Vanth (moon)
On the 13th of November 2005, images from the Hubble Space Telescope revealed a faint dot near the dwarf planet Orcus. Michael Brown and Terry-Ann Suer identified this dot as a moon during their survey for satellites around large trans-Neptunian objects. The Central Bureau for Astronomical Telegrams announced the discovery on the 22nd of February 2007 alongside three other TNO satellites. Brown continued observing the system between October and December 2006 to refine the orbital path. He returned to observe the pair again in November and December 2007. These observations allowed astronomers to calculate the moon's orbit with greater precision than initial detection.
Brown asked readers of his blog to suggest names for the satellite on the 23rd of March 2009. Sonya Taaffe, a fiction writer, proposed Vanth, the winged Etruscan psychopomp who guides souls to the underworld. This suggestion became the most popular name among the many submissions received by the deadline on the 5th of April. The International Astronomical Union approved the name after assessing it against standard procedures for minor planets. The Minor Planet Center published the official naming citation on the 30th of March 2010. Brown selected Vanth because its relationship to Orcus parallels the bond between Pluto and Charon in Greek mythology. Taaffe noted that if Vanth accompanies dead souls to the underworld, her face would always turn toward Orcus, mirroring the likely synchronous orbit of the moon.
Orcus and Vanth revolve around their shared barycenter every 9.54 days in nearly circular orbits. The two bodies sit approximately 6,800 kilometers apart from each other's centers. Vanth orbits farther out at an orbital radius of about 5,870 kilometers while Orcus circles closer at roughly 930 kilometers. This configuration places the system's balance point outside the body of Orcus itself. The north pole of Vanth's orbit points directly toward Earth during observations made between 2005 and 2023. This orientation makes the orbit appear face-on or pole-on from our perspective. Astronomers could not determine the actual inclination until 2015 due to this slow shifting perspective. By the year 2082, the view will shift to edge-on as the system travels along its 247-year journey around the Sun.
A single observatory in Hokkaido, Japan detected a stellar occultation on the 1st of March 2014 that lasted three seconds. This event did not provide meaningful constraints on the moon's diameter because it was a single detection of the star's chord across Vanth. Five observatories participated in observing another occultation on the 7th of March 2017 across the Americas and the Pacific Ocean. Two of these stations made positive detections while others saw no change. The data tightly constrained the possible diameters assuming a spherical shape for the object. These measurements placed an upper bound pressure of one to four microbars for any potential atmosphere. No signs of rings appeared within 6,800 kilometers from Vanth or beyond 13,600 kilometers from Orcus during the event.
Hydrodynamic simulations by Sota Arakawa and colleagues in 2019 suggested an impactor struck Orcus at an oblique angle greater than 45 degrees. This collision likely occurred about 4 billion years ago shortly after the formation of the Solar System. The impact left a large fragment in orbit that eventually became Vanth. Simulations predicted both bodies remained molten for at least 10,000 years allowing tidal interactions to circularize the orbit. Earlier calculations by Michael Brown and his team in 2010 estimated migration took between 150 million and 400 million years. Alternative theories involving gravitational capture have fallen out of favor as understanding of dwarf planet satellites improved. The similarity in density supports the giant impact hypothesis despite uncertainties in current estimates.
The most accurate estimate for Vanth's diameter is 370 kilometers determined from a stellar occultation in 2017. Thermal emission measurements by the Atacama Large Millimeter Array in 2016 previously yielded an estimate of 380 kilometers. Hubble observations from 2007 to 2008 showed the surface appears moderately red with increasing reflectivity over longer wavelengths. Near-infrared spectroscopy by the Very Large Telescope in 2010 confirmed this reddish color but did not conclusively detect water ice. ALMA observations from 2016 determined a geometric albedo of 0.08 based on thermal emission. This low value confirms a dark surface compared to Orcus which has abundant exposed water ice. Vanth lacks volatile ices like ammonia and methane because its gravity cannot prevent gases from escaping into space.
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Common questions
When was the moon Vanth discovered by astronomers?
Astronomers revealed images of Vanth on the 13th of November 2005 using the Hubble Space Telescope. Michael Brown and Terry-Ann Suer identified this faint dot as a satellite during their survey for objects around large trans-Neptunian bodies.
Who named the moon Vanth and when did the International Astronomical Union approve it?
Fiction writer Sonya Taaffe proposed the name Vanth on the 23rd of March 2009 through a blog post by Michael Brown. The International Astronomical Union approved the name after assessing standard procedures, and the Minor Planet Center published the official citation on the 30th of March 2010.
How far apart are Orcus and Vanth from each other's centers?
Orcus and Vanth sit approximately 6,800 kilometers apart from each other's centers while revolving around their shared barycenter every 9.54 days. This configuration places the system's balance point outside the body of Orcus itself with Vanth orbiting at an orbital radius of about 5,870 kilometers.
What event in 2014 provided data on the diameter of Vanth?
A single observatory in Hokkaido Japan detected a stellar occultation on the 1st of March 2014 that lasted three seconds but did not provide meaningful constraints on the diameter. Five observatories participated in observing another occultation on the 7th of March 2017 which tightly constrained possible diameters assuming a spherical shape for the object.
When did the collision that formed Vanth likely occur according to simulations?
Hydrodynamic simulations by Sota Arakawa and colleagues suggest an impactor struck Orcus at an oblique angle greater than 45 degrees about 4 billion years ago shortly after the formation of the Solar System. The collision left a large fragment in orbit that eventually became Vanth while both bodies remained molten for at least 10,000 years allowing tidal interactions to circularize the orbit.