page_title: 225088 Gonggong
section_title: Size
hierarchical_section_title: 225088 Gonggong / Physical characteristics / Size
caption_attribution_description: English: 2007 OR10 compared to Earth and Moon.
context_page_description: 225088 Gonggong is a likely dwarf planet of the Solar System, and a member of the scattered disc beyond Neptune. It has a highly eccentric and inclined orbit during which it ranges from 34–101 astronomical units from the Sun. As of 2019, its distance from the Sun is 88 AU, and it is the sixth-farthest known Solar System object. 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 about 1,230 km in diameter, Gonggong is approximately the size of Pluto's moon Charon, and is the fifth-largest known trans-Neptunian object in the Solar System. It may be sufficiently massive to be gravitationally rounded 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.
context_section_description: As of 2019, Gonggong 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. This would make Gonggong the fifth-largest trans-Neptunian object, after Pluto, Eris, Haumea and Makemake. Gonggong is approximately the size of Pluto's moon Charon, although Gonggong's current size estimate has an uncertainty of 50 km (31 mi).
  The International Astronomical Union (IAU) has not addressed the possibility of officially accepting additional dwarf planets since the acceptance of Makemake and Haumea in 2008, prior to the announcement of Gonggong in 2009. Despite not satisfying the IAU's criterion of having an absolute magnitude brighter than +1, Gonggong is large enough to be considered a dwarf planet by several astronomers. Brown states that Gonggong "must be a dwarf planet even if predominantly rocky", based on the 2013 radiometric measurement of 1,290 km (800 mi). Scott Sheppard and colleagues think that it is likely to be a dwarf planet, based on its minimum possible diameter—580 km (360 mi) under the assumption of a completely reflective surface with an albedo of 1—and what was at the time the expected lower size limit of around 200 km (120 mi) for hydrostatic equilibrium in cold icy-rocky bodies. However, Iapetus is not in equilibrium despite being 1,470 km (910 mi) in diameter, so this remains just a possibility.
  In 2010, astronomer Gonzalo Tancredi initially estimated Gonggong 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 or other information to ascertain its size. Gonggong is too distant to be resolved directly; Brown placed a rough estimate of its diameter ranging from 1,000–1,500 km (620–930 mi), based on an albedo of 0.18 which was the best fit in his model. A survey led by a team of astronomers using the European Space Agency's Herschel Space Observatory in 2012 determined its diameter to be 1280±210 km (795±130 mi), based on the thermal properties of Gonggong observed in the far infrared range. This measurement is consistent with Brown's estimate. Later observations in 2013 using combined thermal emission data from Herschel and the Spitzer Space Telescope suggested a smaller size of 1142+647
  −467 km (710+402
  −290 mi), though this estimate had a larger range of uncertainty.
  In 2016, combined observations from the Kepler spacecraft and archival thermal emission data from Herschel suggested that Gonggong was much larger than previously thought, giving a size estimate of 1535+75
  −225 km (954+46
  −140 mi) based on an assumed equator-on view and a lower estimated albedo of 0.089. This would have made Gonggong the third-largest trans-Neptunian object after Eris and Pluto, larger than Makemake (1,430 km (890 mi)). These observations of Gonggong were part of the Kepler spacecraft's K2 mission which includes studying small Solar System bodies. Subsequent measurements in 2018 revised the size of Gonggong to 1230±50 km (764±31 mi), based on the mass and density of Gonggong derived from the orbit of its satellite and the discovery that the viewing direction was almost pole-on. With this size estimate, Gonggong is again thought to be the fifth-largest trans-Neptunian object.
caption: Gonggong compared to the Earth and the Moon
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