Page Sections

What Is Known
Its Moon, Dysnomia
Name Mythology


Sublimate: Convert directly from a solid to a gas without going through a liquid phase. Ice often does this in dry environments when exposed to enough light.

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Eris (pronounced "E-ris" with a long, hard "e" as in "eat") has a formal designation of 136199 Eris, and it has one known moon, Dysnomia. Eris was one of the originally classified dwarf planets by the International Astronomical Union in 2006.

Little is known about Eris because it is so far away, spending most of its time beyond Pluto, with its highly eccentric orbit taking it up to over three times as far from the sun as Pluto. Because its average distance from the sun is past Neptune, it is known as a "trans-Neptunian object," which makes it a sub-class of dwarf planets known as "plutiods."

It has a highly eccentric orbit and it is both the largest and most massive dwarf planet known. It has been imaged by the Hubble Space Telescope, as shown in the below.

Eris and Dysnomia, Imaged by the Hubble Space Telescope
Photograph from the Hubble Space Telescope (left). It has been annotated (right) to show Eris and its moon, as well as the orbit of the moon. Figure is from the Hubble Site.


Eris was originally discovered on January 5, 2005, by a team led by Mike Brown. The original photographs were taken in 2003, but the object was not identified until 2 years later. It was given the provisional name of 2003 UB313. Subsequent observations revealed a moon in October 2005, which was named Dysnomia two years later.

Once the moon had been discovered, Kepler's Third Law could be used to determine the mass of Eris, which was done in June of 2007. However, before that, astronomers could estimate the diameter of Eris based upon its brightness. The diameter estimates were larger than Pluto, which intensified the controversy surrounding whether or not to name these large Kuiper Belt objects "planets" or to de-classify Pluto as a planet.

In August 2006, the International Astronomical Union voted on the definition of a "planet" as well as creating a new category of "dwarf planet," into which they placed Eris, making Eris the first modern dwarf planet (with Ceres and Pluto having been known for decades).

In mid-2008, it was placed into the newly created sub-class of dwarf planet known as a "plutoid" because its average orbital distance is past Neptune, making it Pluto-like.

What Is Known

The absolute magnitude is -1.15, and the dwarf planet is slightly red in color, with a V-R=0.41 magnitude (Belskaya et al., 2008).

The day of Eris is roughly believed to be about 1.08 Earth days (25.9 hrs). This is calculated by very carefully measuring the brightness of Eris and looking for a pattern of lightness and darkness that repeats. It is assumed that each repetition represents 1 rotation period (Roe, Pike, & Brown, 2008).

Spectroscopic observations have shown that methane is present on the surface of Eris, much like on Pluto and on Neptune's largest moon, Triton. Methane being present on Eris' surface is interesting because it implies that the surface has not been heated since the methane was deposited; if it had heated, the methane would have sublimated and then escaped. This means that either Eris has always been far from the sun, or it has some internal source of methane that replenishes its surface (Gemini Observatory Press Release, 2007).

Other spectroscopic work has refined this estimate such that about 50% of the surface appears to be covered with pure methane ice. The rest of the surface is thought to be a mixture of methane, nitrogen, and water ices. The grain size of the ice is relatively large, modeled to be on the millimeter-sized scale (Dumas et al., 2007).

Its Moon, Dysnomia

On October 2, 2005, it was reported that a moon had been discovered around Eris. Very little is known about it other than the following properties:

Name Mythology

The popular name of the dwarf planet, "Xena," was named for the New Zealand cult television show Xena: Warrior Princess that starred Lucy Lawless as the title character. Fittingly, the moon was given the popular name of Xena's best friend, Gabrielle, played by Renée O'Connor. Although the names were completely unofficial and new names (Eris and Dysnomia) were given by the IAU, "Xena" and "Gabrielle" still remain popular.

Eris is named after the Greek goddess of strife, the Roman version of Discord (or "Discordia" in Latin). Unlike an apparent English pronunciation, the proper way to say the name is "E-ris" with a hard "e" sound as in the word "eat." Although some have accused Dr. Brown for suggesting the name as a protest against the United State's wars in Iraq and Afghanistan, he has stated that it was proposed due to the strife it created within the astronomical community and public at large because it led to the demotion of Pluto as a planet.

Dysnomia is the demon of lawlessness in Greek mythology, a daughter of Eris. The name works on three levels, the first being the daughter (moon) of Eris. The second is that it is an unofficial tribute to the name "Xena" that its primary held for a brief time, since the character was played by Lucy Lawless. Third, dysnomia is a neurological disorder where there is difficulty in retrieving the correct word from memory, affecting speech and/or writing ability. This reflects the IAU's struggle to determine a classification for what are now known as "dwarf planets."


The following table lists data for Pluto and the other dwarf planets for comparison purposes. Data is compiled from NASA's planetary factsheet for Pluto, and from primary sources for the other dwarf planets; see the individual pages on the dwarf planets for references.

Ceres Pluto Haumea Makemake Eris
Perihelion (109 km) 0.381 4.436 5.260 5.761 5.65
Semi-Major Axis (106 km) 0.415 5.906 6.484 6.850 10.12
Aphelion (106 km) 0.449 7.376 7.708 7.940 14.60
Average Orbital Velocity (km/s) 10.587° 17.16° 4.484 4.419 3.436
Orbital Inclination (from Earth's Orbit) 10.587° 17.16° 28.19° 28.96° 44.187°
Orbital Eccentricity 0.080 0.2488 0.18874 0.159 0.44177
Equatorial Radius (km) 975 1195 ~996 x 1518 x 1960 ~2000 1300+200−100
Polar Radius (km) 909 1195 ~2000 1300+200−100
Mass (1021 kg) 0.95 12.5 4.2±0.1 ~40 16.7±0.2
Density (water=1 g/cm3) 2.08 1.75 2.6-3.3 ~2 2.3±0.3
Sidereal Rotation Period (hours) 9.074 153.2820 3.915 ? ~25.9
Sidereal Orbital Period (days) 1679.819 90,588 104,234 113,183 203,600
Apparent Magnitude +6.7 to +9.3 +15.1 +16.7 +17.3 +18.7
Absolute Magnitude +3.36±0.02 -0.7 -0.48 +0.17 -1.15
Number of Moons 0 3 2 0 1
Discoverer G. Piazzi C. Tombaugh M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz
Discovery Date Jan. 1, 1801 Feb. 18, 1930 Dec. 28, 2004 Mar. 31, 2005 Oct. 21, 2003


Belskaya, I. et al. (2008) "Polarimetry of the Dwarf Planet (136199) Eris." Astronomy & Astrophysics, 479, p. 265-269.

Brown, M.E. (2005). "S/2005 (2003 UB313) 1." Central Bureau Electronic Telegrams, 240.

Brown, M.E., and E.L. Schaller. (2007). "The Mass of Dwarf Planet Eris." Science, 316, p. 1585.

Dumans, C., et al., (2007). "Surface Composition of the Largest Dwarf Planet 136199 Eris (2003 UB313)." Astronomy & Astrophysics, 471, p. 331-334.

Gemini Observatory Press Release. (2007). "Gemini Observatory Shows that '10th Planet' Has a Pluto-Like Surface."

Roe, H.G., Pike, R.E., and M.E. Brown. (2008). "Tentative Detection of the Rotation of Eris." Icarus, accepted.