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(225088) 2007 OR10, unofficially known as Gonggong, is a possible dwarf planet[15] orbiting the Sun beyond Neptune. It is a member of the scattered disc, a high-eccentricity population of trans-Neptunian objects (TNOs). It was discovered in July 2007 by American astronomers Megan Schwamb, Michael Brown, and David Rabinowitz at the Palomar Observatory. The discovery was announced in January 2009.

(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]
Designations
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
Eccentricity0.503
553.05 yr (202,003 days)
105.265°
0° 0m 6.415s / day
Inclination30.739°
336.844°
207.546°
Known satellites1[8]
Physical characteristics
Mean diameter
1230±50 km[9]
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]
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])
0.14±0.01[9]
B−V=1.38[12]
V−R=0.86[13]
V−I=1.65±0.03[9]
21.52[14]
2.34[10] · 1.8[1][3] · 2.0[7]

At 1,230 km (760 mi) in diameter, (225088) 2007 OR10 is the fifth-largest known body in the Solar System beyond the orbit of Neptune and is expected to have a gravitationally rounded shape to gain dwarf planet status.[7]

It is currently the largest known body in the Solar System without an official name. In 2019, the discoverers hosted an online poll for the general public to choose among three suggested names. Gonggong was announced as the winner. The discovery team plans to submit the name to the International Astronomical Union.

2007 OR10 is red in color, due to the presence of organic compounds called tholins on its surface. It may support a tenuous methane atmosphere that is slowly escaping into space.[16] Water ice is also present on its surface, which hints at a brief period of cryovolcanic activity in the distant past.[17] 2007 OR10 has one known natural satellite, S/2010 (225088) 1.

Contents

HistoryEdit

DiscoveryEdit

(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 survey conducted to find distant objects in the region of Sedna, beyond 50 astronomical units (AU) from the Sun, using the Samuel Oschin telescope at Palomar Observatory near San Diego, California.[18][19][20] The survey was designed to detect the movements of objects out to at least 1,000 astronomical units from the Sun.[20] Schwamb identified 2007 OR10 by comparing multiple images using the blinking technique.[19] In the discovery images, 2007 OR10 appeared to move slowly, suggesting that it is a distant object.[19][21] The discovery was part of Schwamb's doctoral thesis. At that time, Schwamb was a graduate student of Michael Brown at Caltech.[22][19]

2007 OR10 was formally announced in a Minor Planet Electronic Circular on 7 January 2009.[2] Upon its announcement, the object was given the provisional designation 2007 OR10, which indicates its year of discovery, with the letters further specifying that the discovery took place in the second half of 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 on 19 August 1985.[3][24]

NamingEdit

2007 OR10 is currently the largest known object in the Solar System without an official name.[25] 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.[26][17] By that time, Brown's team had already discovered seven large trans-Neptunian objects which were collectively referred to as the "seven dwarfs":[26][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 very red in color, comparable to Quaoar, so the nickname was dropped.[26][21] On 2 November 2009, two years after its discovery, the Minor Planet Center assigned the minor planet number 225088 to 2007 OR10.[24]

The discoverers were given a 10-year period for naming proposals following the numbering of the object.[27][28] 2007 OR10 was assigned its minor planet number in November 2009, hence the discoverers were given the privilege to propose a name until November 2019.[27][28] 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][29] In 2011, Brown declared that he now 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 choose 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 International Astronomical Union's (IAU's) minor planet naming criteria, which state that objects with orbits like that of 2007 OR10 must be given names related to mythological figures that are associated with creation.[27][28] The three options were also picked because they are associated with water, ice, snow, and the color red, which are all characteristics of 2007 OR10.[30] The voting session ended on 10 May 2019, with Gonggong being the most voted name.[27][19] 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 reserved to the discoverers of the satellite.[27][31]

On 29 May 2019, the discovery team announced Gonggong as the winning name, with a plurality of 46 percent of the 280,000 votes casted during the voting session.[31] The discovery team will propose the winning name to the IAU's Committee of Small Body Nomenclature, the instutution that is responsible for naming minor planets.[31] The name is derived from Gonggong, a Chinese water god depicted as having a red-haired human head and the body of a serpent. In Chinese mythology, Gonggong was responsible for creating chaos, causing flooding, and tilting the Earth, and was sent into exile.[27][31]

Physical characteristicsEdit

Surface and spectraEdit

 
Artist's impression of 2007 OR10 depicting its red surface color.
 
Colors of trans-Neptunian objects. 2007 OR10's color (not shown) is similar to that of Quaoar.

The surface of 2007 OR10 is red in color. It has an albedo (reflectivity) of 0.14±0.01.[9] The surface composition and spectrum of 2007 OR10 is expected to be similar to that of Quaoar, as both objects are red and display signs of water ice and possibly methane in their spectra.[32][33] 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.[16] The observed spectrum of 2007 OR10 exhibits a strong red spectral slope along with broad absorption bands at wavelengths of 1.5 μm and 2 μm. Additional photometric measurements from the Hubble Space Telescope's Wide Field Camera 3 instrument display similar water ice absorption bands at 1.5 μm.[16] These absorption bands are characteristic features of water ice, which is often found on large Kuiper belt objects.[34] The presence of water ice on the surface of 2007 OR10 implies a brief period of cryovolcanism in the distant past when water erupted from its interior, deposited onto its surface, and subsequently froze.[35]

The red color of 2007 OR10 is unexpected for an object with a substantial amount of water ice on its surface.[35][22] Objects with surfaces rich in water ice are typically neutral in color, hence 2007 OR10 was initially nicknamed "Snow White" for its presumed bright and reflective surface.[26][17] 2007 OR10 is among the reddest objects known.[16] Its red color implies that methane is present on its surface, although it was not directly detected in the spectrum of 2007 OR10 due to the low signal-to-noise ratio of the data.[16] The red color results from methane frost that are irradiated by sunlight and cosmic rays.[16] The photolysis of methane on its surface produces reddish organic compounds known as tholins.[16][10]

2007 OR10 is large enough to be able to retain trace amounts of 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.[16] 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.[32][10][25] Like Quaoar, 2007 OR10 is expected to be near the mass limit at which it is able to retain those volatile materials on its surface.[32][22]

BrightnessEdit

The size of an object can be calculated from its absolute magnitude (brightness) and albedo.[36] 2007 OR10 has an absolute magnitude (H) of 2.34,[12][10] which makes it the seventh-brightest trans-Neptunian object known. Other sources give an absolute magnitude of 1.8,[3] which would make it the fifth brightest trans-Neptunian object,[37] brighter than Sedna (H=1.83; D=995 km)[38] and Orcus (H=2.31; D=917 km).[39]

Due to its large distance of 88 astronomical units from the Sun, the apparent magnitude of 2007 OR10 is only 21.5, too dim to be seen from Earth with the naked eye.[27][b] It is dimmer than Sedna, which has an apparent magnitude of 20.9.[41]

AtmosphereEdit

The presence of tholins on the surface of 2007 OR10 implies the existence of a tenuous methane atmosphere slowly escaping into space, analogous to Quaoar.[35][22] Although 2007 OR10 occasionally 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.[16][42] Its low surface albedo may contribute to the loss of volatiles such as nitrogen, since a lower albedo corresponds to more light being absorbed by the surface rather than being reflected, thus resulting in greater surface heating.[42] 2007 OR10 is thought to have had cryovolcanic activity along with a more substantial atmosphere shortly after its formation.[35][22] Such cryovolcanic activity is expected to have been brief, and the resulting atmosphere gradually escaped over time.[35][22] Volatile gases, such as nitrogen and carbon monoxide, were lost, while less volatile gases such as methane are likely to remain in its present tenuous atmosphere.[35][42]

SizeEdit

Size estimates
Year Diameter Method Refs
2010 1,752 km thermal [43]
2011 1,200+300
−200
 km
best fit albedo [16]
2012 1,280±210 km thermal [33]
2013 1,142+647
−467
 km
thermal [44]
2013 1,290 km radiometric [7]
2016 1,834.53 km light curve [13]
2016 1,535+75
−225
 km
thermal [10]
2018 1,230±50 km radiometric [9]
EarthMoonCharonCharonNixNixKerberosStyxHydraHydraPlutoPlutoDysnomiaDysnomiaErisErisNamakaNamakaHi'iakaHi'iakaHaumeaHaumeaMakemakeMakemakeMK2MK22007 OR102007 OR10S/2010 (225088) 1S/2010 (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 would make 2007 OR10 the fifth-largest trans-Neptunian object, 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.[45][46]

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 certainly qualify as a dwarf planet.[7] Scott Sheppard and his colleagues think that it is "likely" to be a dwarf planet,[47] 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.[47]

In 2010, Tancredi initially estimated 2007 OR10 to have a very large diameter of 1,752 km (1,089 mi), though its dwarf planet status was unclear as there was no lightcurve data and other information to ascertain its size.[43] 2007 OR10 is too distant 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 an albedo of 0.18 that is the best fit in his model.[16] 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.[33] This measurement is consistent with Brown's estimate of 1,000–1,500 km (620–930 mi). Later observations in 2013 using combined thermal emission data from Herschel and the Spitzer Space Telescope provided a smaller size estimate of 1,142 km (710 mi), though this estimate had a larger range of uncertainty.[44]

In 2016, combined observations from the Kepler spacecraft and archival thermal emission data from Herschel suggested that 2007 OR10 was much larger than previously thought, giving a size estimate of 1535+75
−225
 km
based on an assumed equator-on view and a lower estimated albedo of 0.089.[10][11] This would have made 2007 OR10 the third-largest trans-Neptunian object after Eris and Pluto, larger than Makemake (1430 km).[11][25][48] These observations of 2007 OR10 were part of the Kepler spacecraft's K2 mission which includes studying small Solar System bodies.[25] 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 and the discovery that the viewing direction was almost pole-on.[9] With this more recent size estimate, 2007 OR10 is again thought to be the fifth-largest trans-Neptunian object.[9]

Mass, density and rotationEdit

2007 OR10 moving among background stars during a 19-day observation period by the Kepler spacecraft.[25]

Based on the orbit of its 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, the 2016 size estimate of 1,535 km (954 mi) would have implied an unexpectedly low (and likely erroneous) density of 0.92+0.46
−0.12
 g/cm3
.[9]

2007 OR10 is the fifth most massive trans-Neptunian object, after Eris, Pluto, Haumea, and Makemake.[9] It is slightly more massive and denser than Charon, which has a mass of 1.586×1021 kg and a density of 1.702 g/cm3.[9][45] Due to its large size, mass, and density, 2007 OR10 is expected to be in 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 first measured in March 2016, by observing variations in its brightness using the Kepler space telescope.[10][27] The Kepler observations provided two possible values of 44.81±0.37 and 22.4±0.18 hours for the rotation period.[10] The value of 22.4±0.18 hours is thought to be the more plausible one.[9] 2007 OR10 rotates slowly compared to other trans-Neptunian objects, which usually have rotation periods between 6 and 12 hours.[9] Due to its slow rotation, it is expected to have a low oblateness of 0.03 or 0.007, for rotation periods of 22.4 or 44.81 hours, respectively.[9] The slow rotation period led astronomers to speculate that the rotation of 2007 OR10 was slowed down by tidal forces exerted by an orbiting satellite, which was confirmed later that same year.[49][8][50]

OrbitEdit

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.
 
Apparent motion of 2007 OR10 through the constellation Aquarius (years 2000 to 2050).

2007 OR10 orbits the Sun at an average distance of 67.4 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, from 101.3 AU at aphelion, its furthest point from the Sun, to around 33.5 AU at perihelion, its closest point to the Sun.[3][27] 2007 OR10 had approached its perihelion in 1857 and is currently moving farther from the Sun, toward its aphelion.[51]

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 (1.319×1010 km) from the Sun[52] and is moving away at a speed of 1.1 kilometers per second (2,500 miles per hour).[53] It is currently the sixth-farthest known Solar System object from the Sun, preceding 2015 TH367 (89.6 AU),[54] 2014 UZ224 (90.4 AU), Eris (96.1 AU), 2018 VG18 (~ 120 AU),[55] and "FarFarOut" (~ 140 AU).[52][56][d] 2007 OR10 is currently more distant than Sedna, which is located 84.8 AU from the Sun as of May 2019.[52] It has been farther from the Sun than Sedna since 2013.[53] 2007 OR10 will be farther than both Sedna and Eris by 2045,[58] and will approach its aphelion in 2130.[53]

ExplorationEdit

It was calculated that a flyby mission to 2007 OR10 could take just under 25 years using a Jupiter gravity assist, based on a launch date of 2030 or 2031. 2007 OR10 would be approximately 95 AU from the Sun when the spacecraft arrives.[59]

SatelliteEdit

2007 OR10 has a single known moon. It was first identified in 2016 by a team of astronomers led by Csaba Kiss, in archival images taken on 18 September 2010 with the Hubble Space Telescope. The discovery was announced on 17 October 2016.[27][60] S/2010 (225088) 1 is estimated to be less than 100 km (62 mi) in diameter, implying an albedo above 0.2.[9]

The absolute magnitude of S/2010 (225088) 1 is estimated to be 6.93±0.15, at least 4.59 magnitudes dimmer than 2007 OR10 (given an absolute magnitude of 2.34 for the primary).[9] It is not yet possible to determine whether the orbit of the satellite is prograde or retrograde.[9] Based on a prograde orbit model, the satellite orbits the primary at a distance of around 24,021 km (14,926 mi) and completes one orbit in 25.22 days.[e] The satellite is believed to be tidally locked to the primary.[9] Using the same prograde orbit model, the discovery team estimates that its orbit is inclined to the ecliptic by about 83 degrees. Assuming that the orbit has a low inclination to the primary's equator, this implies that 2007 OR10 is being viewed at a nearly pole-on configuration.[9] The orbit of S/2010 (225088) 1 has an eccentricity of 0.29.

See alsoEdit

NotesEdit

  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.[40]
  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.[36]
  4. ^ A distant trans-Neptunian object, designated V774104, has been suspected in 2015 to be about 103 AU from the Sun. Due to its short observation arc, its orbit and distance have not been precisely measured.[57]
  5. ^ The values in the retrograde model are similar.

ReferencesEdit

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  2. ^ a b c "MPEC 2009-A42 : 2007 OR10". Minor Planet Center. 7 January 2009. Retrieved 23 May 2019.
  3. ^ a b c d e f g h i j "JPL Small-Body Database Browser: 225088 (2007 OR10)" (2015-09-20 last obs.). Jet Propulsion Laboratory. Retrieved 23 May 2019.
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