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Eris and Dysnomia.jpg
Date of discovery September 10, 2005
Name of discoverer Michael E. Brown[1][2]
Name origin Greek goddess of lawlessness, daughter of Eris.[1]
Orbital characteristics
Primary Eris
Order from primary 1
Perieridon 36,864 km[3]
Apoeridon 37,836 km[3]
Semi-major axis 37,350 km[4]
Orbital eccentricity 0.013
Sidereal month 15.774 da[4]
Inclination 142.3° to Eris's equator
Physical characteristics
Equatorial radius 175 km
Dysnomia, or Eris I, or S/2005 (2003 UB313) 1, is the only (so far) known moon of the dwarf planet Eris.


Astronomers Michael E. Brown at the California Institute of Technology, and Marcos van Dam, Antonin Bouchez, and David Le Mignant of the William M. Keck Observatory in Hawaii, are credited with the discovery of this object. The Keck team used their new Laser Guide Star Adaptive Optics system to examine more closely the newly discovered Kuiper belt object originally named 2003 UB313 and then given the tentative name Xena.[5] (Xena was the title character in an American television action-adventure dramatic series,[6] and also is a name beginning with X, in keeping with the suspicion that this object was the long-sought "Planet X.") Close examination of a faint object near the larger object clearly showed that this object was in motion and was not a star.[7][8]


The initial name chosen for this object was Gabrielle, after the supporting role in the above-mentioned dramatic series. However, the status of object 2003 UB313 (the primary) was the subject of debate, given that other observations[9][10] had already suggested that 2003 UB313 might be more massive even than Pluto, then considered the ninth planet in the solar system. Because planets and Kuiper-Belt objects have different naming conventions, and because scatter disk objects had no naming convention at the time, the names for these two objects remained unofficial.[11]

Finally the International Astronomical Union declared[12] that Pluto and 2003 UB313 were not planets, but belonged to a new category called dwarf planets. Then on September 6, 2006, Mike Brown and his team[13] recognized that the name "Xena" was inappropriate for 2003 UB313 and suggested to the IAU that they name it Eris, after the Greek goddess of discord and strife. They also suggested that the IAU name the satellite Dysnomia, for the Greek goddess of lawlessness and daughter of Eris.[14] Four days later, the IAU officially named the primary Eris and named its satellite Dysnomia.[15]

Artist's conception of Eris and Dysnomia. Dysnomia lies slightly above and to the left of Eris. The bright object in the upper left of the image is the Sun.

Orbital characteristics

Dysnomia orbits Eris in a nearly circular orbit with a sidereal period of 15.774 days and a semi-major axis of 37,350 km. From these properties, Brown and Schaller were able to calculate Eris' mass.[4]

Implications for Other Trans-Neptunian Objects

Brown et al. noted in 2005[2] that Eris, Dysnomia's primary, is one of three of the four brightest Kuiper belt objects that have satellites. (The other two are Pluto and 2003 EL61.) The fourth, 2005 FY9, has no satellite that Earth-based telescopes can presently detect. Most Kuiper belt objects do not have satellites, and that three of the four brightest should have satellites suggests that their origins were significantly different from those of other Kuiper belt objects.

Problem for uniformitarian theories

Brown states[4] that the near-circular orbit of Dysnomia about Eris actually is consistent with Dysnomia's origin as the result of a collision between Eris and another object. But no astronomer has ever explained how such a collision would leave an object in a nearly circular orbit about its primary.

Observation and exploration

The Hubble Space Telescope and the Keck Observatory are the first two telescopes to observe the Eridian system. No deep-space missions are planned.


  1. 1.0 1.1 "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.
  2. 2.0 2.1 Brown, M.E., Van Dam, M. A., Bouchez, A. H., Le Mignant, D., et al. "Satellites of the Largest Kuiper Belt Objects." Astrophys. J. Lett. 639(L43), October 3, 2005. <arXiv:astro-ph/0510029.> Accessed May 15, 2008.
  3. 3.0 3.1 Calculated
  4. 4.0 4.1 4.2 4.3 Brown, Michael E., and Schaller, Emily L. "The Mass of Dwarf Planet Eris." Science, 316(5831):1585, June 15, 2007. <doi:10.1126/science.1139415> Accessed May 15, 2008.
  5. Zabarenko, Deborah. "Planet Xena has moon called Gabrielle." Reuters, quoted by Australian Broadcasting Corporation, October 3, 2005. Accessed May 15, 2008.
  6. The character is supposed to be a princess from a tribe of warriors, either Argive or closely allied with them, in the days of the Mycenean civilization, and a contemporary of, and occasional rival to, Hercules. No historical warrant exists for the existence of such a person or even for a classical poem mentioning that name.
  7. Van Dam, Marcos. Discovery of Dysnomia. William Keck Observatory. Accessed January 22, 2008.
  8. The moon of the 10th planet. California Institute of Technology. Accessed January 22, 2008.
  9. Ingham, Richard. "'Tenth planet' Xena bigger than Pluto." Agence France-Presse, quoted by Australian Broadcasting Corporation, February 2, 2006. Accessed May 15, 2008.
  10. Eris (2003 UB313) and Dysnomia. Accessed January 22, 2008.
  11. Tytell, David. "All Hail Eris and Dysnomia." Sky and Telescope, September 14, 2006. Accessed May 15, 2008.
  12. "IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting," International Astronomical Union, 2005. Accessed January 14, 2008.
  13. Brown, Mike. "The discovery of Eris, the largest known dwarf planet." California Institute of Technology. Accessed January 22, 2008.
  14. Dysnomia, meaning "lawlessness," is also a play on the name of the American actress who portrayed Xena, Lucy Lawless.
  15. IAU Circular No. 8747, International Astronomical Union, September 10, 2006. Accessed May 15, 2008.