Eris (Dwarf Planet/Plutoid)

Eris, [1] named after the Greek goddess of strife and discord, is a late addition to the solar family. It was discovered by M. E. Brown, C. A. Trujillo and D. L. Rabinowitz on January 5, 2005, from images taken on October 21, 2003, and officially designated as a "dwarf planet" [2] by the International Astronomical Union on August 24, 2006, and as a "plutoid" [3] on June 11, 2008. [4]

A highly eccentric orbit places Eris as close as 37.9 AU from the Sun at perihelion and as far as 97.65 AU from the Sun at aphelion (in conventional terms 5.65 to 14.6 billion kilometers). Earth by comparison orbits at an average distance of 1 AU or approximately 150 million kilometers.

With a diameter of approximately 2,326 kilometers Eris is the second largest, after Pluto, of the dwarf planets (though 27% more massive). This latest and most accurate determination of its size is the result of measurements taken by French astronomers during a stellar occultation in November 2010.

Spectroscopic analysis of the dwarf planet seems to indicate the presence of methane ice and it's possible that at its closest approach to the Sun the methane sublimates and forms a tenuous atmosphere. Surface temperatures are estimated to vary between -217.2 and -243.2 degrees Celsius.

To date Eris has one known moon: Dysnomia.

[1] "Xena," inspired by the lead character in the television show Xena: Warrior Princess, was an early moniker for Eris although Persephone the wife of Pluto was also considered.

[2] A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape), (c) has not cleared the neighborhood around its orbit and (d) is not a satellite.

[3] A "plutoid" is a trans-Neptunian dwarf planet.

Due to its extreme elliptical orbit, it's a split decision, Eris is referred to as a KBO (Kuiper Belt object) by NASA and an SDO (scattered disk object) [5] by the IAU. Which is correct? An interesting head-scratcher for the experts. My view on the subject: although often referred to as a KBO it's technically an SDO.

[5] The scattered disk is an area of rock-ice objects with highly erratic trajectories (steeply inclined to the ecliptic plane) orbiting beyond but approaching Neptune at perihelion (30 AU to as much as 150 AU at aphelion) and therefore subject to continued perturbation by that giant's gravity. As is obvious there is overlap with the Kuiper belt [6] and in fact many astronomers consider the scattered disk to be not separate but rather an extension of that more densely populated inner region.

[6] The Kuiper belt is a region of the outer Solar System populated by billions of rock-ice objects. It reaches from the orbit of Neptune (30 AU) outward to approximately 55 AU.

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