Orcus (dwarf planet)
A Kuiper belt dwarf planet and the second-largest plutino.
Orcus (minor-planet number 90482) is a dwarf planet in the Kuiper belt, accompanied by a single large moon called Vanth. Its estimated diameter ranges from 870 to 960 km (540 to 600 mi), making it roughly the size of the inner Solar System dwarf planet Ceres. The surface is relatively bright, with an albedo of about 23 percent, neutral in color, and rich in water ice. Most of this ice is in crystalline form, possibly due to past cryovolcanic activity, and other compounds like methane or ammonia may also be present. American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz discovered Orcus on 17 February 2004.
Orcus is a plutino—a trans-Neptunian object in a 2:3 orbital resonance with Neptune, meaning it completes two orbits around the Sun for every three of Neptune's. This is similar to Pluto, but the phase of Orcus's orbit is opposite: Orcus reached aphelion (its farthest point from the Sun) most recently in 2019, while Pluto reached perihelion (its closest point) in 1989, and the pattern reverses. Orcus is the second-largest known plutino, after Pluto. Its perihelion is about 120° from Pluto's, and their orbital eccentricities and inclinations are similar. Because of these parallels and contrasts—along with its large moon Vanth, comparable to Pluto's moon Charon—Orcus has been called the "anti-Pluto." This influenced its name: Orcus is the Roman/Etruscan equivalent of Pluto, the Roman/Greek underworld deity.
The object was named after Orcus, the Etruscan god of the underworld and punisher of broken oaths. The name was published by the Minor Planet Center on 26 November 2004. It also served as a private reference to Orcas Island, where Brown's wife had lived as a child. On 30 March 2010, Orcus's moon was named Vanth, after a winged female entity from Etruscan mythology who guided the deceased to the underworld. No astronomical symbol for Orcus has been used in the literature, though astrologers sometimes use a symbol (⟨⟩) designed to resemble a skull and an orca's gape, or a rarer inverted Pluto symbol reflecting its "anti-Pluto" status.
Orcus's orbit has a period of 245 years and is moderately inclined at 20.6° to the ecliptic. Though its orbit approaches Neptune's, the 2:3 resonance ensures they are always separated by more than 60°, and over a 14,000-year period Orcus stays more than 18 AU from Neptune. Orcus last reached aphelion in 201
- discovered
- 17 February 2004
- discoverers
- Michael Brown, Chad Trujillo, David Rabinowitz
- estimated_diameter
- 870 to 960 km (540 to 600 mi)
- orbital_period
- 245 years
- moon
- Vanth
- classification
- dwarf planet, plutino
- albedo
- 21–25%
Lore & Background
Orcus was discovered on 17 February 2004 by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz. Precovery images from the Palomar Observatory as early as 8 November 1951 were later obtained from the Digitized Sky Survey. The minor planet was named after Orcus, the Roman/Etruscan god of the underworld and punisher of broken oaths, a name published by the Minor Planet Center on 26 November 2004. The name also served as a private reference to Orcas Island, where Brown's wife had lived as a child. On 30 March 2010, Orcus's moon Vanth was named after a winged female entity of the Etruscan underworld who acted as a psychopomp.
Reader's Guide
Orcus is significant as a large trans-Neptunian object that helps illuminate the diversity of Kuiper belt bodies and the dynamics of plutinos. Its contrasting orbit to Pluto—being at aphelion when Pluto is at perihelion—earned it the nickname 'anti-Pluto' and influenced its naming after an underworld deity. The presence of crystalline water ice on its surface, which should be amorphized by radiation within about 10 million years, suggests recent or ongoing cryovolcanic activity. The Orcus–Vanth binary system provides a valuable analog to the Pluto–Charon system, allowing comparative studies of binary formation and evolution in the outer Solar System. Its relatively high albedo and neutral color distinguish it from many redder Kuiper belt objects, and its infrared spectrum shows the deepest water-ice absorption bands of any Kuiper belt object not associated with the Haumea collisional family.
Did You Know?
- Orcus is the second-largest known plutino, after Pluto itself.
- Orcus has been called the 'anti-Pluto' because its orbit is opposite in phase to Pluto's.
- The surface of Orcus shows crystalline water ice, which may indicate past cryovolcanic activity.
- Orcus's moon Vanth has a diameter of about 442.5 km, determined by a stellar occultation in 2017.
More in Discoveries by Michael E. Brown 1-24
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