Orcus (dwarf planet)

Orcus (minor-planet number 90482) is a dwarf planet candidate located in the Kuiper belt, with one large moon, Vanth. It has an estimated diameter of 870 to 960 km (540 to 600 mi), comparable to the Inner Solar System dwarf planet Ceres. The surface of Orcus is relatively bright with albedo reaching 23 percent, neutral in color, and rich in water ice. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present on its surface. Orcus was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on 17 February 2004. Orcus is a plutino, a trans-Neptunian object that is locked in a 2:3 orbital resonance with the ice giant Neptune, making two revolutions around the Sun to every three of Neptune's. This is much like Pluto, except that the phase of Orcus's orbit is opposite to Pluto's: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa. Orcus is the second-largest known plutino, after Pluto itself. The perihelion of Orcus's orbit is around 120° from that of Pluto, while the eccentricities and inclinations are similar. Because of these similarities and contrasts, along with its large moon Vanth that can be compared to Pluto's large moon Charon, Orcus has been dubbed the "anti-Pluto". This was a major consideration in selecting its name, as the deity Orcus was the Roman/Etruscan equivalent of the Roman/Greek Pluto.

History

Discovery

Orcus was discovered on 17 February 2004, by American astronomers Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images taken by the Palomar Observatory as early as 8 November 1951 were later obtained from the Digitized Sky Survey.

Name and symbol The minor planet Orcus was named after one of the Roman gods of the underworld, Orcus. While Pluto (of Greek origin) was the ruler of the underworld, Orcus (of Etruscan origin) was a punisher of the condemned. The name was published by the Minor Planet Center on 26 November 2004 (M.P.C. 53177). Under the guidelines of the International Astronomical Union's (IAU) naming conventions, objects with a similar size and orbit to that of Pluto are named after underworld deities. Accordingly, the discoverers suggested naming the object after Orcus, the Etruscan god of the underworld and punisher of broken oaths. The name was also a private reference to the homonymous Orcas Island, where Brown's wife had lived as a child and that they visit frequently. On 30 March 2010, Orcus's moon, Vanth, was named after a winged female entity, Vanth, of the Etruscan underworld. She could be present at the moment of death, and frequently acted as a psychopomp, a guide of the deceased to the underworld. The usage of planetary symbols is no longer recommended in astronomy, so Orcus never received a symbol in the astronomical literature. A symbol ⟨⟩, used mostly among astrologers, is included in Unicode as U+1F77F. The symbol was designed by Denis Moskowitz, a software engineer in Massachusetts; it is an OR monogram, designed to resemble both a skull and an orca's gape. There is a rarer symbol , an inverted astrological Pluto symbol, reflecting Orcus as the anti-Pluto: it was designed by Melanie Reinhart.

Orbit Orcus is in a 2:3 orbital resonance with Neptune, having an orbital period of 245 years, and is classified as a plutino. Its orbit is moderately inclined at 20.6° to the ecliptic. Orcus's orbit is similar to Pluto's (both have perihelia above the ecliptic), but is oriented differently. Although at one point its orbit approaches that of Neptune, the resonance between the two bodies means that Orcus itself is always a great distance away from Neptune (there is always an angular separation of over 60° between them). Over a 14,000-year period, Orcus stays more than 18 AU from Neptune. Because their mutual resonance with Neptune constrains Orcus and Pluto to remain in opposite phases of their otherwise very similar motions, Orcus is sometimes described as the "anti-Pluto". Orcus last reached its aphelion (farthest distance from the Sun) in 2019 and will come to perihelion (closest distance to the Sun) around 12 January 2143. Simulations by the Deep Ecliptic Survey show that over the next 10 million years Orcus may acquire a perihelion distance (qmin) as small as 27.8 AU.

Physical characteristics

Size and magnitude

The absolute magnitude of Orcus is approximately 2.3. The detection of Orcus by the Spitzer Space Telescope in the far infrared and by Herschel Space Telescope in submillimeter estimates its diameter at 958.4 km (595.5 mi), with an uncertainty of 22.9 km (14.2 mi). Orcus appears to have an albedo of about 21–25%, which may be typical of trans-Neptunian objects approaching the 1,000 km (620 mi) diameter range. The magnitude and size estimates were made under the assumption that Orcus is a singular object. The presence of a relatively large satellite, Vanth, may change them considerably. The absolute magnitude of Vanth is estimated at 4.88, which means that it is about 1/11 as bright as Orcus itself. The ALMA submillimeter measurements taken in 2016 showed that Vanth has a relatively large size of 475 km (295 mi) with an albedo of about 8 percent while Orcus's has a slightly smaller size of 910 km (570 mi). Using a stellar occultation by Vanth in 2017, Vanth's diameter has been determined to be 442.5 km (275.0 mi), with an uncertainty of 10.2 km (6.3 mi).

Mass and density Orcus and Vanth are known to constitute a binary system. The mass of the system has been estimated to be (6.348±0.019)×1020 kg, approximately equal to that of the Saturnian moon Tethys (6.175×1020 kg). The mass of the Orcus system is about 3.8 percent that of Eris, the most massive known dwarf planet (1.66×1022 kg). The ratio of the mass of Vanth to that of Orcus was measured astrometrically with the ALMA submillimeter telescope and is 0.16±0.02 with Vanth containing 13.7%±1.3% of the total system mass. This also means that the densities of both bodies are about the same at ~1.5 g/cm3.

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