Orcus (dwarf planet)

Orcus, also known by its minor-planet designation 90482 Orcus, is a dwarf planet found in the Kuiper belt, a region beyond the orbit of Neptune. It has one large moon called Vanth. Orcus is about the same size as Ceres, another dwarf planet closer to the Sun.

The surface of Orcus is bright and appears neutral in color. It is made up of water ice and might have other compounds like methane or ammonia on its surface.

Orcus was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on February 17, 2004. It is a plutino, which means it is a trans-Neptunian object that follows a special path around the Sun related to Neptune. Orcus has been called the "anti-Pluto" because of its similarities and differences with Pluto, and because of its large moon Vanth. This idea influenced the choice of its name, as Orcus was the Roman and Etruscan equivalent of the Roman and Greek god Pluto.

History

Discovery

Orcus was discovered on February 17, 2004, by American astronomers Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Earlier pictures from November 8, 1951, taken by the Palomar Observatory, were found later.

Name and symbol

The minor planet Orcus was named after Orcus, a Roman god of the underworld. The name was chosen because objects similar in size and orbit to Pluto are named after underworld gods. Orcus's moon, Vanth, was named after Vanth, an Etruscan figure who guided people to the underworld.

The use of planetary symbols is not common today. However, a symbol ⟨.svg)⟩ is included in Unicode and is mostly used by astrologers. It was designed to look like both a skull and an orca's mouth. There is also a rarer symbol .svg), an upside-down Pluto symbol.

Orbit and rotation

Orcus travels around the Sun in a special pattern with Neptune. For every two times Orcus goes around the Sun, Neptune goes around three times. Orcus takes about 245 years to complete one orbit. Its path is tilted about 20.6° compared to the plane where most planets orbit, called the ecliptic. Orcus last reached its farthest point from the Sun in 2019 and will come closest to the Sun around January 10, 2143.

We are not exactly sure how long it takes Orcus to spin once on its axis. Some measurements show it might spin every 7 to 21 hours, while others show almost no change in brightness. It is thought that Orcus's spin axis might line up with the orbit of its moon, Vanth. One study suggests Orcus might spin once every about 10.5 hours.

Physical characteristics

The dwarf planet Orcus is in a far part of space called the Kuiper belt. It has one large moon named Vanth. Scientists think Orcus is about as wide as a big city.

Orcus's surface reflects sunlight and looks grayish. It has water ice and some darker materials on its surface.

Satellite

Main article: Vanth (moon)

Orcus has one moon named Vanth. It was discovered in 2005 using the Hubble Space Telescope and announced in 2007. Vanth is large, about 475 kilometers across, and is one of the biggest moons of a dwarf planet. It helps make Orcus sometimes called the "anti-Pluto."