Orcus
What is Orcus?
Dwarf planets· Diameter ~910 km. The 'anti-Pluto' — a dwarf-planet candidate locked in the same 2:3 resonance with Neptune as Pluto but in the opposite phase, so it sits at aphelion when Pluto is at perihelion. Named for the Roman god of the underworld, with a large moon, Vanth. Compare its orbit with Pluto's in the orbit view.
The orbit of Orcus at this moment: https://nowsky.org/solar/orbit?fw=orcus&sp=110 — replayed live, no login.
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.
Frequently asked
- How big is Orcus?
- ~910 km.
- How far is Orcus from Earth?
- 30~48 AU.
- What is notable about Orcus?
- anti-Pluto — same orbit type, opposite phase.
Vanth, the moon that makes Orcus a double
Orcus's moon Vanth is about 440 km across, close to half the size of Orcus itself (about 910 km) — a double-body arrangement like Pluto and Charon, the two facing each other as they circle every 9.5 days. Vanth was found in Hubble images in 2005 and is named for the winged Etruscan goddess who guides the dead to the underworld, a fitting partner for Orcus, the Roman god of the dead.
Vanth's surface is redder and darker than Orcus's, so the two may have different origins — either Orcus captured its moon, or the pair split in a collision and their surfaces evolved apart. Their large mass ratio and nearly circular orbit suggest tidal evolution has run its course, which means Vanth has been where it is for hundreds of millions of years at least.
The dwarf planet in depth
Orcus was discovered in 2004 by Michael Brown's team and named for the Roman god of the underworld who punished oath-breakers. Estimated at about 910 km across, it travels between 30 and 48 AU on a 245-year orbit. Like Pluto it circles the Sun twice for every three Neptune orbits — a 2:3 resonance that makes it a 'plutino'.
What makes Orcus fascinating is that its orbit nearly mirrors Pluto's while sitting in exactly the opposite phase. When Pluto is closest to the Sun, Orcus is farthest, and the two always lie on opposite sides of Neptune — hence the nickname 'anti-Pluto'. Its moon Vanth, found by Hubble in 2005 and named for an Etruscan guide of the dead, is about 440 km wide, nearly half of Orcus, making the pair almost a double body like Pluto and Charon.
Track Orcus and then Pluto in the orbit view to compare their mirror-image paths. Crystalline water ice and ammonia detected on its surface suggest that, despite its modest size, Orcus may once have been geologically active inside.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.