Gonggong

When Megan Schwamb, Mike Brown and David Rabinowitz found a large, distant object in July 2007, Brown gave it the nickname “Snow White”, expecting it to be pale and icy. It turned out to be very red. For more than a decade it went by its catalogue designation, 2007 OR10.

A name chosen by vote

In 2019 the discoverers let the public choose. Of about 280 000 votes, 46% went to Gonggong, a Chinese water god said to have caused floods, chaos and the tilt of the Earth. The name became official in February 2020. Its moon was named Xiangliu, after Gonggong’s nine-headed serpent servant.

A world with a moon

Gonggong is about 1 230 kilometres across, one of the largest known objects beyond Neptune. Its moon, Xiangliu, was spotted in Hubble images and announced in 2016. Xiangliu is only about 100 kilometres across and circles Gonggong every 25 days. Watching that orbit let astronomers weigh Gonggong: about 1.75 × 10²¹ kilograms, roughly a seventh of Pluto’s mass, with a density that suggests a mix of rock and ice.

Gonggong turns slowly for a body of its size, probably once every 22 hours.

How big it really is

Gonggong’s size was only settled once its moon gave its mass. In 2016 András Pál and colleagues combined brightness measurements from the Kepler space telescope with infrared data from the Herschel space observatory and made it about 1 535 kilometres across, which would have put it among the three largest known objects beyond Neptune. That estimate assumed Gonggong is seen from the side, along its equator. In 2019 Csaba Kiss and colleagues showed that such a large body, with the mass measured from Xiangliu’s orbit, would have a density of only about 0.92 grams per cubic centimetre, unexpectedly low for a dwarf planet. Assuming instead that Xiangliu circles above Gonggong’s equator, they arrived at 1 230 kilometres and a density of about 1.75 grams per cubic centimetre.

Far and cold

Its orbit is long and tilted. At its closest it comes within 33 AU of the Sun, just beyond Neptune; at its farthest it is about 101 AU away. One orbit takes about 547 years. The tilt is about 31° to the plane of the planets. Gonggong belongs to the scattered disc, and it is locked in a 3:10 resonance with Neptune, going round the Sun three times for every ten Neptune orbits. It was last at its closest to the Sun in the 1850s and will reach its farthest point around 2134.

Its surface has water ice. Its red colour was once thought to come partly from methane, but the James Webb Space Telescope found none. It did find ethane, a hydrocarbon that forms when sunlight and cosmic rays break down methane.