Vanth (formal designation (90482) Orcus I) is the only known moon of the large trans-Neptunian dwarf planet Orcus. It was discovered by Michael Brown and Terry-Ann Suer using images taken by the Hubble Space Telescope on 13 November 2005. The moon has a diameter of 443 km (275 mi), making it about half the size of Orcus and the third-largest moon of a trans-Neptunian object. Vanth is massive enough that it shifts the barycenter of the Orcus–Vanth system outside of Orcus, forming a binary system in which the two bodies revolve around the barycenter, much like the Pluto–Charon system. It is hypothesized that both systems formed similarly, most likely by a giant impact early in the Solar System's history. Compared to Orcus, Vanth has a darker and slightly redder surface that supposedly lacks exposed water ice, resembling primordial Kuiper belt objects.
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Vanth (moon) in the context of Orcus (dwarf planet)
Orcus (minor-planet designation: 90482 Orcus) is a dwarf planet 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.
Vanth (moon) in the context of Charon (moon)
Charon (/ˈkɛərɒn, -ən/ KAIR-on, -ən or /ˈʃærən/ SHARR-ən), formal designation (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).
With half the diameter and one-eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto, and the two bodies are tidally locked to each other. The dwarf planet systems Pluto–Charon and Eris–Dysnomia and the dwarf planet candidate system Salacia-Actaea are the only known examples of mutual tidal locking in the Solar System, though it is likely that Orcus–Vanth is another.
Vanth (moon) in the context of Solar System object
The following is a list of Solar System objects by orbit, ordered by increasing distance from the Sun. Most named objects in this list have a diameter of 500 km or more.
- The Sun, a spectral class G2V main-sequence star
- The inner Solar System and the terrestrial planets
- Mercury
- Venus
- Earth
- Mars
- Asteroids in the asteroid belt, between the orbits of Mars and Jupiter
- Ceres, a dwarf planet
- Vesta
- Pallas
- Hygiea
- Europa (asteroid)
- Davida
- Sylvia
- Asteroids number in the hundreds of thousands. For longer lists, see list of exceptional asteroids, list of asteroids, or list of Solar System objects by size.
- A number of smaller groups distinct from the asteroid belt
- The outer Solar System with the giant planets, their satellites, trojan asteroids and some minor planets
- Jupiter
- Saturn
- Uranus
- Rings of Uranus
- Complete list of Uranus's natural satellites
- Uranus trojan (2011 QF99)
- Uranus-crossing minor planets
- Neptune
- Non-trojan minor planets
- Trans-Neptunian objects (beyond the orbit of Neptune)
- Kuiper-belt objects (KBOs)
- Plutinos
- Orcus, a dwarf planet
- Pluto, a dwarf planet
- Complete list of Pluto's natural satellites
- Ixion
- Achlys
- Twotinos
- Cubewanos (classical objects)
- Plutinos
- Scattered-disc objects
- Detached objects
- (612911) 2004 XR190
- 2012 VP113 (possibly inner Oort cloud)
- Sedna, a dwarf planet (possibly inner Oort cloud)
- Oort cloud (hypothetical)
- Kuiper-belt objects (KBOs)
Vanth (moon) in the context of Actaea (moon)
Actaea, formal designation (120347) Salacia I, is the only known moon of the classical Kuiper belt object 120347 Salacia. Its diameter is estimated to be 393 km (244 mi), which is approximately one-half the diameter of Salacia; thus, Salacia and Actaea are viewed by William Grundy et al. to be a binary system. Assuming that the following size estimates are correct, Actaea is about the fifth-biggest known moon of a trans-Neptunian object, after Charon (1212 km), Dysnomia (615 km), Vanth (443 km), and Ilmarë (403 km).