Vanth, full designation , is the single known
natural satellite
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
of the
plutino
In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the lar ...
and likely
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
90482 Orcus
Orcus (minor-planet designation 90482 Orcus, provisional designation ) is a trans-Neptunian object, trans-Neptunian dwarf planet with a large moon, Vanth (moon), Vanth. It has a diameter of . The surface of Orcus is relatively bright with albed ...
. With a diameter of about 440 km, it is half the size of Orcus and probably the third-largest known moon of a known trans-Neptunian object, after
Pluto I Charon and
Eris I Dysnomia, though it is possible that the poorly resolved
Varda I Ilmarë or
Haumea I Hiiaka might be comparable in size. Vanth was discovered by
Michael Brown and T.-A. Suer using discovery images taken by the
Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
on 13 November 2005.
The discovery was announced in an
IAU Circular
The International Astronomical Union Circulars (IAUCs) are notices that give information about astronomical phenomena. IAUCs are issued by the International Astronomical Union's Central Bureau for Astronomical Telegrams (CBAT) at irregular interval ...
notice published on 22 February 2007.
Using observations with the Hubble Space Telescope from 13 November 2005, Michael Brown and T. A. Suer detected a natural satellite.
[ The discovery of a satellite of Orcus was reported in an IAU Circular notice published on 22 February 2007.] The satellite was given the designation S/2005 (90482) 1 before later being named ''Vanth''. It orbits Orcus in a nearly face-on circular orbit with an eccentricity
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off-center, in geometry
* Eccentricity (graph theory) of a v ...
of about 0.007, and an orbital period of 9.54 days. Vanth orbits only from Orcus and is too close to Orcus for ground-based spectroscopy to determine the surface composition of the satellite.
Brown suspects that like the Pluto–Charon system, which are similar in their relative sizes, Orcus and Vanth are tidally locked
Tidal locking between a pair of co-orbiting astronomical body, astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where ...
. Vanth does not resemble known collisional satellites because its spectrum is very different from that of its primary, and it may be a captured Kuiper belt object. Vanth could also have originated as a result of rotational fission of the primordial Orcus, which would have rotated much faster than now.
Name
Upon discovery, Vanth was issued a provisional designation, . On 23 March 2009, Brown asked readers of his weekly column to suggest possible names for the satellite, with the best one to be submitted to the International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU) on 5 April. The name Vanth
Vanth is a chthonic figure in Etruscan mythology shown in a variety of forms of funerary art, such as in tomb paintings and on sarcophagi.
Vanth is a female demon in the Etruscan underworld that is often accompanied either by additional Vanth fi ...
, the winged Etruscan __NOTOC__
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** Etrusca ...
psychopomp
Psychopomps (from the Greek word , , literally meaning the 'guide of souls') are supernatural creatures, spirits, entities, angels, demons or deities in many religions whose responsibility is to escort newly deceased souls from Earth to the afte ...
who guides the souls of the dead to the underworld, was chosen from among a large pool of submissions. Vanth was the only suggestion that was purely Etruscan in origin. It was the most popular submission, first suggested by Sonya Taaffe
Sonya Taaffe is an American author of short fiction and poetry based out of Massachusetts. She grew up in Arlington and Lexington, Massachusetts and graduated from Brandeis University in 2003 where she received a B.A. and M.A. in Classical Studi ...
. This submission was assessed and approved by the IAU's Committee for Small Body Nomenclature, in accordance with the naming procedures for minor planets and satellites. The official naming citation was announced in a Minor Planet Circular notice published on 30 March 2010.
The Etruscan Vanth is frequently portrayed in the company of Charun
In Etruscan mythology, Charun (also spelled Charu, or Karun) acted as one of the psychopompoi of the underworld (not to be confused with the god of the underworld, known to the Etruscans as Aita). He is often portrayed with Vanth, a winged figu ...
(Charon), and so as the name of the moon of Orcus (nicknamed the "anti-Pluto" because resonance with Neptune keeps it on the opposite side of the Sun from Pluto), it is an allusion to the parallels between Orcus and . Brown quoted Taaffe as saying that if Vanth "accompanies dead souls from the moment of death to the underworld itself, then of course her face is turned always toward Orcus", a reference to the likely synchronous orbit of Vanth about Orcus.
Characteristics
Vanth was found at 0.25 arcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
s from Orcus with magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
difference of . Estimates made in 2009 by Brown show that the apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of Vanth is which is magnitudes fainter than Orcus. Assuming equal albedos this would mean a diameter of , or 2.9 times smaller than the primary. However, the dissimilar colors of Orcus (neutral) and Vanth (red
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
) suggest that Vanth could have an albedo a factor of two lower than Orcus. Should Vanth have an albedo of only 0.12, Vanth could be as large as with Orcus being in diameter. The mass of Vanth also depends on its albedo and can vary from 3 to 9 percent of the total system mass. In 2016, Brown and Butler used resolved ALMA
Alma or ALMA may refer to:
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* ''Alma'' (film), a 2009 Spanish short animated film
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* ''Alma'' (Le Clézio novel), 2017
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images and estimated Vanth's diameter at , about half Orcus's diameter, making Orcus and Vanth a smaller analog of Pluto and Charon. They also measured Vanth's albedo as only , almost 3 times darker than Orcus.
For 7 March 2017, a stellar occultation
Stellar means anything related to one or more stars (''stella''). The term may also refer to:
Arts, entertainment, and media
* ''Stellar'' (magazine), an Irish lifestyle and fashion magazine
* Stellar Loussier, a character from ''Mobile Suit Gu ...
by Orcus had been predicted to take place in the Americas and over the Pacific Ocean. Observations were made at five sites in North and South America, and two solid body chords were observed, with the more precise one being . Using speckle imaging, the occulted star was revealed to be a close double star
In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.
This occurs because the pair either forms a binary star (i.e. a bi ...
, and a reconstruction of the orbits of Orcus and Vanth showed that both chords were from Vanth (occulting either of the two stars) rather than Orcus. A non-detection at a nearby site placed an upper limit of () on the diameter of Vanth. The occultation data is consistent with a pressure of at most 4 microbar
The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea leve ...
for a global atmosphere of Vanth.
Vanth does not resemble other known collisional satellites because its spectrum is very different from that of its primary. Vanth may thus be a captured Kuiper belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
object. In that respect, it may be similar to the satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
of another possible dwarf planet, Gonggong
Gonggong () is a Chinese water god who is depicted in Chinese mythology and folktales as having a copper human head with an iron forehead, red hair, and the body of a serpent, or sometimes the head and torso are human, with the tail of a serpe ...
, which also has a very different colour compared to its primary.
Notes
References
{{Trans-Neptunian objects
90482 Orcus
Trans-Neptunian satellites
Astronomical objects discovered in 2005
Discoveries by Michael E. Brown
Objects observed by stellar occultation