Orcus (minor-planet designation:90482 Orcus) is adwarf planet located in theKuiper belt, with one large moon,Vanth.[7] It has an estimated diameter of 870 to 960 km (540 to 600 mi), comparable to theInner Solar System dwarf planetCeres. The surface of Orcus is relatively bright withalbedo reaching 23 percent, neutral in color, and rich in water ice. The ice is predominantly in crystalline form, which may be related to pastcryovolcanic activity. Other compounds likemethane orammonia may also be present on its surface. Orcus was discovered by American astronomersMichael Brown,Chad Trujillo, andDavid Rabinowitz on 17 February 2004.
Orcus is aplutino, atrans-Neptunian object that is locked in a 2:3orbital resonance with theice giantNeptune, making two revolutions around the Sun to every three of Neptune's.[5] This is much likePluto, except that thephase of Orcus's orbit is opposite to Pluto's: Orcus is ataphelion (most recently in 2019) around when Pluto is atperihelion (most recently in 1989) and vice versa.[18] 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 moonVanth that can be compared to Pluto's large moonCharon, Orcus has been dubbed the "anti-Pluto".[19] This was a major consideration in selecting its name, as the deityOrcus was theRoman/Etruscan equivalent of the Roman/GreekPluto.[19]
Theminor planet Orcus was named after one of the Roman gods of the underworld,Orcus. WhilePluto (of Greek origin) was the ruler of the underworld, Orcus (of Etruscan origin) was a punisher of the condemned. The name was published by theMinor Planet Center on 26 November 2004 (M.P.C. 53177).[21] Under the guidelines of theInternational Astronomical Union's (IAU)naming conventions, objects with a similar size andorbit to that ofPluto are named afterunderworld deities. Accordingly, the discoverers suggested naming the object afterOrcus, the Etruscan god of the underworld and punisher of broken oaths. The name was also a private reference to the homonymousOrcas Island, where Brown's wife had lived as a child and that they visit frequently.[22]
On 30 March 2005, 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 apsychopomp, a guide of the deceased to the underworld.[23]
The usage ofplanetary symbols is no longer recommended in astronomy, so Orcus never received a symbol in the astronomical literature. A symbol⟨⟩, used mostly among astrologers,[24] is included inUnicode as U+1F77F.[25] The symbol was designed by Denis Moskowitz, a software engineer in Massachusetts; it is an ORmonogram, designed to resemble both a skull and anorca's gape.[26] There is a rarer symbol, an invertedastrological Pluto symbol, reflecting Orcus as the anti-Pluto: it was designed by Melanie Reinhart.[24]
Orcus is in a 2:3 orbital resonance withNeptune, having an orbital period of 245 years,[5][1] and is classified as aplutino.[2] Its orbit is moderatelyinclined at 20.6° to theecliptic.[1] Orcus's orbit is similar to Pluto's (both haveperihelia above theecliptic), 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 anangular separation of over 60° between them). Over a 14,000-year period, Orcus stays more than 18 AU from Neptune.[18] 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".[19] Orcus last reached itsaphelion (farthest distance from the Sun) in 2019 and will come to perihelion (closest distance to the Sun) around 10 January 2143.[10] Simulations by theDeep Ecliptic Survey show that over the next 10 million years Orcus may acquire a perihelion distance (qmin) as small as 27.8 AU.[5]
The rotation period of Orcus is uncertain, as different photometric surveys have produced different results. Some show low amplitude variations with periods ranging from 7 to 21 hours, whereas others show no variability.[27] The rotational axis of Orcus probably coincides with the orbital axis of its moon, Vanth. This means that Orcus is currently viewed pole-on, which could explain the near absence of anyrotational modulation of its brightness.[27][28] AstronomerJosé Luis Ortiz and colleagues have derived a possible rotation period of about 10.5 hours, assuming that Orcus is nottidally locked with Vanth.[28] If, however, the primary is tidally locked with the satellite, the rotational period would coincide with the 9.7-day orbital period of Vanth.[28]
The orbits of Orcus (blue), Pluto (red) and Neptune (grey). Orcus and Pluto are shown in the April 2006 positions. The dates of theirperihelia (q) andaphelia (Q) are also marked.
Orcus compared toEarth and theMoonLong-exposure photograph of Orcus at visual magnitude 19.2
The absolute magnitude of Orcus is approximately 2.3.[12] The detection of Orcus by theSpitzer Space Telescope in thefar infrared[29] and byHerschel Space Telescope insubmillimeter estimates its diameter at 958.4 km (595.5 mi), with an uncertainty of 22.9 km (14.2 mi).[12] Orcus appears to have analbedo of about 21–25%,[12] which may be typical of trans-Neptunian objects approaching the 1,000 km (620 mi) diameter range.[30] 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.[17] TheALMA 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).[11] Using astellar 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).[31]
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,[7] approximately equal to that of theSaturnian moonTethys (6.175×1020 kg).[32] The mass of the Orcus system is about 3.8 percent that ofEris, the most massive known dwarf planet (1.66×1022 kg).[17][33]
The ratio of the mass of Vanth to that of Orcus was measured astrometrically with theALMA submillimeter telescope and is0.16±0.02 with Vanth containing13.7%±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.[13]
The first spectroscopic observations in 2004 showed that the visible spectrum of Orcus is flat (neutral in color) and featureless, whereas in thenear-infrared there were moderately strongwater absorption bands at 1.5 and 2.0 μm.[34] The neutral visible spectrum and strong water absorption bands of Orcus showed that Orcus appeared different from other trans-Neptunian objects, which typically have a red visible spectrum and often featurelessinfrared spectra.[34] Further infrared observations in 2004 by theEuropean Southern Observatory and theGemini telescope gave results consistent with mixtures of waterice andcarbonaceous compounds, such astholins.[15] The water and methane ices can cover no more than 50 percent and 30 percent of the surface, respectively.[35] This means the proportion of ice on the surface is less than onCharon, but similar to that onTriton.[35]
Later in 2008–2010 new infrared spectroscopic observations with a highersignal-to-noise ratio revealed additional spectral features. Among them is a deep water ice absorption band at 1.65 μm, which is evidence of thecrystalline water ice on the surface of Orcus, and a new absorption band at 2.22 μm. The origin of the latter feature is not completely clear. It can be caused either byammonia/ammonium dissolved in the water ice or bymethane/ethane ices.[14] Theradiative transfer modeling showed that a mixture of water ice, tholins (as a darkening agent), ethane ice, and ammonium ion (NH4+) provides the best match to the spectra, whereas a combination of water ice, tholins, methane ice and ammonia hydrate gives a slightly inferior result. On the other hand, a mixture of only ammonia hydrate, tholins and water ice failed to provide a satisfactory match.[27] As of 2010, the only reliably identified compounds on the surface of Orcus are crystalline water ice and, possibly, dark tholins. A firm identification of ammonia, methane, and otherhydrocarbons requires a better infrared spectra.[27]
Orcus sits at the threshold for trans-Neptunian objects massive enough to retainvolatiles such as methane on the surface.[27] The reflectance spectrum of Orcus shows the deepest water-ice absorption bands of any Kuiper belt object that is not associated with theHaumea collisional family.[17] Thelarge icy satellites of Uranus have infrared spectra quite similar to that of Orcus.[17] Among other trans-Neptunian objects, the large plutino208996 Achlys and Pluto's moon Charon both have similar surface spectra to Orcus,[14] with flat, featureless visible spectra and moderately strong water ice absorption bands in the near-infrared.[27]
Crystalline water ice on the surfaces of trans-Neptunian objects should be completelyamorphized by the galactic and Solar radiation in about 10 million years.[14] Thus the presence ofcrystalline waterice, and possiblyammonia ice, may indicate that a renewal mechanism was active in the past on the surface of Orcus.[14] Ammonia so far has not been detected on any trans-Neptunian object or icy satellite of the outer planets other thanMiranda.[14] The 1.65 μm band on Orcus is broad and deep (12%), as on Charon,Quaoar,Haumea, and icy satellites of thegiant planets.[14] Some calculations indicate thatcryovolcanism, which is considered one of the possible renewal mechanisms, may indeed be possible for trans-Neptunian objects larger than about 1,000 km (620 mi).[27] Orcus may have experienced at least one such episode in the past, which turned the amorphous water ice on its surface into crystalline. The preferred type of volcanism may have been explosive aqueous volcanism driven by an explosive dissolution of methane from water–ammonia melts.[27]
Orcus has one known moon, Vanth (formal designation(90482) Orcus I). It was discovered byMichael Brown and T.-A. Suer using discovery images taken by theHubble Space Telescope on 13 November 2005.[36] The discovery was announced in anIAU Circular notice published on 22 February 2007.[37] A spatially resolved submillimeter imaging of the Orcus–Vanth system in 2016 showed that Vanth has a relatively large size of 475 km (295 mi), with an uncertainty of 75 km (47 mi).[11] That estimate for Vanth is in good agreement with the size of about 442.5 km (275.0 mi) derived from astellar occultation in 2017.[31] Like Charon compared to Pluto, Vanth is quite large compared to Orcus, and is one reason for characterizing Orcus as the 'anti-Pluto'. Vanth is the third-largest known dwarf-planet moon, after Charon andDysnomia. The ratio of masses of Orcus and Vanth is uncertain, possibly anywhere from 1:33 to 1:12.[38]
^abcOrtiz, J. L.; Cikota, A.; Cikota, S.; Hestroffer, D.; Thirouin, A.; Morales, N.; Duffard, R.; Gil-Hutton, R.; Santos-Sanz, P.; De La Cueva, I. (2010). "A mid-term astrometric and photometric study of trans-Neptunian object (90482) Orcus".Astronomy & Astrophysics.525: A31.arXiv:1010.6187.Bibcode:2011A&A...525A..31O.doi:10.1051/0004-6361/201015309.S2CID56051949.
^Stansberry, J.; Grundy, W.; Brown, M.; Cruikshank, D.; Spencer, J.; Trilling, D.; Margot, J.-L. (2008). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from the Spitzer Space Telescope". In M. A. Barucci; H. Boehnhardt; D. P. Cruikshank; A. Morbidelli (eds.).The Solar System Beyond Neptune. Tucson: University of Arizona Press. pp. 161–179.Bibcode:2008ssbn.book..161S.ISBN978-0816527557.
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