
Grundy et al's assessmentEdit
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 400–1000 km have oddly low densities, in the range of 1.0–1.2 g/cm3, that are substantially less than dwarf planets such as Pluto, 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 possibly 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, low-density 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 900–1000 km.
If correct, then among known bodies in the outer Solar System this would leave only Pluto–Charon, Eris, Haumea, 2007 OR10, Makemake, Quaoar, Orcus and high-albedo Sedna (but probably not low-albedo 2002 MS4) as being likely to have achieved hydrostatic equilibrium at some point in their histories, and thus to possibly be dwarf planets at present.
Likeliest dwarf planetsEdit
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, the largest body known to not be in hydrostatic equilibrium, is included for comparison.
|Per IAU||Per Tancredi
|Per Brown||Per Grundy
|134340 Pluto||2377±3||(measured, in equilibrium)||2:3 resonant|
|136108 Haumea||1596 to 1632||
|S VIII Iapetus||1469±6||(measured, not in equilibrium)||(moon of Saturn)|
|(225088) 2007 OR10||1230±50||NA||SDO|
|90377 Sedna||900 to 1200||detached|
|(307261) 2002 MS4||934±47||NA||3:5 resonant|
|1 Ceres||946±2||(measured, in equilibrium)||asteroid|
|120347 Salacia||854±45||3:5 resonant|
|(532037) 2013 FY27||740+90
|(208996) 2003 AZ84||727+62
By Brown's criteriaEdit
The following trans-Neptunian objects have estimated diameters at least 400 kilometres (250 mi) and so are considered "possible" dwarf planets by Brown's criteria. 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.
The default sort is per Brown's size estimate. The IAU-recognised dwarf planets have bold names. Explanations and sources for the measured masses and diameters can be found in the corresponding articles linked in column "Designation" of the table.
per assumed albedo
|136199 Eris||−1.1||2330||99||16600||−1.1||2326±12||90||2206||11028||accepted (measured)||SDO||2326|
|134340 Pluto||−0.7||2329||64||13030||−0.76||2376±3.2||63||1886||9430||accepted (measured)||2:3 resonant||2376|
|(225088) 2007 OR10||2||1290||19||1750||1.8||1230±50||22||580||2901||SDO||1230|
|50000 Quaoar||2.7||1092||13||1400||2.82||1110±5||11||363||1813||accepted (and recommended)||cubewano||1110|
|90377 Sedna||1.8||1041||32||1.83||995±80||33||572||2861||accepted (and recommended)||detached||995|
|25||459||2293||accepted (and recommended)||2:3 resonant||910|
|(307261) 2002 MS4||4||960||5||3.6||934±47||7||253||1266||3:5 resonant||934|
|1 Ceres||939||3.36||946±2||9||283||1414||asteroid belt||946|
|120347 Salacia||4.2||921||4||438||4.25||854±45||5||188||939||possible||3:5 resonant||854|
|2018 VG18||3.6||253||1266||SDO||895|
|(208996) 2003 AZ84||3.7||747||11||3.74||727+62
|(532037) 2013 FY27||3.5||721||14||3.15||740+90
|(55637) 2002 UX25||3.9||704||11||125||3.87||665±29||11||224||1118||3:5 resonant||665|
|(90568) 2004 GV9||4.2||703||8||4.25||680±34||8||188||939||accepted||3:5 resonant||680|
|(145452) 2005 RN43||3.9||697||11||3.89||679+55
|(55565) 2002 AW197||3.8||693||12||3.3||768+39
|(202421) 2005 UQ513||4||643||11||3.5||498+63
|(523794) 2015 RR245||4.2||615||10||3.8||231||1155||SDO||615|
|(523692) 2014 EZ51||4.2||615||10||3.8||231||1155||detached||615|
|(84522) 2002 TC302||4.2||591||12||3.9||584+106
|(78799) 2002 XW93||5.4||584||4||5.5||106||528||SDO||584|
|(523759) 2014 WK509||4.5||574||9||4.4||175||876||detached||574|
|2017 OF69||4.6||160||799||2:3 resonant||565|
|(42301) 2001 UR163||4.6||561||9||4.1||201||1006||possible||SDO||561|
|(523639) 2010 RE64||4.6||561||9||4.4||175||876||SDO||561|
|(523671) 2013 FZ27||4.6||561||9||4.4||175||876||1:2 resonant||561|
|(230965) 2004 XA192||4.6||549||9||4.2||339+120
|(84922) 2003 VS2||4.1||537||15||4.1||523+35
|15||201||1006||not accepted||2:3 resonant||523|
|(455502) 2003 UZ413||4.7||536||8||4.3||183||917||2:3 resonant||536|
|(120348) 2004 TY364||4.7||536||8||4.52||512+37
|10||166||829||not accepted||2:3 resonant||512|
|(482824) 2013 XC26||4.8||524||8||4.4||175||876||3:5 resonant||524|
|(145451) 2005 RM43||4.8||524||8||4.4||175||876||possible||SDO||524|
|(470443) 2007 XV50||4.8||524||8||4.4||175||876||cubewano||524|
|(470308) 2007 JH43||4.9||513||8||4.5||167||837||2:3 resonant||513|
|(278361) 2007 JJ43||4.9||513||8||4.5||167||837||cubewano||513|
|(82075) 2000 YW134||4.9||513||8||4.5||167||837||detached||513|
|(523681) 2014 BV64||4.9||513||8||4.7||153||763||cubewano||513|
|(523742) 2014 TZ85||5||501||7||4.8||146||729||4:7 resonant||501|
|(499514) 2010 OO127||5||501||7||4.6||160||799||3:5 resonant||501|
|(523645) 2010 VK201||5||501||7||5||133||665||cubewano||501|
|(315530) 2008 AP129||5.1||490||7||4.7||153||763||cubewano||490|
|(470599) 2008 OG19||5.1||490||7||4.7||153||763||SDO||490|
|(523635) 2010 DN93||5.1||490||7||4.8||146||729||detached||490|
|(444030) 2004 NT33||5.1||490||7||4.8||423+87
|(119979) 2002 WC19||5.1||490||7||4.7||153||763||1:2 resonant||490|
|(523693) 2014 FT71||5.1||490||7||5||133||665||4:7 resonant||490|
|(472271) 2014 UM33||5.1||490||7||4.7||153||763||cubewano||490|
|(175113) 2004 PF115||4.5||482||12||4.54||406+98
|(307982) 2004 PG115||5.2||479||7||4.8||146||729||SDO||479|
|(48639) 1995 TL8||5.2||479||7||4.8||146||729||TNO||479|
|(523752) 2014 VU37||5.2||479||7||5.1||127||635||cubewano||479|
|(495603) 2015 AM281||5.2||479||7||4.8||146||729||detached||479|
|(26375) 1999 DE9||5.2||474||7||4.9||461±45||9||139||696||possible||2:5 resonant||461|
|(35671) 1998 SN165||5.7||473||4||5.5||393+39
|(145480) 2005 TB190||4.4||469||15||4.4||464±62||14||175||876||detached||464|
|(26181) 1996 GQ21||5.3||468||7||4.9||139||696||SDO||468|
|(119951) 2002 KX14||4.9||468||10||4.86||445±27||10||142||709||cubewano||445|
|2014 JP80||5.3||468||7||5.1||127||635||2:3 resonant||468|
|2014 JR80||5.3||468||7||5.1||127||635||2:3 resonant||468|
|(523750) 2014 US224||5.3||468||7||5||133||665||cubewano||468|
|(523757) 2014 WH509||5.3||468||7||5.2||121||606||4:7 resonant||468|
|(120132) 2003 FY128||5.1||467||8||4.6||460±21||12||160||799||SDO||460|
|38628 Huya||5||466||8||5.04||406±16||10||130||652||accepted||2:3 resonant||406|
|(84719) 2002 VR128||5.6||459||5||5.58||449+42
|(307616) 2003 QW90||5.4||457||6||5||133||665||cubewano||457|
|(469306) 1999 CD158||5.4||457||6||5||133||665||4:7 resonant||457|
|(523640) 2010 RO64||5.4||457||6||5.2||121||606||cubewano||457|
|(445473) 2010 VZ98||5.4||457||6||4.8||146||729||SDO||457|
|(523653) 2011 OA60||5.4||457||6||5.1||127||635||cubewano||457|
|(308379) 2005 RS43||5.4||457||6||5||133||665||1:2 resonant||457|
|(523772) 2014 XR40||5.4||457||6||5.2||121||606||cubewano||457|
|2003 QX111||6.8||453||2||7.1||51||253||2:3 resonant||453|
|(471137) 2010 ET65||5.5||447||6||5.1||127||635||SDO||447|
|2010 EL139||5.5||447||6||5.6||101||504||2:3 resonant||447|
|(471288) 2011 GM27||5.5||447||6||5.1||127||635||cubewano||447|
|(471165) 2010 HE79||5.5||447||6||5.1||127||635||2:3 resonant||447|
|2014 FY71||5.5||447||6||5.4||111||553||4:7 resonant||447|
|2014 CO23||5.5||447||6||5.3||116||579||4:7 resonant||447|
|(523690) 2014 DN143||5.5||447||6||5.3||116||579||cubewano||447|
|(532093) 2013 HV156||5.5||447||6||5.2||121||606||1:2 resonant||447|
|(523773) 2014 XS40||5.5||447||6||5.4||111||553||3:5 resonant||447|
|(523738) 2014 SH349||5.5||447||6||5.4||111||553||cubewano||447|
|(469372) 2001 QF298||5.4||421||7||5.43||408+40
|(303775) 2005 QU182||3.8||415||33||3.8||416±73||31||231||1155||SDO||416|
|(144897) 2004 UX10||4.8||409||14||4.75||361+124
- The measured diameter, else Brown's estimated diameter, else the diameter calculated from H using an assumed albedo of 8%.
- Diameters with the text in red indicate that Brown's bot derived them from heuristically expected albedo.
- This is the total system mass (including moons), except for Pluto and Ceres.
- The geometric albedo is calculated from the measured absolute magnitude and measured diameter via the formula:
- Mike Brown. "The Dwarf Planets". Retrieved 2008-01-20.
- "Today we know of more than a dozen dwarf planets in the solar system [and] it is estimated that the ultimate number of dwarf planets we will discover in the Kuiper Belt and beyond may well exceed 10,000".The PI's Perspective
- W.M. Grundy, K.S. Noll, M.W. Buie, S.D. Benecchi, D. Ragozzine & H.G. Roe, 'The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ((229762) 2007 UK126)', Icarus (forthcoming, available online 30 March 2019) DOI: 10.1016/j.icarus.2018.12.037,
- Dan Bruton. "Conversion of Absolute Magnitude to Diameter for Minor Planets". Department of Physics & Astronomy (Stephen F. Austin State University). Archived from the original on 2010-03-23. Retrieved 2008-06-13.
- "IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. Archived from the original on 2007-01-03. Retrieved 2008-01-26.
- "Dwarf Planets". NASA. Retrieved 2008-01-22.
- "Plutoid chosen as name for Solar System objects like Pluto" (Press release).
- Savage, Don; Jones, Tammy; Villard, Ray (1995-04-19). "Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta". Hubble Site News Release STScI-1995-20. Retrieved 2006-10-17.
- "Iapetus' peerless equatorial ridge". www.planetary.org. Retrieved 2 April 2018.
- "DPS 2015: First reconnaissance of Ceres by Dawn". www.planetary.org. Retrieved 2 April 2018.
- Tancredi, G. (2010). "Physical and dynamical characteristics of icy "dwarf planets" (plutoids)". Icy Bodies of the Solar System: Proceedings IAU Symposium No. 263, 2009.
- Michael E. Brown. "How many dwarf planets are there in the outer solar system? (updates daily)". California Institute of Technology. Retrieved 2 June 2017.
- "AstDys (47171) 1999TC36 Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2009-12-07.
- "List Of Trans-Neptunian Objects". Minor Planet Center.
- "List Of Centaurs and Scattered-Disk Objects". Minor Planet Center.
- How many dwarf planets are there in the outer solar system? (updates daily) (Mike Brown)
- Details on the dwarf planet size calculations (Mike Brown)
- Which are the Dwarfs in the Solar System? Tancredi, G.; Favre, S. Icarus, Volume 195, Issue 2, p. 851–862.
- NASA JPL Small-Body Database Search Engine