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 and is estimated to be between 870 to 960 kilometers (540 to 600 miles) in diameter, which is about the same size as Ceres, another dwarf planet closer to the Sun. The surface of Orcus is bright, with a reflectivity called albedo reaching 23 percent, and it appears neutral in color. It is rich in water ice, which is mostly in a crystalline form, possibly due to past cryovolcanic activity. Other compounds like methane or ammonia might also be present 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 locked in a 2:3 orbital resonance with the ice giant Neptune. This means Orcus makes two revolutions around the Sun for every three of Neptune's, similar to Pluto. However, Orcus's orbit is positioned opposite to Pluto's. When Orcus is farthest from the Sun, Pluto is closest, and vice versa. Because of these similarities and contrasts with Pluto, and because of its large moon Vanth, which can be compared to Pluto's large moon Charon, Orcus has been called the "anti-Pluto." 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 images 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 no longer common in astronomy. 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, reflecting Orcus as the opposite of Pluto.

Orbit and rotation

Orcus travels around the Sun in a special pattern with Neptune, called a 2:3 orbital resonance. This means that 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. Even though Orcus sometimes gets close to Neptune's path, they never get too near each other because of this resonance. 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, which could explain why we see little change in brightness from Orcus. One study suggests Orcus might spin once every about 10.5 hours, but if it is locked in position with Vanth, its spin would match Vanth's 9.7-day orbit around it.

Physical characteristics

The dwarf planet Orcus is found in a distant area of space called the Kuiper belt. It has one big moon named Vanth. Scientists think Orcus is about as wide as the distance across a big city, maybe a little bigger or smaller than that.

Orcus's surface reflects quite a bit of sunlight and looks a neutral grayish color. It is known to have water ice on its surface along with some darker materials. The presence of fresh, crystalline water ice suggests that some process may have renewed its surface in the past.

Satellite

Main article: Vanth (moon)

Orcus has one known moon named Vanth. It was found in 2005 using the Hubble Space Telescope and announced in 2007. Vanth is quite large, about 475 kilometers across, which makes it similar in size to some other big moons. Like the moon Charon to the dwarf planet Pluto, Vanth is a big part of why Orcus is sometimes called the "anti-Pluto." It is the third-largest known moon of a dwarf planet.