Orcus

Trans-Neptunian object · Dwarf-planet candidate (plutino) · orbits Sun

Orcus is Pluto’s mirror image, nicknamed the “anti-Pluto”: it is locked in the same 2:3 resonance with Neptune, but permanently phased so that it sits on the opposite side of the dance. It even has an outsized moon, Vanth, echoing Pluto and Charon.

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Quick facts

Its orbital period of ~247 years is within months of Pluto’s, yet the two can never meet.
Orcus was a Roman-Etruscan god of the underworld - a counterpart of Pluto; its moon Vanth is named for a guide of souls.
Vanth is nearly half Orcus’ diameter, making the pair a smaller echo of the Pluto-Charon double system.

The anti-Pluto

A resonance is a rhythm, and rhythms have phases. Orcus and Pluto each complete two orbits for every three of Neptune’s, but half a cycle apart: when Pluto is near perihelion, Orcus is near aphelion, and vice versa. Their orbits are near-twins - about 248 years at ~39 AU - yet Neptune’s clockwork keeps the two worlds forever on opposite sides of it.

Physical characteristics

Diameter	910 km
Mass	6.300e+20 kg
Geometric albedo	0.23
Known moons	1

Atmosphere and composition

Atmosphere	None detected.
Composition	Water ice (partly crystalline) with possible ammonia - hints of past cryovolcanic resurfacing.

Discovery

Discovered by	Mike Brown, Chad Trujillo & David Rabinowitz
Year	2004
How	Palomar Observatory 48-inch Samuel Oschin Telescope

What is still uncertain

Mass is measured for the Orcus-Vanth pair together; Vanth may carry close to a tenth of it.