(225088) 2007 OR10

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(225088) 2007 OR10
2007 OR10 and its moon.png
2007 OR10 and its moon seen by the Hubble Space Telescope in 2010
Discovery [1][2]
Discovered byM. E. Schwamb
M. E. Brown
D. L. Rabinowitz
Discovery sitePalomar Obs.
Discovery date17 July 2007 [a]
MPC designation(225088) 2007 OR10
2007 OR10
TNO[3] · SDO[4]
3:10 res.[5][6]
 · p-DP[7]
Orbital characteristics[3]
Epoch 27 April 2019 (JD 2458600.5)
Uncertainty parameter 4
Observation arc30.09 yr (10,989 days)
Earliest precovery date19 August 1985
Aphelion101.259 AU
Perihelion33.494 AU
67.376 AU
553.05 yr (201,863 days)
0° 0m 6.415s / day
Known satellites1[8]
Physical characteristics
Mean diameter
1230±50 km[9]
(assuming equator-on view, but implying a density of 0.92+0.46

1280±210 km[12]
Flattening0.03 (for a rotation period of 22.4 h)[9]
0.007 (for a rotation period of 44.81 h)[9]
Mass(1.75±0.07)×1021 kg[9]
Mean density
1.74±0.16 g/cm3[9][9]
Equatorial surface gravity
≈ 0.3
Equatorial escape velocity
≈ 0.61
22.40±0.18 h or 44.81±0.37 h
(ambiguous,[10][11] but 22.4 h more likely[9])

(assuming equator-on view)[10][11]
B−V=1.38, V−R=0.86[13][14]
1.8[1][3] · 2.34[10]

(225088) 2007 OR10 is a trans-Neptunian object orbiting the Sun in the scattered disc, approximately 1,230 km (760 mi) in diameter. It is the fifth-largest known body in the Solar System beyond the orbit of Neptune,[9][10][11][16] and is the largest known body in the Solar System without a name.[17] According to estimates as of September 2018, it is slightly larger than Charon, and is hence almost certainly a dwarf planet.[7][6] It has one known satellite, S/2010 (225088) 1, which likely has a diameter of less than 100 km (60 mi).[9]



1991 precovery image of 2007 OR10.

(225088) 2007 OR10 was discovered by American astronomers Megan Schwamb, Michael Brown and David Rabinowitz on 17 July 2007.[1] The discovery was part of the Palomar Distant Solar System Survey, a conducted survey to find distant objects in the region of Sedna using the Samuel Oschin telescope at Palomar Observatory near San Diego, California.[18][19][20] The sensitive Samuel Oschin telescope used during the survey was designed to detect the movements of distant Sedna-like objects at least 1,000 astronomical units from the Sun.[20] Schwamb identified 2007 OR10 by comparing multiple images using the blinking technique to show movements of objects.[19] From the blink comparison of images, 2007 OR10 appeared to move slowly, suggesting that it is a distant object.[19][21] The discovery was part of Schwamb's PhD thesis, who at that time was graduate student of Michael Brown at Caltech.[22][19]

2007 OR10 was formally announced in a Minor Planet Circular notice published on 7 January 2009.[2] Upon its announcment, the object was given the provisional designation 2007 OR10, which indicates its year of discovery and the letters further specifying that its discovery took place on 17 July.[2][23] It has been observed 230 times over 13 oppositions and has been identified in two precovery images, with the earliest image taken by the European Southern Observatory in 19 August 1985.[3][24]


2007 OR10 is currently the largest known object in the Solar System without an official name.[16] Initially after the discovery of 2007 OR10, Brown nicknamed the object "Snow White" for its presumed white color, because it would have to be very large or very bright to be detected by their survey.[25][17] By that time, Brown's team had already discovered seven large trans-Neptunian objects which were collectively referred as the "seven dwarfs":[25][17] Quaoar in 2002, Sedna in 2003, Haumea, Salacia and Orcus in 2004, and Makemake and Eris in 2005. However, 2007 OR10 turned out to be one of the reddest objects in the Kuiper belt, comparable only to Quaoar, so the nickname was dropped.[25][21] Two years after its discovery in 2007, the Minor Planet Center assigned the minor planet number 225088 to 2007 OR10 on 2 November 2009.[24]

The discoverers of 2007 OR10, including Schwamb, Brown, and Rabinowitz were given a 10-year period for naming proposals following the numbering of the object.[26][27] 2007 OR10 was assigned its minor planet number in November 2009, hence the discoverers were given the privilege to submit a naming proposal by November 2019.[26][27] Upon the discovery and announcement of 2007 OR10, Brown did not consider naming it as he regarded it to be an unremarkable object despite its large size.[17][28] In 2011, Brown later decided he had enough information to justify giving it a name, in consideration of the discovery of water ice and the possibility of methane on its surface which made it noteworthy enough to warrant further study.[22] In 2016, in response to the larger size revisions of 2007 OR10 made in that same year, Schwamb remarked:

In 2019, the discoverers of 2007 OR10 hosted an online poll for the general public to decide between three possible names they thought appropriate: Gonggong (Chinese), Holle (German), and Vili (Norse). These names were selected by the discoverers in accordance to the minor planet naming criteria, which states that objects with similar orbits to 2007 OR10 must be given names associated with mythological figures associated with creation.[26][27] These names were also picked as they are associated with water, ice, snow, as well as the color red, which are all characteristics of 2007 OR10.[29] The voting session ended on 10 May 2019 at 11:59 PDT (11 May 2019 at 6:59 UTC).[26][19] The discovery team plans to formally submit the naming proposal with the most voted name to the International Astronomical Union after the end of the voting session.[26] The satellite of 2007 OR10 will not be named by the hosts of the naming poll as the naming privilege for the satellite of 2007 OR10 is only reserved to the discoverers of the satellite.[26]

Physical characteristics[edit]

Surface and spectra[edit]

Artist's impression of 2007 OR10 depicting its red surface color.

The surface composition and spectrum of 2007 OR10 is similar to that of Quaoar, as both objects are red in color and display signs of water ice and possibly methane in their spectra.[30][31] Due to this similarity in composition and spectrum, 2007 OR10 was assumed to have a Quaoar-like albedo (reflectivity) of 0.18.[30][12] The spectrum of 2007 OR10 was first measured in 2011, at near-infrared wavelengths using the Folded port InfraRed Echellette (FIRE) spectrograph on the Magellan Baade Telescope at the Las Campanas Observatory in Chile.[30] The observed spectrum of 2007 OR10 exhibits a strong red spectral slope along with absorption signatures at wavelengths of 1.5 μm and 2 μm.[30] These absorption signatures are characteristic features of water ice, which is often found on large Kuiper belt objects.[30][32] Additional photometric measurements from the Hubble Space Telescope's Wide Field Camera 3 instrument display similar water ice absorption signatures at 1.5 μm.[30] The presence of water ice on the surface of 2007 OR10 implies a brief period of cryovolcanism in its distant past where water erupted from its interior, deposited onto its surface, and subsequently froze.[33][22][26] The red color of 2007 OR10 implies that methane is present on its surface, although it was not directly detected in the spectrum of 2007 OR10.[30] 2007 OR10 is large enough to be able to retain volatile methane on its surface, even when at its closest distance to the Sun (33.5 AU)[3] where temperatures are higher than that of Quaoar.[30] In particular, the large size of 2007 OR10 means that it is likely to retain other volatiles including ammonia, carbon monoxide, and possibly nitrogen, which almost all trans-Neptunian objects lose over the course of their existence.[31][10][16] Like Quaoar, 2007 OR10 is believed to be near the mass limit in which it is able to retain volatile materials such as methane on its surface.[31][22]

2007 OR10 is among the reddest objects known.[30] Its red color is likely in part due to methane frosts being irradiated by sunlight and cosmic rays.[30] The irradiation of methane on its surface produces reddish organic compounds known as tholins.[30][10] The presence of tholins on the surfaces of both 2007 OR10 and Quaoar implies the existence of a tenuous methane atmosphere on both objects, slowly evaporating into space. Although 2007 OR10 comes closer to the Sun than Quaoar, and is thus warm enough that a methane atmosphere should evaporate, its larger mass makes retention of an atmosphere just possible.[30][34]


The size of an object can be calculated from its absolute magnitude (brightness) and albedo.[35] 2007 OR10 has an absolute magnitude (H) of 1.8,[3] which makes it the fifth-brightest trans-Neptunian object known,[36] brighter than Sedna (H=1.83; D=995 km).[37] and Orcus (H=2.2; D≈900 km).[38] Upon the discovery of 2007 OR10, its observed apparent magnitude was 21.4,[18] too dim to be seen from Earth with the naked eye.[26][b] The apparent magnitude of 2007 OR10 is dimmer than that of Sedna, which had an apparent magnitude of 20.7 at that time.[18]


Size estimates
Year Diameter Method Refs
2010 1,752 km thermal [40]
2011 1,200+300
best fit albedo [30]
2012 1,280±210 km thermal [12]
2013 1,142+647
thermal [41]
2013 1,290 km radiometric [7]
2016 1,834.53 km light curve [14]
2016 1,535+75
thermal [10]
2018 1,230±50 km radiometric [9]
EarthMoonCharonCharonNixNixKerberosStyxHydraHydraPlutoPlutoDysnomiaDysnomiaErisErisNamakaNamakaHi'iakaHi'iakaHaumeaHaumeaMakemakeMakemakeMK2MK22007 OR102007 OR10S/(225088) 1S/(225088) 1WeywotWeywotQuaoarQuaoarSednaSedna2002 MS42002 MS4VanthVanthOrcusOrcusActaeaActaeaSalaciaSalaciaFile:EightTNOs.png
Artistic comparison of Pluto, Eris, Haumea, Makemake, 2007 OR10, Quaoar, Sedna, 2002 MS4, Orcus, Salacia, and Earth along with the Moon.
2007 OR10 compared to the Earth and the Moon.

As of 2019, 2007 OR10 is estimated to have a diameter of 1,230 km (760 mi), derived from radiometric measurements, its calculated mass, and assuming a density similar to other similar bodies.[9] This size would make 2007 OR10 the fifth-largest dwarf planet, after Pluto, Eris, Haumea, and Makemake.[9] 2007 OR10 is probably larger than both Pluto's moon Charon and the large Kuiper belt object Quaoar, which have diameters of 1,212 km (753 mi) and 1,110 km (690 mi), respectively.[42][43] Due to its large size, it is very likely a dwarf planet.[7] The International Astronomical Union has not addressed the possibility of accepting additional dwarf planets since before the announcement of the discovery of 2007 OR10. Brown states that 2007 OR10 "must be a dwarf planet even if predominantly rocky", as his 2013 radiometric measurement of 1,290 km (800 mi) is large enough to centainly qualify as a dwarf planet.[7] Scott Sheppard and his colleagues think that it is "likely" to be a dwarf planet,[44] based on its minimum possible diameter (580 km under the assumption of an albedo of 1)[c] and the expected lower size limit of around 200 km (120 mi) for hydrostatic equilibrium in cold icy-rocky bodies.[44]

In 2010, Tancredi initially estimated 2007 OR10 to have a very large diameter of 1,752 km (1,089 mi), though the certainty of its dwarf planet status was undeclared as there was no lightcurve data and other information to ascertain its size.[40] 2007 OR10 is too distant for its diameter to be resolved directly; Brown placed a rough estimate of its diameter ranging from 1,000 km (620 mi) to 1,500 km (930 mi), based on calculating a Quaoar-like albedo of 0.18 that is the best fit in his model.[30] A survey led by a team of astronomers using the European Space Agency's Herschel Space Observatory in 2012 determined its diameter to be 1,280 km (800 mi) with an uncertainty of 210 km (130 mi), based on the thermal properties of 2007 OR10 observed in the far infrared range.[12] This measurement made by observations with Herschel is consistent with Brown's constrained estimate of 1,000–1,500 km (620–930 mi).[12] Later observations in 2013 using combined thermal measurement data from Herschel and the Spitzer Space Telescope provided a smaller size estimate of 1,142 km (710 mi), though this estimate has a larger range of uncertainty.[41]

In 2016, combined observations from the Kepler spacecraft and archival data from Herschel suggested that 2007 OR10 was much larger than previously thought, giving a size estimate of 1535+75
based on an assumed equator-on view and a lower albedo estimate of 0.089.[10][11] This estimated size would make 2007 OR10 the third-largest trans-Neptunian object after Eris and Pluto, being larger than that of Makemake (1430 km).[11][16][45] These observations of 2007 OR10 were part of the Kepler spacecraft's K2 mission which includes studying small Solar System bodies.[16] Subsequent measurements in 2018 revised the size of 2007 OR10 to 1,230 km (760 mi), based on the mass and density of 2007 OR10 derived from the orbit of its satellite.[9] With this more recent size estimate, 2007 OR10 is again thought to be the fifth-largest trans-Neptunian object.[9]

Mass, density and rotation[edit]

Based on the orbit of its small satellite, the mass of 2007 OR10 has been calculated to be 1.75×1021 kg, with a density of 1.72±0.16 g/cm3.[9] From these mass and density estimates, the size of 2007 OR10 was calculated to be about 1,230 km (760 mi), smaller than the previous 2016 size estimate of 1,535 km (954 mi).[9] Given the mass of 1.75×1021 kg for the 2016 size estimate of 1,535 km (954 mi), it would imply an unexpectedly low (and likely erroneous) density of 0.92+0.46
for its size.[9] It is the fifth most massive trans-Neptunian object, after Eris, Pluto, Haumea, and Makemake.[9] In comparison to Pluto's moon Charon, which has a mass of 1.586×1021 kg and a density of 1.702 g/cm3, 2007 OR10 is slightly more massive and dense than Charon.[9][42] Due to its large size, mass, and density, 2007 OR10 is expected to have a spheroidal shape from hydrostatic equilibrium.[9][10] Its shape is described as a MacLaurin spheroid, being slightly flattened due to its rotation.[9][10]

The rotation period of 2007 OR10 was measured by András Pál along with collaborators using the Kepler spacecraft by observing variations in brightness and measuring its rotational light curve over a 19-day period.[10][26] Kepler observations of 2007 OR10 in 2016 provided two ambiguous values of 44.81±0.37 and 22.4±0.18 hours.[10] The rotation period estimate of 22.4±0.18 is thought to be the most plausible value.[9] Despite the ambiguity of its rotation period, 2007 OR10 rotates slowly compared to other trans-Neptunian objects, which usually have rotation periods less than 24 hours.[10][16][46] Due to its slow rotation, it is expected to have low oblateness values of 0.03 or 0.007, for rotation periods of 22.4 or 44.81 hours, respectively.[9] Astronomers have suspected in 2016 that the rotation of 2007 OR10 was slowed down by tidal forces exerted by an orbiting satellite, which was discovered in that same year.[47][8][46]


Polar view of the orbits of 2007 OR10, Eris, and Pluto.
Ecliptic view of the highly inclined orbits of 2007 OR10 and Eris.
A preliminary motion analysis of 2007 OR10 librating in a 3:10 resonance with Neptune. This animation consists of 16 frames covering 26,000 years.[5] Neptune (white dot) is held stationary.

2007 OR10 orbits the Sun at an average distance of 67.4 astronomical units (AU) and completes a full orbit in 553 years.[3] The orbit of 2007 OR10 is highly inclined to the ecliptic, with an orbital inclination 30.7 degrees.[3] Its orbit is also highly eccentric, with a measured orbital eccentricity of 0.503.[3] Due to its highly eccentric orbit, the distance of 2007 OR10 from the Sun varies greatly over the course of its orbit around the Sun; its aphelion, or furthest distance from the Sun, is around 101.3 AU while its perihelion, its closest distance from the Sun, is around 33.5 AU.[3][26] 2007 OR10 had approached its perihelion in 1857 and is currently moving farther from the Sun, toward its aphelion.[48] The Minor Planet Center lists it as a scattered disc object for its eccentric and distant orbit.[4] The Deep Ecliptic Survey shows the orbit of 2007 OR10 to be in a 3:10 resonance with Neptune; 2007 OR10 completes three orbits around the Sun for every ten orbits completed by Neptune.[5]

As of May 2019, 2007 OR10 is located 88.2 AU from the Sun[49] and is moving away at a speed of 1.1 kilometers per second (2,500 miles per hour).[50] It is currently the sixth-farthest known Solar System object from the Sun, preceding 2015 TH367 (89.6 AU),[51] 2014 UZ224 (90.4 AU), Eris (96.1 AU), 2018 VG18 (~ 120 AU),[52] and "FarFarOut" (~ 140 AU).[49][53][d] 2007 OR10 is currently more distant than Sedna, which is located 84.8 AU from the Sun as of May 2019.[49] It has been farther from the Sun than Sedna since 2013.[50] 2007 OR10 will be farther than both Sedna and Eris by 2045,[55] and will approach its aphelion in 2130.[50]


S/2010 (225088) 1
Moon Around the Dwarf Planet 2007 OR10.jpg
Hubble image of a moon around trans-Neptunian object 2007 OR10.[56]
Discovered byGábor Marton
Csaba Kiss
Thomas Müller
Discovery dateSeptember 2010[57]
MPC designation(225088) 2007 OR10
Orbital characteristics[9]
Epoch 8 December 2014 (JD 2457000.0)
24021±202 km
25.22073±0.000357 d
Inclination83.08°±0.86° (assumed to be inclined ~ 0° to equator)
Satellite of(225088) 2007 OR10
Physical characteristics
Equatorial radius
< 50 km[e]
Albedo> 0.2
Hubble image sequence of 2007 OR10 and its satellite.

The slow rotation rate of 2007 OR10, compared to the other TNOs, raised the possibility of a satellite that slowed down the rotation via tidal dissipation. In 2016, analysis of Hubble images of 2007 OR10 taken in 2010 revealed a satellite orbiting at a distance of at least 15,000 km (9,300 mi). Assuming that satellite and primary had equal albedos, the satellite was initially estimated to be around 300 km (190 mi) in diameter. It was announced at the DPS48 meeting on 17 October 2016.[8][45] The satellite is probably too small and dark to affect size estimates for 2007 OR10.[8] Further analysis from May 2017 confirmed this orbiting moon.[46][47]

Based on Hubble observations taken between October and December 2017,[58] the orbit and physical properties of the satellite could be further constrained. The orbit has a relatively high eccentricity of 0.31, either resulting from an intrinsically eccentric orbit and slow tidal evolution, or caused by the Kozai mechanism. The dynamics of the orbit suggest that the satellite is small, less than 100 kilometres (60 mi) in diameter, indicating an albedo much higher than that of 2007 OR10. The satellite is far less red than 2007 OR10. The color difference of ΔV-R=0.43±0.17 mag between primary and satellite is the largest among all known binary trans-Neptunian objects.[9]

See also[edit]


  1. ^ Discovery was announced two years later on 7 January 2009.
  2. ^ The unaided human eye can detect objects with a visual magnitude of around +8 or lower.[39]
  3. ^ The resulting minimum diameter of 580 km is derived from the equation , where is the absolute magnitude of 2007 OR10, and is the albedo of 2007 OR10, which in this case is assumed to be 1.[35]
  4. ^ A distant trans-Neptunian object, designated V774104, has been suspected to be about 103 AU from the Sun in 2015. Due to its short observation arc, its orbit and distance have not been precisely measured.[54]
  5. ^ The minimum radius is 18 km, corresponding to an albedo of 1.[35] The 50 km corresponds to an albedo of 0.2.[9]


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