2007 OR10

The celestial body designated 2007 OR10 is a large trans-Neptunian object (TNO) located in the distant regions of the Solar System. It is regarded as a probable dwarf planet and was later officially named Gonggong. Its size, orbit, and surface characteristics make it one of the most intriguing objects beyond Neptune, contributing significantly to the understanding of the Solar System’s outermost domains.

Discovery and Designation

The object was discovered on 17 July 2007 by astronomers Megan E. Schwamb, Michael E. Brown, and David L. Rabinowitz as part of the Palomar Distant Solar System Survey. Initially assigned the provisional designation 2007 OR10 by the Minor Planet Center, it was later numbered (225088). In February 2020, it received the official name Gonggong, following a naming campaign. The name is derived from Chinese mythology, where Gonggong is a water god associated with chaos and floods, symbolising the object’s reddish appearance and water-ice features.

Orbital Characteristics

2007 OR10 follows a highly elongated and inclined orbit around the Sun. Its semi-major axis is about 66.9 astronomical units (AU), meaning it orbits at an average distance nearly 67 times that between the Earth and the Sun. The perihelion distance—its closest point to the Sun—is roughly 33 AU, while the aphelion extends beyond 100 AU.
The orbital period of the object is approximately 547 Earth years, reflecting the vast scale of its orbit. It possesses an inclination of about 30° relative to the ecliptic plane, marking it as a distinctly tilted and distant member of the outer Solar System. Astronomers classify 2007 OR10 as a scattered-disc object or possibly a resonant trans-Neptunian object, due to its 3:10 orbital resonance with Neptune.

Physical Properties

The estimated diameter of 2007 OR10 is around 1,230 kilometres, though earlier assessments suggested values as high as 1,500 kilometres. Its size places it among the largest known trans-Neptunian objects, comparable to or slightly smaller than Eris, Pluto, and Makemake. The surface albedo is relatively low, ranging between 0.09 and 0.14, indicating that it reflects only a small fraction of sunlight.
The object’s surface colour is exceptionally red, making it one of the reddest bodies observed in the outer Solar System. This hue is thought to result from organic compounds known as tholins, which are created when solar and cosmic radiation act on ices composed of methane, carbon monoxide, and nitrogen. Spectroscopic observations have revealed the presence of water-ice and possibly methane-ice on the surface, suggesting a complex chemical evolution.
The rotation period remains uncertain, with analyses suggesting either 22.4 hours or 44.8 hours. This slow rotation could be the result of tidal interactions with its moon. Based on satellite data, the estimated mass is about 1.75 × 10²¹ kilograms, and the density is approximately 1.75 grams per cubic centimetre, consistent with a mixture of rock and ice.

The Satellite Xiangliu

A satellite orbiting 2007 OR10 was discovered in Hubble Space Telescope images taken in 2009 and 2010. The moon, named Xiangliu, honours a nine-headed serpent from Chinese mythology that served the deity Gonggong.
The satellite’s diameter is estimated to be under 100 kilometres, although earlier estimates placed it closer to 230 kilometres. The gravitational interaction between 2007 OR10 and Xiangliu allows scientists to estimate the system’s mass and density more accurately. The relatively slow rotation of 2007 OR10 may be the result of tidal locking processes with its satellite, similar to the gravitational interactions seen between Pluto and Charon.

Classification as a Dwarf Planet

Given its size, spherical shape, and mass sufficient to achieve hydrostatic equilibrium, 2007 OR10 meets most scientific criteria for classification as a dwarf planet. It is generally accepted within the astronomical community as one, though it awaits full confirmation by the International Astronomical Union (IAU).