# Orcus (dwarf planet) <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Orcus in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Orcus&embed=1" data-title="Orcus in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Orcus`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: drag the year slider to 2019 to find Orcus at the far end of its orbit, about 48 AU from the Sun; set the speed to 100 years/s and count two of its laps against three of Neptune's, noticing that Neptune is never nearby when Orcus comes in close; press l to show the labels and compare Orcus's path with Pluto's.* **Orcus** (minor-planet number 90482) is a [[Dwarf_planet|dwarf planet]] candidate of the [[Kuiper_belt|Kuiper belt]], about 900–960 km across, with an unusually large moon, Vanth.[^fornasier2013][^brown2018] It was discovered on 17 February 2004 by Michael Brown, Chad Trujillo and David Rabinowitz.[^mpc] Its surface is fairly bright, neutral grey in colour and rich in crystalline water ice, which may record past cryovolcanism; ammonia or methane may also be present.[^barucci2008][^delsanti2010] Orcus is a [[Plutino|plutino]]: like [[Pluto]], it is locked in a 2:3 resonance with [[Neptune]], completing two orbits for every three of Neptune's.[^buie2007] Its orbit resembles Pluto's in size, tilt and eccentricity, but it runs in the opposite phase: Orcus was at aphelion in 2019, thirty years after Pluto passed perihelion.[^mpec2004][^horizons] That symmetry, together with a large moon comparable to Pluto's [[Charon_(moon)|Charon]], earned it the nickname "anti-Pluto", and it guided the choice of its name, a Roman god of the underworld.[^brown2009a] The explorer at the top of this page is locked on Orcus: it draws the orbit from JPL's Small-Body Database, a plutino path with a semi-major axis of 39.4 [[Astronomical_unit|AU]], in the same resonance with Neptune as Pluto; Vanth is not drawn at this scale.[^sbdb] ## History ### Discovery Brown, Trujillo and Rabinowitz found Orcus on 17 February 2004 in images from their survey of the outer Solar System at Palomar Observatory. Earlier images turned up quickly once the orbit was known, the oldest from a Palomar plate of 8 November 1951 scanned for the Digitized Sky Survey.[^mpc] The first Minor Planet Center announcement, MPEC 2004-D09, credited the discovery to the NEAT asteroid survey, and a correction was issued a few days later.[^mpec2004d13] ### Name and symbol Objects of Pluto's size and orbital type are, by IAU convention, named after underworld deities. The discoverers chose Orcus, a god of the underworld of Etruscan origin in Roman religion, remembered as a punisher of those who broke oaths, whereas Pluto ruled the underworld as a whole. The name, published by the Minor Planet Center on 26 November 2004, fits the object's role as a mirror image of Pluto. It was also a private reference: Brown's wife grew up on Orcas Island in Washington state, which the family often visits.[^mpc-arch][^brown2009b] The moon was named Vanth on 30 March 2010, after a winged female figure of the Etruscan underworld who attends the dying and guides souls to the afterlife.[^johnston] Brown had invited readers of his blog to suggest names for the moon.[^brown2009a] Planetary symbols are no longer used in astronomical research, and Orcus never received one in the scientific literature. A symbol designed by the software engineer Denis Moskowitz, an OR monogram shaped to suggest both a skull and the open jaws of an orca, was added to Unicode in 2022 and is used mainly by astrologers.[^unicode2022][^miller2021] ## Orbit Orcus orbits the Sun every 247 years or so, between about 30.7 AU at perihelion and 48.1 AU at aphelion, on an orbit with an eccentricity of 0.22 inclined 20.6° to the [[Ecliptic|ecliptic]].[^sbdb] Its period is one and a half times Neptune's, the 2:3 mean-motion [[Resonance|resonance]] that defines the plutinos, and the Deep Ecliptic Survey classifies it as a member of that group.[^buie2007] It passed aphelion in 2019 and will return to perihelion around 12 January 2143.[^horizons] As with Pluto, the resonance keeps Orcus safe even though its perihelion lies near Neptune's orbit. The timing of the two orbits is locked so that Neptune is always well away, more than 60° of longitude, when Orcus reaches its closest point to the Sun; over a 14,000-year integration Orcus stays more than 18 AU from the planet.[^mpec2004] Orcus's perihelion lies about 120° in longitude from Pluto's (derived), while the eccentricities and inclinations of the two orbits are similar.[^sbdb][^fs-pluto] Because both bodies are held in the same resonance, they stay in opposite phases, one near perihelion when the other is near aphelion: Pluto passed perihelion in 1989, Orcus aphelion in 2019.[^mpec2004][^brown2009a] Over the next 10 million years, Buie's simulations show that its perihelion could shrink to 27.8 AU.[^buie2007] The explorer draws the present osculating orbit from JPL elements, so the resonance can be seen at work: run the speed up and Neptune circles three times while Orcus goes round twice. The body's size is enlarged for visibility.[^sbdb] ## Physical characteristics ### Size and magnitude Orcus has an absolute magnitude of about 2.3.[^fornasier2013] Thermal measurements, first from the Spitzer Space Telescope in the far-infrared and then from Herschel in the submillimetre, give a diameter of 958.4 ± 22.9 km and a geometric albedo of about 21–25 percent, which may be typical of trans-Neptunian objects approaching 1,000 km.[^stansberry2008][^fornasier2013] Those values treat the system as one body, however. Vanth, with an absolute magnitude of about 4.88, is about one-eleventh as bright as Orcus itself.[^brown2010] ALMA submillimetre images from 2016 resolved the pair and gave Orcus a diameter of about 910 km and Vanth about 475 km, with a much darker albedo of around 8 percent; a 2017 stellar occultation by Vanth measured its diameter at 442.5 ± 10.2 km.[^brown2018][^sickafoose2019] ### Mass and density The orbit of Vanth gives the mass of the pair through [[Kepler's_laws_of_planetary_motion|Kepler's third law]]: 6.348 × 10²⁰ kg, close to the mass of Saturn's moon Tethys, 6.175 × 10²⁰ kg, and about 3.8 percent of the mass of [[Eris_(dwarf_planet)|Eris]] (derived).[^grundy2019][^jacobson2006][^brown2007] Brown and Butler measured the wobble of Orcus about the system's centre of mass with ALMA and found a Vanth-to-Orcus mass ratio of about 0.16, so that Vanth holds 13.7 percent of the total; both bodies then have similar densities, about 1.5 g/cm³.[^brown2023] ### Spectra and surface The first spectra, taken in 2004, found a flat, featureless visible spectrum, neutral in colour rather than the red of most trans-Neptunian objects, and moderately strong water-ice absorption bands at 1.5 and 2.0 μm in the near-infrared.[^fornasier2004] Further spectra from ESO and Gemini fitted mixtures of [[Water|water]] ice with dark carbon-rich compounds such as tholins; Trujillo and colleagues limited water ice to no more than half the surface and methane ice to no more than 30 percent, less ice than on Charon but similar to [[Triton_(moon)|Triton]].[^debergh2005][^trujillo2005] Better spectra from 2008–2010 revealed a deep band at 1.65 μm, the signature of crystalline water ice, and a new band at 2.22 μm whose origin is uncertain; ammonia or ammonium in the ice, or methane or ethane, could produce it.[^barucci2008] In radiative-transfer models by Delsanti and colleagues, water ice with tholins, ethane ice and ammonium ions fit best, and water ice, tholins, methane and ammonia hydrate slightly less well; as of 2010 only crystalline water ice, and probably tholins, were firmly identified.[^delsanti2010] Orcus lies near the size threshold at which a trans-Neptunian object can keep volatile ices such as methane. Outside the [[Haumea]] family, its water-ice bands are the deepest known in the Kuiper belt, and its spectrum resembles those of the large icy moons of [[Uranus]] and of Charon.[^delsanti2010][^brown2010] ### Cryovolcanism Radiation from the Sun and the galaxy should convert crystalline water ice on a trans-Neptunian surface to the amorphous form within about 10 million years, so the crystalline ice on Orcus, and possibly ammonia, points to a mechanism that renewed the surface in the past.[^barucci2008] Its 1.65 μm band is broad and deep, about 12 percent, like those of Charon, [[Quaoar]], Haumea and the icy moons of the giant planets.[^barucci2008] Calculations suggest that cryovolcanism may be possible in trans-Neptunian objects larger than about 1,000 km, so Orcus is a borderline case. Delsanti and colleagues suggested that at least one episode of explosive, water-based volcanism, driven by methane coming out of solution from a water–ammonia melt, could have turned the surface ice crystalline.[^delsanti2010] ### Rotation The rotation period is uncertain. Some photometric studies found small brightness variations with periods from about 7 to 21 hours, others none at all.[^delsanti2010] If Orcus's spin axis is aligned with the orbit of Vanth, the system is at present seen nearly pole-on, which would explain why the light barely varies.[^ortiz2011] Ortiz and colleagues derived a possible rotation period of about 10.5 hours, assuming Orcus is not tidally locked to Vanth; if it is locked, the rotation would match Vanth's orbital period of about 9.7 days.[^ortiz2011] Brown and Butler's ALMA results make Orcus and Vanth a candidate for mutual tidal locking, a state confirmed for Pluto and Charon.[^brown2023] ## Satellite Orcus has one known moon, Vanth, formally (90482) Orcus I. Michael Brown and T.-A. Suer found it in Hubble Space Telescope images taken on 13 November 2005, and the discovery was announced in an IAU Circular on 22 February 2007.[^iauc8812] At roughly 440–475 km across, about half the diameter of Orcus, Vanth is proportionally as large as Charon is to Pluto, one of the reasons for the "anti-Pluto" label. Its diameter was first estimated from ALMA imaging at 475 ± 75 km and then measured more precisely during the 2017 occultation.[^brown2018][^sickafoose2019] Early estimates of the mass ratio ranged widely, from 1:33 to 1:12; the ALMA astrometry, which gives 0.16, has since narrowed it considerably.[^carry2011][^brown2023] Vanth is much darker than its primary: the ALMA measurements give it an albedo of about 8 percent, against roughly 23 percent for Orcus.[^brown2018][^fornasier2013] Grundy and colleagues measured the orientation of its orbit as part of a survey of trans-Neptunian binaries.[^grundy2019] ## See also - [[Plutino]] · [[Resonant_trans-Neptunian_object]] - [[Pluto]] · [[Charon_(moon)]] - [[Dwarf_planet]] · [[Trans-Neptunian_object]] - [[Kuiper_belt]] - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived numbers are computed from the cited values: the orbital period from the JPL value of about 90,300 days, ≈ 247 years; the separation of the perihelia from JPL and NASA elements, Orcus 268° + 73.6° ≈ 342° against Pluto's longitude of perihelion of about 224°, ≈ 118°; the ratio 6.348 × 10²⁰ / 1.66 × 10²² ≈ 0.038 for the system mass relative to Eris; and Orcus's mean orbital motion against Neptune's, 164.8 / 247 ≈ 0.67, near the 2:3 ratio. ## References [^sbdb]: JPL Small-Body Database, "90482 Orcus (2004 DW)" (orbital elements fetched 2026-09-18). https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=90482 [^horizons]: JPL Solar System Dynamics. "Horizons" ephemeris for 90482 Orcus near perihelion, 12 January 2143. https://ssd.jpl.nasa.gov/horizons/ [^fs-pluto]: Williams, D. R. "Pluto Fact Sheet" (mean orbital elements, J2000). NASA NSSDCA, last updated 11 January 2024. https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html [^mpc]: Minor Planet Center. "90482 Orcus (2004 DW)". http://www.minorplanetcenter.net/db_search/show_object?object_id=90482 [^mpc-arch]: Minor Planet Center. "MPC/MPO/MPS Archive" (M.P.C. 53177, 26 November 2004). http://www.minorplanetcenter.net/iau/ECS/MPCArchive/MPCArchive_TBL.html [^mpec2004]: Minor Planet Center (20 February 2004). "MPEC 2004-D15: 2004 DW". http://www.minorplanetcenter.net/mpec/K04/K04D15.html [^mpec2004d13]: Minor Planet Center (2004). "MPEC 2004-D13" (correction to MPEC 2004-D09). https://minorplanetcenter.net/mpec/K04/K04D13.html [^buie2007]: Buie, M. W. (22 December 2007). "Orbit fit and astrometric record for 90482". Southwest Research Institute. http://www.boulder.swri.edu/~buie/kbo/astrom/90482.html [^brown2009a]: Brown, M. E. (23 March 2009). "S/1 90482 (2005) needs your help". *Mike Brown's Planets*. http://www.mikebrownsplanets.com/2009/03/s1-90482-2005-needs-your-help.html [^brown2009b]: Brown, M. E. (6 April 2009). "Orcus Porcus". *Mike Brown's Planets*. http://www.mikebrownsplanets.com/2009/04/orcus-porcus.html [^johnston]: Johnston, W. R. "(90482) Orcus and Vanth". Johnston's Archive. http://www.johnstonsarchive.net/astro/astmoons/am-90482.html [^unicode2022]: Anderson, D. (4 May 2022). "Out of this world: New astronomy symbols approved for the Unicode Standard". Unicode blog. http://blog.unicode.org/2022/05/out-of-this-world-new-astronomy-symbols.html [^miller2021]: Miller, K. (26 October 2021). "Unicode request for dwarf-planet symbols". Unicode proposal L2/21-224. https://www.unicode.org/L2/L2021/21224-dwarf-planet-syms.pdf [^fornasier2013]: Fornasier, S.; Lellouch, E.; Müller, T.; et al. (2013). "'TNOs are cool': A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of nine bright targets at 70–500 μm". *Astronomy & Astrophysics* 555: A15. https://doi.org/10.1051/0004-6361/201321329 [^stansberry2008]: Stansberry, J.; Grundy, W.; Brown, M.; et al. (2008). "Physical properties of Kuiper belt and Centaur objects: Constraints from the Spitzer Space Telescope". In *The Solar System Beyond Neptune*. University of Arizona Press, pp. 161–179. Bibcode 2008ssbn.book..161S. [^brown2010]: Brown, M. E.; Ragozzine, D.; Stansberry, J.; Fraser, W. C. (2010). "The size, density, and formation of the Orcus–Vanth system in the Kuiper belt". *The Astronomical Journal* 139: 2700–2705. https://doi.org/10.1088/0004-6256/139/6/2700 [^brown2018]: Brown, M. E.; Butler, B. J. (2018). "Medium-sized satellites of large Kuiper belt objects". *The Astronomical Journal* 156: 164. https://doi.org/10.3847/1538-3881/aad9f2 [^sickafoose2019]: Sickafoose, A. A.; Bosh, A. S.; Levine, S. E.; et al. (2019). "A stellar occultation by Vanth, a satellite of (90482) Orcus". *Icarus* 319: 657–668. https://doi.org/10.1016/j.icarus.2018.10.016 [^grundy2019]: Grundy, W. M.; Noll, K. S.; Roe, H. G.; et al. (2019). "Mutual orbit orientations of transneptunian binaries". *Icarus* 334: 62–78. https://doi.org/10.1016/j.icarus.2019.03.035 [^jacobson2006]: Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; et al. (2006). "The gravity field of the Saturnian system from satellite observations and spacecraft tracking data". *The Astronomical Journal* 132: 2520–2526. https://doi.org/10.1086/508812 [^brown2007]: Brown, M. E.; Schaller, E. L. (2007). "The mass of dwarf planet Eris". *Science* 316: 1585. https://doi.org/10.1126/science.1139415 [^brown2023]: Brown, M. E.; Butler, B. J. (2023). "Masses and densities of dwarf planet satellites measured with ALMA". *The Planetary Science Journal* 4: 193. https://doi.org/10.3847/PSJ/ace52a [^fornasier2004]: Fornasier, S.; Dotto, E.; Barucci, M. A.; Barbieri, C. (2004). "Water ice on the surface of the large TNO 2004 DW". *Astronomy & Astrophysics* 422: L43–L46. https://doi.org/10.1051/0004-6361:20048004 [^debergh2005]: de Bergh, C.; Delsanti, A.; Tozzi, G. P.; et al. (2005). "The surface of the transneptunian object 90482 Orcus". *Astronomy & Astrophysics* 437: 1115–1120. https://doi.org/10.1051/0004-6361:20042533 [^trujillo2005]: Trujillo, C. A.; Brown, M. E.; Rabinowitz, D. L.; Geballe, T. R. (2005). "Near-infrared surface properties of the two intrinsically brightest minor planets: (90377) Sedna and (90482) Orcus". *The Astrophysical Journal* 627: 1057–1065. https://doi.org/10.1086/430337 [^barucci2008]: Barucci, M. A.; Merlin, F.; Guilbert, A.; et al. (2008). "Surface composition and temperature of the TNO Orcus". *Astronomy & Astrophysics* 479: L13–L16. https://doi.org/10.1051/0004-6361:20079079 [^delsanti2010]: Delsanti, A.; Merlin, F.; Guilbert-Lepoutre, A.; et al. (2010). "Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of plutino (90482) Orcus". *Astronomy & Astrophysics* 520: A40. https://doi.org/10.1051/0004-6361/201014296 [^ortiz2011]: Ortiz, J. L.; Cikota, A.; Cikota, S.; et al. (2011). "A mid-term astrometric and photometric study of trans-Neptunian object (90482) Orcus". *Astronomy & Astrophysics* 525: A31. https://doi.org/10.1051/0004-6361/201015309 [^iauc8812]: Green, D. W. E. (22 February 2007). "IAUC 8812: Satellites of 2003 AZ84, (50000), (55637), (90482)". *IAU Circular*. http://www.cbat.eps.harvard.edu/iauc/08800/08812.html [^carry2011]: Carry, B.; Hestroffer, D.; DeMeo, F. E.; et al. (2011). "Integral-field spectroscopy of (90482) Orcus–Vanth". *Astronomy & Astrophysics* 534: A115. https://doi.org/10.1051/0004-6361/201117486 ## External links - JPL Small-Body Database: 90482 Orcus. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=90482 - Minor Planet Center: 90482 Orcus. http://www.minorplanetcenter.net/db_search/show_object?object_id=90482 - MPEC 2004-D15: 2004 DW. http://www.minorplanetcenter.net/mpec/K04/K04D15.html - NASA Science: Kuiper belt. https://science.nasa.gov/solar-system/kuiper-belt/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Orcus_(dwarf_planet)) : [Wikitube](https://en.wikitube.io/wiki/Orcus_(dwarf_planet)) · pinned revision [1373458670](https://en.wikipedia.org/w/index.php?oldid=1373458670) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. 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