Orcus
A dwarf-planet candidate about 917 km across on an orbit much like Pluto's, in the same resonance with Neptune, with a large moon, Vanth, about half its size.
At a glance
- Status
- Candidate: likely large enough to be round, not formally classified[3]Not among the five dwarf planets the International Astronomical Union recognizes
- Size
- About 917 km across[1]917 km, give or take 25, from the Herschel Space Observatory, assuming the moon reflects light as Orcus does; its surface reflects about 23 per cent of sunlight
- Orbit
- 30.7 to 48.1 au from the Sun, once every 247 years or so[4]Semi-major axis 39.4 au, tilted 20.6° to the plane of the planets; a trans-Neptunian object in JPL's catalogue
- Day
- About 13.2 hours[4]From its light curve
- Moons
- One, Vanth, about 475 km across[2]475 km, give or take 75, from the ALMA radio telescope array; roughly half the size of Orcus, with a much darker surface
- Density
- About 1.53 g/cm³[1]For the Orcus and Vanth system together
Timeline
- February 17, 2004Discovered[4]Mike Brown, Chad Trujillo and David Rabinowitz at the Palomar Observatory, California
- 2013Size measured[1]The Herschel Space Observatory's infrared measurements separate Orcus, at 917 km, from Vanth, at 276 km, if both reflect light alike
- October 11, 2016ALMA resolves Orcus and Vanth[2]Four observations in October and November 2016 give Vanth its own size and albedo for the first time
Orcus is a dwarf-planet candidate in the Kuiper belt about 917 km across, with a moon, Vanth, about half its size.[1][2] Its orbit closely resembles Pluto's, and it shares Pluto's resonance with Neptune. The International Astronomical Union has not classified it; the five dwarf planets it recognizes are Ceres, Pluto, Haumea, Makemake and Eris.[3]
How it was found
Mike Brown, Chad Trujillo and David Rabinowitz discovered Orcus on 17 February 2004 at the Palomar Observatory in California.[4] It is named for the Roman god of death, the counterpart of the Greek Pluto, who collected the dead and carried them to the underworld he ruled.[4]
Orbit
JPL's catalogue gives Orcus a path from 30.7 to 48.1 astronomical units, with a semi-major axis of 39.4 astronomical units and a tilt of 20.6 degrees to the plane of the planets, and one orbit takes about 247 years.[4] Those numbers are close to Pluto's, and for the same reason: Orcus is a plutino, one of the objects locked in a 3:2 resonance with Neptune, circling the Sun twice for every three orbits of Neptune, a group so populous that astronomers gave it Pluto's name.[1][5] It rotates once every 13.2 hours or so.[4]
Size and surface
Before the Herschel Space Observatory, Orcus was known as a body about 1,000 km in size with a surface rich in water ice, in both its amorphous and crystalline forms.[1] Herschel's measurement of the heat the pair give off, combined with a thermophysical model, gave a diameter of 917 km, give or take 25, for Orcus and 276 km for Vanth, on the assumption that both reflect light alike, and a surface reflecting about 23 per cent of sunlight, brighter than the average plutino.[1] The density of the system is about 1.53 grams per cubic centimeter, which points to a mix of rock and ice.[1]
The moon
The assumption that Vanth is as bright as Orcus turned out to be wrong. In October and November 2016 the ALMA radio telescope array in Chile resolved the two bodies separately for the first time at thermal wavelengths, and found Vanth to be 475 km across, give or take 75, about half the diameter of Orcus but with a surface reflecting only about 8 per cent of sunlight, three times darker; Orcus itself came out at about 910 km.[2] So large a moon is unusual among the biggest Kuiper belt objects, whose satellites are mostly small, faint fragments; Vanth looks instead like an ordinary Kuiper belt object of its size, which bears on how such moons form.[2] Without knowing which body holds how much of the mass, the density of Orcus itself is uncertain, between about 1.4 and 2.0 grams per cubic centimeter.[2]
What is not yet known
No spacecraft has visited Orcus. How Vanth came to orbit it, the separate densities of the two bodies, and Orcus's formal status all remain open.[2][3]
Sources
- [1]arXiv (Astronomy & Astrophysics, 2013), TNOs are Cool: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of 9 bright targets at 70–500 micron — doi.org, retrieved November 5, 2026
- [2]arXiv (The Astronomical Journal, 2018), Medium-sized satellites of large Kuiper belt objects — doi.org, retrieved November 5, 2026
- [3]NASA, Pluto & Dwarf Planets — science.nasa.gov, retrieved November 5, 2026
- [4]NASA/JPL Small-Body Database, 90482 Orcus (2004 DW) — ssd.jpl.nasa.gov, retrieved November 5, 2026
- [5]NASA, Kuiper Belt: Facts — science.nasa.gov, retrieved November 5, 2026
Related
- What is a dwarf planet?The definition that made Pluto a dwarf planet in 2006, the five the IAU recognises, why the list grows so slowly, and the candidates waiting to join it.
- The Kuiper belt and beyondThe ring of icy worlds past Neptune, the scattered disk and the Oort cloud beyond it, how many objects are known, and how new ones are found.
- PlutoThe largest known body in the Kuiper belt and the first dwarf planet, counted as the ninth planet from 1930 to 2006 and visited by New Horizons in July 2015.
- QuaoarA dwarf-planet candidate about 1,110 km across on a nearly circular orbit beyond Pluto, with a moon, Weywot, and two rings where none were expected.
- GonggongA dwarf-planet candidate about 1,230 km across on a tilted, elongated orbit far beyond Pluto, with a small moon, Xiangliu, found in Hubble images.
- SednaA dwarf-planet candidate about 995 km across on one of the most distant orbits known, never closer to the Sun than 76 au and about 12,700 years around.
- 2026 ED138A dwarf-planet candidate found by the Rubin Observatory beyond Neptune and announced on 27 October 2026, an icy world that may be 600 to 1,500 km across.