Orcus
What is Orcus?
Dwarf planets· Diameter ~910 km. Called the 'anti-Pluto' because it moves in the same rhythm as Pluto but on the opposite side of its orbit. It is named after a Roman god of the underworld and has a large moon, Vanth.
Orbit and size
| Diameter | ~910 km |
|---|---|
| Distance | 30~48 AU |
| Year | 245 yr |
| Note | anti-Pluto — same orbit type, opposite phase |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Vanth, the outsized partner of Orcus
Vanth is the moon of Orcus. At about 440 km across it is nearly half the size of Orcus, which is about 910 km. That is very large for a moon, so the pair is close to being a double, like Pluto and Charon. Vanth circles Orcus about 9,000 km away, once every 9.5 days.
Vanth was found in Hubble Space Telescope images in November 2005. Its discoverer, Michael Brown, asked readers of his blog to suggest names. The one chosen was Vanth, the winged goddess of Etruscan myth who guides the dead to the underworld, a good partner for Orcus, a god of the dead.
Vanth's surface is redder than that of Orcus, so the two may have different origins. Orcus may have captured a passing body, or the two may have split apart in a collision and then changed differently. Which is true is not yet known. Orcus passed its farthest point from the Sun in 2019, and it will next be at its closest in the 2140s.
The dwarf planet in depth
Orcus is a body beyond Neptune discovered in 2004 by Michael Brown's team. It is named after the Roman god of the underworld who punished those who broke their oaths. It is estimated to be about 910 km across. It moves between 30 and 48 AU from the Sun, taking 245 years per lap, and is considered a likely dwarf planet.
Like Pluto, Orcus goes around the Sun twice for every three orbits of Neptune. Bodies with this rhythm are called 'plutinos', meaning little Plutos. But Orcus sits on the opposite side of its orbit from Pluto: when Pluto is near its closest point to the Sun, Orcus is near its farthest. That is how it earned the nickname 'anti-Pluto'.
Crystalline water ice and ammonia have been found on its surface. Particles from space tend to break down the crystal structure of such ice over time, so some scientists think fresh ice may once have risen from inside. In the orbit view, follow Orcus and then Pluto to compare their similar paths.
Frequently asked
- How big is Orcus?
- The diameter of Orcus is ~910 km.
- How far is Orcus from Earth?
- The distance of Orcus is 30~48 AU.
- What is notable about Orcus?
- The note of Orcus is anti-Pluto — same orbit type, opposite phase.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.