Dwarf planet in the scattered-disc
Gonggong (minor-planet designation: 225088 Gonggong) is a dwarf planet and a member of the scattered disc beyond Neptune. It has a highly eccentric and inclined orbit during which it ranges from from the Sun. As of 2019[ [update]], its distance from the Sun is , and it is the sixth-farthest known Solar System object. According to the Deep Ecliptic Survey, Gonggong is in a 3:10 orbital resonance with Neptune, in which it completes three orbits around the Sun for every ten orbits completed by Neptune. Gonggong was discovered in July 2007 by American astronomers Megan Schwamb, Michael Brown, and David Rabinowitz at the Palomar Observatory, and the discovery was announced in January 2009.
At approximately in diameter, Gonggong is similar in size to Pluto's moon Charon, making it the fifth-largest known trans-Neptunian object (apart possibly from Charon). It may be sufficiently massive to be in hydrostatic equilibrium and therefore a dwarf planet. Gonggong's large mass makes retention of a tenuous atmosphere of methane just possible, though such an atmosphere would slowly escape into space. The object is named after "Gònggōng", a Chinese water god responsible for chaos, floods and the tilt of the Earth. The name was chosen by its discoverers in 2019, when they hosted an online poll for the general public to help choose a name for the object, and the name Gonggong won.
Gonggong is red, likely due to the presence of organic compounds called tholins on its surface. Water ice is also present on its surface, which hints at a brief period of cryovolcanic activity in the distant past. With a rotation period of around 22 hours, Gonggong rotates slowly compared to other trans-Neptunian objects, which typically have periods of less than 12 hours. The slow rotation of Gonggong may have been caused by tidal forces from its natural satellite, named Xiangliu.
History.
Discovery.
Gonggong was discovered by American astronomers Megan Schwamb, Michael Brown and David Rabinowitz on 17 July 2007. The discovery was part of the Palomar Distant Solar System Survey, a survey conducted to find distant objects in the region of Sedna, beyond from the Sun, using the Samuel Oschin telescope at Palomar Observatory near San Diego, California. The survey was designed to detect the movements of objects out to at least 1,000 AU from the Sun. Schwamb identified Gonggong by comparing images using the blinking technique. In the discovery images, Gonggong appeared to move slowly, suggesting that it is a distant object. The discovery was part of Schwamb's doctoral thesis. At that time, Schwamb was a graduate student of Michael Brown at the California Institute of Technology.
Gonggong was formally announced in a Minor Planet Electronic Circular on 7 January 2009. It was then given the provisional designation 2007 OR10 because it was discovered during the second half of July 2007. The last letter and numbers of its designation indicate that it is the 267th object discovered during the latter half of July. As of April 2017[ [update]], it has been observed 230 times over 13 oppositions, and has been identified in two precovery images, with the earliest image taken by the La Silla Observatory on 19 August 1985.
Name and symbol.
The object is named after Gonggong, a water god in Chinese mythology. Gonggong is depicted as having a copper-and-iron, red-haired human head (or sometimes torso) and the body or tail of a serpent. Gonggong was responsible for creating chaos and catastrophe, causing flooding and tilting the Earth, until he was sent into exile. Gonggong is often accompanied by his minister, Xiangliu, a nine-headed poisonous snake monster who was also responsible for causing flooding and destruction.
Before its official naming, Gonggong was the largest known unnamed object in the Solar System. Initially after the discovery of Gonggong, Brown nicknamed the object "Snow White" for its presumed white color based on his assumption that it may be a member of the icy Haumea collisional family. The nickname also fit because, by that time, Brown's team had discovered seven other large trans-Neptunian objects which were collectively referred to as the "seven dwarfs": Quaoar in 2002, Sedna in 2003, Haumea, Salacia and Orcus in 2004, and Makemake and Eris in 2005. However, Gonggong turned out to be very red in color, comparable to Quaoar, so the nickname was dropped. On 2 November 2009, two years after its discovery, the Minor Planet Center assigned the minor planet number 225088 to Gonggong.
When Gonggong's discovery was first announced, Brown did not name it, as he considered it to be an unremarkable object, despite its large size. In 2011, he declared that he now had enough information to justify naming it, because of the discovery of water ice and the possibility of methane on its surface, which made it noteworthy enough to warrant further study. Following the "Kepler" spacecraft's large revision of Gonggong's size in 2016, Schwamb justified that Gonggong was eligible for naming, an acknowledgement of its large size and that its characteristics were known with enough certainty for a name to be given to reflect them.
In 2019, the discoverers of Gonggong hosted an online poll for the general public to choose between three possible names: Gonggong (Chinese), Holle (German), and Vili (Norse). These were selected by the discoverers in accordance with the International Astronomical Union's (IAU's) minor planet naming criteria, which state that objects with orbits like that of Gonggong must be given names related to mythological figures that are associated with creation. The three options were chosen because they were associated with water, ice, snow, and the color red—all characteristics of Gonggong—and because they had associated figures that could later provide a name for Gonggong's satellite. The name for Gonggong's satellite was not chosen by the hosts of the naming poll, as this privilege is reserved for its discoverers.
Having gained 46 percent of the 280,000 votes, on 29 May 2019, the discovery team announced Gonggong as the winning name. The name was proposed to the IAU's Committee on Small Body Nomenclature (CSBN), which is responsible for naming minor planets. The name was accepted by the CSBN and was announced by the Minor Planet Center on 5 February 2020.
As planetary symbols are no longer used regularly in astronomy, Gonggong never received a symbol in the astronomical literature. A symbol ⟨⟩, used mostly among astrologers, is included in Unicode as . The symbol was designed by Denis Moskowitz, a software engineer in Massachusetts; it combines the Chinese character 共 "gòng" with a snake's tail.
Orbit.
Gonggong orbits the Sun at an average distance of , and completes a full orbit in 554 years. The orbit of Gonggong is highly inclined to the ecliptic, with an orbital inclination of 30.7 degrees. Its orbit is also highly eccentric, with a measured orbital eccentricity of 0.50. Due to its highly eccentric orbit, the distance of Gonggong from the Sun varies greatly over the course of its orbit, from at aphelion, its furthest point from the Sun, to around at perihelion, its closest point to the Sun. Gonggong last reached perihelion in 1857, and is currently moving farther from the Sun, toward its aphelion. Gonggong will reach aphelion by 2134.
The period, inclination and eccentricity of Gonggong's orbit are all rather extreme compared to other large bodies in the Solar System. Among likely dwarf planets, its period is the third-longest, at 554 years compared to 558 years for Eris and the ca. 11,400 years of Sedna. Its 31° inclination is second, after 44° for Eris, and its 0.50 eccentricity is also (a rather distant) second, after Sedna at 0.84.
The Minor Planet Center lists it as a scattered disc object for its eccentric and distant orbit. The Deep Ecliptic Survey shows the orbit of Gonggong to be in a 3:10 resonance with Neptune; Gonggong completes three orbits around the Sun for every ten orbits completed by Neptune.
As of 2021[ [update]], Gonggong is about from the Sun and is moving away at a speed of . It is the eleventh-farthest known Solar System object from the Sun, preceding (89.5 AU), (89.6 AU), (90.3 AU), (92.4 AU), Eris (95.9 AU), (97.2 AU), (99.0 AU), (111.0 AU), (123.5 AU), and (~ 132 AU). Gonggong is more distant than Sedna, which is located 84.3 AU from the Sun as of 2021[ [update]]. It has been farther from the Sun than Sedna since 2013, and it will surpass Eris in distance by 2045.
Brightness.
Gonggong has an absolute magnitude (H) of 2.34, which makes it the seventh-brightest trans-Neptunian object known. It is dimmer than Orcus (H=2.31; D=917 km) but brighter than Quaoar (H=2.82; D=1,110 km). The Minor Planet Center and the Jet Propulsion Laboratory Small-Body Database assume a brighter absolute magnitude of 1.6 and 1.8, respectively, which would make it the fifth brightest trans-Neptunian object.
Being 88 AU from the Sun, the apparent magnitude of Gonggong is only 21.5, and so it is too dim to be seen from Earth with the naked eye. Although closer to the Sun than the dwarf planet Eris, Gonggong appears dimmer, as Eris has a higher albedo and an apparent magnitude of 18.8.
Physical characteristics.
Surface and spectra.
The surface of Gonggong has an albedo (reflectivity) of 0.14. The surface composition and spectrum of Gonggong is expected to be similar to that of Quaoar, as both objects are red in color and display signs of water ice and possibly methane in their spectra. The reflectance spectrum of Gonggong was first measured in 2011 at near-infrared wavelengths, with the Folded port InfraRed Echellette (FIRE) spectrograph on the Magellan Baade Telescope at the Las Campanas Observatory in Chile. Gonggong's spectrum exhibits a strong red spectral slope along with broad absorption bands at wavelengths of 1.5 μm and 2 μm, meaning that Gonggong reflects more light at these wavelengths. Additional photometric measurements from the Hubble Space Telescope's Wide Field Camera 3 instrument display similar absorption bands at 1.5 μm, which are characteristic features of water ice, a substance often found on large Kuiper belt objects. The presence of water ice on the surface of Gonggong implies a brief period of cryovolcanism in the distant past, when water erupted from its interior, deposited onto its surface, and subsequently froze.
Gonggong is among the reddest trans-Neptunian objects known, especially in the visible and near-infrared. Its red color is unexpected for an object with a substantial amount of water ice on its surface, which are typically neutral in color, hence why Gonggong was initially nicknamed "Snow White". Gonggong's color implies that methane is present on its surface, although it was not directly detected in the spectrum of Gonggong due to the low signal-to-noise ratio of the data. The presence of methane frost would account for its color, as a result of the photolysis of methane by solar radiation and cosmic rays producing reddish organic compounds known as tholins. Observations of Gonggong's near-infrared spectrum in 2015 revealed an absorption feature at 2.27 μm, indicating the presence of methanol along with its irradiation products on its surface.
Gonggong 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 (33.7 AU), where temperatures are higher than that of Quaoar. In particular, the large size of Gonggong means that it is likely to retain trace amounts of other volatiles, including ammonia, carbon monoxide, and possibly nitrogen, which almost all trans-Neptunian objects lose over the course of their existence. Like Quaoar, Gonggong is expected to be near the mass limit at which it is able to retain those volatile materials on its surface.
In 2022, low resolution near-infrared (0.7–5 μm) spectroscopic observations by the James Webb Space Telescope (JWST) revealed the presence of significant amounts of ethane ice (C2H6) on the surface of Gonggong, though there appears to be less ethane on Gonggong than on Sedna. The JWST spectra also contain evidence of presence of small amounts of carbon dioxide (CO2) complexed with either dark surface material or some ices as well as complex organics. On the other hand no evidence of presence of methane (CH4) and methanol (CH3OH) was found at variance with the earlier observations.
Atmosphere.
The presence of tholins on the surface of Gonggong implies the possible existence of a tenuous methane atmosphere, analogous to Quaoar. Although Gonggong occasionally comes closer to the Sun than Quaoar, where it becomes warm enough that a methane atmosphere should evaporate, its larger mass could make the retention of methane just possible. During aphelion, methane along with other volatiles would condense on Gonggong's surface, allowing for