Eris (dwarf planet)

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Eris (centre) and Dysnomia (left of centre).
Hubble Space Telescope
Discovered by
Discovery date January 5, 2005[2][a]
MPC designation 136199 Eris
Pronunciation /ˈɪərɨs/ or /ˈɛrɨs/[b]
Named after
2003 UB313[3]
Minor planet category
Adjectives Eridian
Orbital characteristics[3]
Epoch 2013-Apr-18
(JD 2456400.5)
  • 97.661 AU
  • 14.602×109 km
  • 38.255 AU
  • 5.723×109 km
  • 67.958 AU
  • 10.166×109 km
Eccentricity 0.437083
  • 204624 d
  • 560.23 years
3.4338 km/s
Inclination 43.8853°
Known satellites Dysnomia
Physical characteristics
Mean radius
1163±6 km[8][9]
17000000 km2
Mean density
2.52±0.05 g/cm3[8][11]
0.827 m/s2
0.084 g[12]
1.384 km/s
25.9±8 hr[3]
Albedo 0.96+0.09
Surface temp. min mean max
(approx) 30 K 42.5 K 55 K
Spectral type
B−V=0.78, V−R=0.45[13]
40 milli-arcsec[15]

Eris (minor-planet designation 136199 Eris) is the most massive known dwarf planet in the Solar System and the ninth most massive body known to directly orbit the Sun.[c][d] It is estimated to be 2,326 (±12) km in diameter,[9] and 27% more massive than Pluto, or about 0.27% of the Earth's mass.[10][17]

Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. It is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known as the scattered disc. It has one known moon, Dysnomia. As of 2014, its distance from the Sun is 96.4 AU,[14] roughly three times that of Pluto. With the exception of some comets, Eris and Dysnomia are currently the most distant known natural objects in the Solar System.[2][e]

Because Eris appeared to be larger than Pluto, its discoverers[20] and NASA initially described it as the Solar System's tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under the IAU definition approved on August 24, 2006, Eris is a "dwarf planet", along with objects such as Pluto, Ceres, Haumea and Makemake.[21] However, observations of a stellar occultation by Eris in 2010 showed that its diameter was only 2326±12 km, not significantly different from the size of Pluto.[22] Given the uncertainties in their size estimates, there is a reasonable chance that Eris is actually smaller than Pluto.[16][f] However, Pluto's atmosphere makes determining its diameter difficult, and until the New Horizons probe arrives at Pluto the diameter is expected to remain unknown. Because of that, many scientists believe that the knowledge of whether Pluto or Eris is bigger will be unknown until then.[23]


Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz[2] on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea,[24] due in part to events that would later lead to controversy about the latter. The search team had been systematically scanning for large outer Solar System bodies for several years, and had been involved in the discovery of several other large TNOs, including 50000 Quaoar, 90482 Orcus, and 90377 Sedna.


Distribution of trans-Neptunian objects


More observations released in October 2005 revealed that Eris had a moon, later named Dysnomia. Observations of Dysnomia's orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be (1.66±0.02)×1022 kg, 27% greater than Pluto's.


Eris is a plutoid, that is, a trans-Neptunian dwarf planet.[25] Its orbital characteristics more specifically categorize it a scattered-disk object (SDO), or a TNO that is believed to have been "scattered" from the Kuiper belt into more distant and unusual orbits following gravitational interactions with Neptune as the Solar System was forming. Although its high orbital inclination is unusual among the known SDOs, theoretical models suggest that objects that were originally near the inner edge of the Kuiper belt were scattered into orbits with higher inclinations than objects from the outer belt.[26] Inner-belt objects are expected to be generally more massive than outer-belt objects, and so astronomers expect to discover more large objects like Eris in high-inclination orbits, which have traditionally been neglected.

Because Eris may be larger than Pluto, it was initially described as the "tenth planet" by NASA and in media reports of its discovery.[27] In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the term planet to decide the issue. This was announced as the IAU's Definition of a Planet in the Solar System, adopted on August 24, 2006. At this time, both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of planet.[28] Brown has since stated his approval of this classification.[29] The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.[30]


Athenian painting of Eris, circa 550 BC

Eris is named after the goddess Eris (Greek Ἔρις), a personification of strife and discord.[31] The name was assigned on September 13, 2006, following an unusually long period in which the object was known by the provisional designation 2003 UB313, which was granted automatically by the IAU under their naming protocols for minor planets. The regular adjectival form of Eris is Eridian.


Due to uncertainty over whether the object would be classified as a planet or a minor planet, because different nomenclature procedures apply to these different classes of objects,[32] the decision on what to name the object had to wait until after the August 24, 2006, IAU ruling.[30] As a result, for a time the object became known to the wider public as Xena.


We chose it since it started with an X (planet "X"), it sounds mythological (OK, so it's TV mythology, but Pluto is named after a cartoon, right?),[g] and (this part is actually true) we've been working to get more female deities out there (i.e. Sedna). Also, at the time, the TV show was still on TV, which shows you how long we've been searching![34]

"We assumed [that] a real name would come out fairly quickly, [but] the process got stalled," Mike Brown said in interview,


Choosing an official name[edit]



Brown had also speculated that Persephone, the wife of the god Pluto, would be a good name for the object.[2] The name had been used several times in science fiction,[38] and was popular with the public, having handily won a poll conducted by New Scientist magazine ("Xena", despite only being a nickname, came fourth).[39] However, this was not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399 Persephone.[2] Because IAU regulations require a name from creation mythology for objects with orbital stability beyond Neptune's orbit, the team had also been considering such possibilities.[40]



The orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and Pluto (white/grey). The arcs below the ecliptic are plotted in darker colours, and the red dot is the Sun. The diagram on the left is a polar view whereas the diagrams on the right are different views from the ecliptic.

Eris has an orbital period of 557 years, and as of 2011 lies at 96.6 AUs from the Sun,[14] almost its maximum possible distance (its aphelion is 97.5 AU). It came to perihelion between 1698[5] and 1699,[42] to aphelion around 1977,[42] and will return to perihelion around 2256[42] to 2258.[43] Eris and its moon are currently the most distant known objects in the Solar System apart from long-period comets and space probes.[44] However, approximately forty known TNOs, most notably 2006 SQ372, 2000 OO67 and Sedna, though currently closer to the Sun than Eris, have greater average orbital distances than Eris' semimajor axis of 67.7 AU.[4]


Eris currently has an apparent magnitude of 18.7, making it bright enough to be detectable to some amateur telescopes. A 200 mm telescope with a CCD can detect Eris under favorable conditions.[h] The reason it had not been noticed until now is its steep orbital inclination; most searches for large outer Solar System objects concentrate on the ecliptic plane, where most bodies are found.

Eris is now in the constellation Cetus. It was in Sculptor from 1876 until 1929 and Phoenix from roughly 1840 until 1875. In 2036 it will enter Pisces and stay there until 2065, when it will enter Aries.[42] It will then move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173. Also, because of the high inclination of its orbit, Eris only passes through a few constellations of the traditional Zodiac.

Size, mass and density[edit]

Size estimates
Year Radius (diameter) Source
2005 1,199 (2,397) km[46] Hubble
2007 1,300 (2,600) km[47] Spitzer
2011 1,163 (2,326) km[8] Occultation

In 2005, the diameter of Eris was measured to be 2397±100 km, using images from the Hubble Space Telescope (HST).[46][48] The size of an object is determined from its absolute magnitude (H) and the albedo (the amount of light it reflects). At a distance of 97 AU, an object with a diameter of 3,000 km would have an angular size of 40 milliarcseconds,[15] which is directly measurable with the Hubble Space Telescope. Although resolving such small objects is at the very limit of the telescope's capabilities,[i] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.[j]

This makes Eris around the same size as Pluto, which is 2368±20 km across. It also indicates an albedo of 0.96, higher than that of any other large body in the Solar System except Enceladus.[8] It is speculated that the high albedo is due to the surface ices being replenished because of temperature fluctuations because Eris's eccentric orbit takes it closer and farther from the Sun.[50]

In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave an estimate of Eris' diameter of 2600+400
.[47] The Spitzer and Hubble estimates overlap in the range of 2,400–2,500 km, 4–8% larger than Pluto. However, astronomers now suspect that Eris' spin axis is pointing toward the sun, at the moment—a possibility that would keep the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values.[9] So the outcome from the 2010 Chile occultation is actually more in line with the Hubble result from 2005.[9]

In November 2010, Eris was the subject of one of the most distant stellar occultations yet achieved from Earth.[9] Preliminary data from this event cast doubt on previous size estimates.[9] The teams announced their final results from the occultation in October 2011, with an estimated diameter of 2326+12
.[8] However, when using data from this event for comparison to Pluto, there is a range of figures available for Pluto's radius/diameter that can be selected.[51] This is due in part to Pluto's atmosphere, which interferes with making measurements of its solid surface (as opposed to gaseous haze).[51] The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period—15.774 days—[10][52] Eris is 27 percent more massive than Pluto. If the 2011 occultation results are used, then Eris has a density of 2.52±0.05 g/cm, substantially denser than Pluto, and thus must be composed largely of rocky materials.[8]

Models of internal heating via radioactive decay suggest that Eris may be capable of sustaining an internal ocean of liquid water at the mantle-core boundary.[53]

Surface and atmosphere[edit]

Artist's impression of Eris and Dysnomia. Eris is the main object, Dysnomia the small grey sphere just above it. The flaring object top-left is the Sun.



Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey.[2] Pluto's reddish color is believed to be due to deposits of tholins on its surface, and where these deposits darken the surface, the lower albedo leads to higher temperatures and the evaporation of methane deposits. In contrast, Eris is far enough away from the Sun that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly over the surface reduces any albedo contrasts and would cover up any deposits of red tholins.[55]



Main article: Dysnomia (moon)

In 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive optics system.[57] Images taken on September 10 revealed a moon in orbit around Eris. In keeping with the "Xena" nickname already in use for Eris, Brown's team nicknamed the moon "Gabrielle", after the television warrior princess' sidekick. When Eris received its official name from the IAU, the moon received the name Dysnomia, after the Greek goddess of lawlessness who was Eris' daughter. Brown says he picked it for similarity to his wife's name, Diane. The name also retains an oblique reference to Eris' old informal name Xena, portrayed on TV by Lucy Lawless.[58]

See also[edit]


  1. ^ Images were taken on October 21, 2003, however the object was not detected and identified until 2005.
  3. ^ It is only the 16th most massive known body in the Solar System, because seven moons are more massive than any known dwarf planet.
  4. ^ According to most recent estimates, Eris is ~1% larger[8] and 25% more massive than Pluto.[10] Given the error bars in the different diameter estimates, it is currently unknown whether Eris or Pluto has the larger diameter.[16] Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.[16]
  5. ^ As of 2013, Sedna is 86.4 AU from the Sun,[18] whereas Eris is 96.5 AU from the Sun.[14] Eris is still close to its 1977 aphelion (furthest distance from the Sun), whereas Sedna is nearing its 2076 perihelion (closest approach to the Sun).[19] Sedna will overtake Eris as the farthest presently known spherical minor planet in 2114.[19]
  6. ^ For r(Eris) = 1163±6 and r(Pluto) = 1153±10, and assuming normal distributions, the chance that Eris is smaller than Pluto is \Phi\left(\frac{r_P-r_E}{\sqrt{\sigma_P^2 + \sigma_E^2}}\right) = \frac12\left[1 + \operatorname{erf}\left(\frac{r_P-r_E}{\sqrt{\sigma_P^2 + \sigma_E^2}\sqrt{2}}\right)\right], where Φ is the cumulative distribution function and erf is the error function. Erf(−0.6064) = −0.6088, and Φ = 0.1956, or an approximately 20% chance.


  1. ^ Staff (2007-05-01). "Discovery Circumstances: Numbered Minor Planets". IAU: Minor Planet Center. Retrieved 2007-05-05. 
  2. ^ a b c d e f g h Brown, Mike (2006). "The discovery of 2003 UB313 Eris, the largest known dwarf planet". Retrieved 2007-05-03. 
  3. ^ a b c d "JPL Small-Body Database Browser: 136199 Eris (2003 UB313)". 2009-11-20 last obs. Retrieved 2012-08-08.  Archived from the original on 2012-08-08
  4. ^ a b "List Of Centaurs and Scattered-Disk Objects". Minor Planet Center. Retrieved 2008-09-10. 
  5. ^ a b Buie, Marc (2007-11-06). "Orbit Fit and Astrometric record for 136199". Deep Ecliptic Survey. Retrieved 2007-12-08. 
  6. ^ "Define Eris". Random House. Retrieved 2012-10-04. 
  8. ^ a b c d e f g h Sicardy, B.; Ortiz, J. L.; Assafin, M.; Jehin, E.; Maury, A.; Lellouch, E.; Gil-Hutton, R.; Braga-Ribas, F.; Colas, F.; Widemann (2011). "Size, density, albedo and atmosphere limit of dwarf planet Eris from a stellar occultation". European Planetary Science Congress Abstracts 6: 137. Bibcode:2011epsc.conf..137S. Retrieved 2011-09-14. 
  9. ^ a b c d e f Beatty, Kelly (November 2010). "Former 'tenth planet' may be smaller than Pluto". Sky and Telescope. Retrieved 2011-10-17. 
  11. ^ Beatty, Kelly (2010-11-07). "Eris Gets Dwarfed (Is Pluto Bigger?)". Sky & Telescope (News Blog). Retrieved 2011-10-17. 
  12. ^ Calculated based on the known parameters
  13. ^ Snodgrass, C.; Carry, B.; Dumas, C.; Hainaut, O. (February 2010). "Characterisation of candidate members of (136108) Haumea's family". Astronomy and Astrophysics 511: A72. arXiv:0912.3171. Bibcode:2010A&A...511A..72S. doi:10.1051/0004-6361/200913031.  edit
  14. ^ a b c d "AstDys (136199) Eris Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2009-03-16. 
  16. ^ a b c Brown, Mike (2010-11-22). "How big is Pluto, anyway?". Mike Brown's Planets. Retrieved 2010-11-23.  (Franck Marchis on 2010-11-08)
  17. ^ "Dwarf Planet Outweighs Pluto". 2007. Retrieved 2007-06-14. 
  20. ^ Brown, Mike (2006). "The discovery of 2003 UB313 Eris, the 10th planet largest known dwarf planet". Caltech. Retrieved 2010-01-05. 
  21. ^ "The IAU draft definition of "planet" and "plutons"" (Press release). IAU. 2006-08-16. Retrieved 2006-08-16. 
  23. ^ Mike Brown 2010, How I Killed Pluto and Why It Had It Coming
  24. ^ Thomas H. Maugh II and John Johnson Jr. (2005-10-16). "His Stellar Discovery Is Eclipsed". Los Angeles Times. Retrieved 2008-07-14. 
  25. ^ "Pluto Now Called a Plutoid". 2008-06-11. Retrieved 2008-06-11. 
  26. ^ Gomes R. S., Gallardo T., Fernández J. A., Brunini A. (2005). "On the origin of the High-Perihelion Scattered Disk: the role of the Kozai mechanism and mean motion resonances". Celestial Mechanics and Dynamical Astronomy 91 (1–2): 109–129. Bibcode:2005CeMDA..91..109G. doi:10.1007/s10569-004-4623-y. 
  27. ^ "NASA-Funded Scientists Discover Tenth Planet". Jet Propulsion Laboratory. 2005. Retrieved 2007-05-03. 
  28. ^ "IAU 2006 General Assembly: Resolutions 5 and 6". IAU. 2006-08-24. 
  29. ^ Robert Roy Britt (2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". Retrieved 2007-05-03. 
  30. ^ a b Green, Daniel W. E. (September 13, 2006). "(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia)". IAU Circular 8747. Retrieved 2012-01-12. 
  31. ^ Blue, Jennifer (2006-09-14). "2003 UB 313 named Eris". USGS Astrogeology Research Program. Retrieved 2007-01-03. 
  32. ^ "International Astronomical Association homepage". Archived from the original on September 30, 2007. Retrieved 2007-01-05. 
  33. ^ "The girl who named a planet". BBC News. 2006-01-13. Retrieved 2007-06-21. 
  34. ^ "Xena and Gabrielle" (PDF). Status. January 2006. Retrieved 2007-05-03. 
  36. ^ "Faraway Eris is Pluto's Twin". ESO Science Release. 26 October 2011. Retrieved 28 October 2011. 
  38. ^ "Planet X Marks the Spot" (PDF). TechRepublic. 2006. Retrieved 2008-07-13. 
  39. ^ O'Neill, Sean (2005). "Your top 10 names for the tenth planet". NewScientist. Retrieved 2008-06-28. 
  40. ^ a b "The Discovery of Eris, the Largest Known Dwarf Planet". California Institute of Technology, Department of Geological Sciences. Retrieved 2007-01-05. 
  41. ^ "IAU0605: IAU Names Dwarf Planet Eris". International Astronomical Union News. 2006-09-14. Retrieved 2007-01-05. 
  42. ^ a b c d Yeomans, Donald K. "Horizons Online Ephemeris System". California Institute of Technology, Jet Propulsion Laboratory. Retrieved 2007-01-05. 
  43. ^ Johnston, Wm. Robert (2007-08-21). "(136199) Eris and Dysnomia". Johnston's Archive. Retrieved 2007-07-27. 
  44. ^ Peat, Chris. "Spacecraft escaping the Solar System". Heavens-Above. Retrieved 2008-01-25. 
  46. ^ a b "Hubble Finds 'Tenth Planet' Slightly Larger Than Pluto". NASA. 2006-04-11. Retrieved 2008-08-29. 
  47. ^ a b John Stansberry, Will Grundy, Mike Brown, John Spencer, David Trilling, Dale Cruikshank, Jean-Luc Margot (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". arXiv:astro-ph/0702538 [astro-ph].
  48. ^ "Comment on the recent Hubble Space Telescope size measurement of 2003 UB313 by Brown et al.". Max Planck Institute. 2006. Retrieved 2007-05-03. 
  50. ^ M. E. Brown, E.L. Schaller, H.G. Roe, D. L. Rabinowitz, C. A. Trujillo (2006). "Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope" (PDF). The Astronomical Journal 643 (2): L61–L63. arXiv:astro-ph/0604245. Bibcode:2006ApJ...643L..61B. doi:10.1086/504843. 
  51. ^ a b Young, Eliot F.; Young, L. A.; Buie, M.; Buie (2007). "Pluto's Radius". Bulletin of the American Astronomical Society 39: 541. Bibcode:2007DPS....39.6205Y. 
  52. ^ Brown, Mike (2007). "Dysnomia, the moon of Eris". CalTech. Retrieved 2007-06-14. 
  54. ^ "Gemini Observatory Shows That "10th Planet" Has a Pluto-Like Surface". Gemini Observatory. 2005. Retrieved 2007-05-03. 
  55. ^ M. E. Brown, C. A. Trujillo, D. L. Rabinowitz (2005). "Discovery of a Planetary-sized Object in the Scattered Kuiper Belt". The Astrophysical Journal 635 (1): L97–L100. arXiv:astro-ph/0508633. Bibcode:2005ApJ...635L..97B. doi:10.1086/499336. 
  56. ^ J. Licandro, W. M. Grundy, N. Pinilla-Alonso, P. Leisy (2006). "Visible spectroscopy of 2003 UB313: evidence for N2 ice on the surface of the largest TNO". Astronomy and Astrophysics 458 (1): L5–L8. arXiv:astro-ph/0608044. Bibcode:2006A&A...458L...5L. doi:10.1051/0004-6361:20066028. 
  58. ^ Tytell, David (2006). "All Hail Eris and Dysnomia". Sky and Telescope. Retrieved 2010-01-05. 

External links[edit]