Gonggong

Gonggong, long known by its provisional designation 2007 OR10, is one of the largest bodies found beyond Neptune and a dwarf-planet candidate: at about 1,230 km across it is almost certainly round, but the International Astronomical Union has not formally classified it, and the five it recognizes remain Ceres, Pluto, Haumea, Makemake and Eris.^[1][2] NASA's Hubble team is less cautious, calling it a member of an exclusive club of nine dwarf planets, of which only Pluto and Eris are larger.^[3]

How it was found

It was discovered on 17 July 2007 by Meg Schwamb, Mike Brown and David Rabinowitz with the Samuel Oschin Telescope at the Palomar Observatory in California, in a survey for distant bodies.^[4][3] It went unnamed for more than twelve years; the name Gonggong, for a Chinese water god with red hair and a serpent's tail, known for causing chaos and floods, was announced in February 2020.^[4]

Orbit

Like Pluto, Gonggong follows an eccentric orbit, and it is currently about three times farther from the Sun than Pluto.^[3] JPL's catalogue gives a path from 33.1 to 101 astronomical units, with a semi-major axis of 66.9 astronomical units, tilted 30.9 degrees to the plane of the planets; one orbit takes about 550 years.^[4] Its rotation is unusually slow for a Kuiper belt object: NASA's Kepler telescope measured a period of 45 hours, where typical periods are under 24, although JPL's catalogue lists 22.4 hours.^[3][4]

Size and interior

Its size has been revised more than once. NASA's 2017 release put it at about 950 miles, roughly 1,530 km, which would have made it the third-largest dwarf planet.^[3] A 2019 study that combined Hubble, Kepler and Herschel data with the moon's orbit found a diameter of 1,230 km, give or take 50, and a density of about 1.75 grams per cubic centimeter; the earlier, larger figure would have implied a density of 0.92, unexpectedly low for a body of its size.^[1] With a system mass of 1.75 × 10²¹ kg it is the fifth most massive known dwarf planet, after Eris, Pluto, Haumea and Makemake.^[1]

The moon

The moon was found in two Hubble observations a year apart, from 6 November 2009 and 18 September 2010, after Kepler's measurement of the slow rotation prompted astronomers to look for a companion that might have slowed the spin.^[3] NASA's release estimated it at 150 to 250 miles across, roughly 240 to 400 km, while the 2019 orbit study argues from the orbit's dynamics that it is small, under 100 km.^[3][1] Its orbit is noticeably eccentric, which may reflect an intrinsically eccentric orbit with slow tidal evolution, or the Sun's gravitational influence through what is called the Kozai mechanism.^[1] The moon is named Xiangliu, the nine-headed poisonous snake who served Gonggong in Chinese mythology.^[4]

What is not yet known

Gonggong has never been visited by a spacecraft. Its exact size, the size of Xiangliu and the reason for its slow rotation are all open, and its formal status waits on the International Astronomical Union.^[1][2]