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Eris (dwarf planet)

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Time-lapse images showing the movement of Eris (circled) against the stars.

Eris ([ˈirɪs] or [ˈɛrɪs]), officially designated 136199 Eris , is the largest known dwarf planet in the solar system. Previously designated 2003 UB313, it was initially nicknamed "Xena" by its discoverers. It is a trans-Neptunian object (TNO), orbiting the Sun in a region of space known as the Kuiper Belt, and accompanied by at least one moon, Dysnomia. Mike Brown, who led the Mount Palomar-based discovery team, announced in April 2006 that the Hubble Telescope has measured Eris' diameter to be 2400 km, slightly larger than Pluto.[1]

Eris's size resulted in its discoverers and NASA labelling it the solar system's tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, stimulated the International Astronomical Union (IAU) to define the term "planet" more precisely. Under a new definition approved on August 24, 2006, Eris was designated a "dwarf planet".[2] Brown has since stated his approval of the new "dwarf planet" label.[3]

Discovery

Eris was discovered by the team of Michael E. Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as two other large TNOs, (136108) 2003 EL61 and (136472) 2005 FY9. The search team has been systematically scanning for large outer solar system bodies for several years, and had previously been involved in the discovery of several other very large TNOs, including 50000 Quaoar, 90482 Orcus, and 90377 Sedna.

Routine observations were taken by the team on October 21, 2003, using the 48-inch (122 cm) Samuel Oschin reflecting telescope at Mount Palomar Observatory, California, but the object captured on the images was not discovered at that point due to its very slow motion across the sky: the team's automatic image-searching software excluded all objects moving at less than 1.5 arcseconds per hour to reduce the number of false positives returned. When 90377 Sedna was discovered, it was moving at 1.75 arcsec/hr, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the false positives by eye. In January 2005, the re-analysis revealed Eris' slow motion against the background stars.

File:2003 UB313 distance comparison.jpg
Comparing the distances of the planets to the dwarf planets Pluto and Eris

Follow-up observations were then carried out to make a preliminary determination of its orbit, which allowed its distance and size to be estimated. The team had planned to delay announcing their discovery until further observations had been made which would have allowed more accurate determinations of the body's size and mass, but were forced to bring forward the announcement when the discovery of another object they had been tracking (2003 EL61) was announced by another group in Spain. Brown's group later accused the Spanish group of a serious breach of ethics in connection with the discovery of 2003 EL61 and asked that they be stripped of credit for its discovery (see the 2003 EL61 or Michael E. Brown articles for details).

Yet more observations released in October 2005 revealed that the object had a moon, Dysnomia, nicknamed "Gabrielle" at the time. Scientists plan to use this information to determine the mass of Eris.

Classification

Animation showing the movement of Eris on the images used to discover it. Eris is located on the left side, slightly above the middle of the image. The three frames were taken over a period of three hours.

Eris is classified as a dwarf planet and a scattered disk object (SDO). The latter is a category of TNO that are 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 are scattered into orbits with higher inclinations than objects from the outer belt. 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.

As Eris is larger than Pluto, it was initially described as the "tenth planet" by NASA and in media reports of its discovery. In response to the uncertainty over its status, and because of continuing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a new definition of the term planet. This definition was clarified under the new International Astronomical Union definition of a planet, adopted on 24 August 2006. Eris has been termed a dwarf planet by the IAU. It may also be under consideration as a member of "a new class of trans-Neptunian objects" yet to be defined by that body. It is not, however, considered to be a planet.

Name

File:2003 UB313 NASA illustration 3.jpg
NASA illustration of Eris

136199 Eris is named after the Greek goddess Eris (Ἔρις in Greek), a personification of strife and discord. The contemporary religion of Discordianism also worships this goddess.

This name was assigned on September 13, 2006 following an unusually long period in which it was best known by the provisional designation 2003 UB313, granted automatically according to the International Astronomical Union's naming protocols for minor planets. The delay in assigning a name was due to uncertainty over whether the nomenclature of the object should reflect prior practices associated with planetary nomenclature or the procedures applied to asteroids and other minor planets.[4] The permanent name was assigned within a few weeks of the August 24, 2006 IAU ruling defining it as a dwarf planet.[5] The name in part reflects the discord in the astronomy community caused by the debate over the object's nature.[6]

Unofficial names used in popular media for the object included:

  • "Lila", a misunderstanding of the phrase planetlila which formed part of the URL of the discovery web page, named after Michael E. Brown's newborn daughter, Lilah.
  • "Xena", an informal name used by the discovery team, inspired by the television series Xena: Warrior Princess. This name was reportedly saved for the first body discovered that was larger than Pluto. The team has said that they chose the name because "We have always wanted to name something Xena",[7] apparently implying that the name was chosen without any reference to Planet X.

These names were not among those proposed to the IAU. Brown's speculation on the name to be given the object included:

It appears that Brown's proposals along these lines were not accepted, and that the name Eris represents a new approach under different rules.[10]

Orbit

Position of Eris on 29 July 2005. On the left is the view from "above" the plane of the solar system, while on the right is the view from "in front". Darker blue indicates the part of the orbit below the ecliptic plane. Also shown are Saturn, Uranus, Neptune, and Pluto.

Eris has an orbital period of 556.7 years, and currently lies at almost its maximum possible distance from the Sun (aphelion). It is currently the most distant known solar system object from the Sun at a distance of roughly 97 AU. Its semimajor axis is 67.669 astronomical units, its perihelion distance is 37.78 AU, and its aphelion distance is 97.56 AU. Approximately forty known TNOs (most notably 2000 OO67 and Sedna), while currently closer to the Sun than Eris, have greater average orbital distances.[11]

Its orbit is highly eccentric, and brings it to within 37.8 AU of the Sun (a typical perihelion for scattered objects), still safe from direct interaction with Neptune (at ~30 AU). For comparison, Pluto, like other dwarf planets, follows a less inclined and less eccentric orbit and, protected by orbital resonance, it can cross Neptune’s orbit. Unlike the terrestrial planets and gas giants, whose orbits all lie roughly in the same plane as the Earth's, Eris' orbit is very inclined — it is tilted at an angle of about 44 degrees to the ecliptic.

The object currently has an apparent magnitude of about 19, making it bright enough to be detectable even in some amateur telescopes. While it would be a difficult object to spot visually, a telescope with an 8" lens or mirror and a CCD can image Eris in dark skies.[12] The reason it had not been noticed until now is because of its steep orbital inclination: most searches for large outer solar system objects concentrate on the ecliptic plane, in which most solar system material is found.

Distribution of trans-Neptunian Objects.

Size

Optical measurement from HST pictures

The diameter of Eris has been measured to be 2400 km using images from the Hubble Space Telescope. The brightness of an object depends both on its size and the amount of light it reflects (its albedo). At a distance of 67 AU, an object with a radius of 3000 km would have an angular size of 40 milliarcseconds, which is directly measurable with HST; although resolving such small objects is at the very limit of Hubble's capabilities,[13] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.[14]

File:EightTNOs.png
Eris compared to Pluto, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar, Varuna, and Earth.

This revised estimate of the diameter makes Eris only 4% larger than Pluto (According to Hubble, Eris' diameter measures 2,397 km (1,490 mi), give or take 100 km (60 mi). Pluto is about 2,306 km (1,433 mi) across. It also indicates that the albedo is 0.86, higher than any other large body in the Solar system other than Enceladus. It is speculated that the high albedo is due to the surface ices being replenished due to temperature fluctuations as Eris' eccentric orbit takes it closer and farther from the Sun.

Thermal measures

Previous observations of the thermal emission of Eris at a wavelength of 1.2 mm, where the object's brightness depends only on temperature and surface area, indicated a diameter of 3000+270-100 km, about a third larger than Pluto.[15] If the object rotates quickly, resulting in a more even heat distribution and a temperature of 23 to 24 kelvins, a likely diameter would be in the higher portion of the range (best fit 3090 km); if it rotates slowly, the visible surface would be warmer (about 27 K) and a likely diameter would be in the smaller end of the range (best fit 2860 km). The 2860 km figure implies a Pluto-like albedo of 60%, consistent with its Pluto-like spectral signature.

Possible explanation of the inconsistent results

The apparent inconsistence of the HST PSF results (2400 ± 100 km) with the above IRAM results (3000 ± 370 km) will be certainly studied in more length. Brown explains it by a slightly lower magnitude than the one assumed by Bertoldi (−1.12 ± 0.01 versus −1.16 ± 0.1, resulting by itself in almost 100 km difference in diameter). Assuming further the highest diameter (2500 km) and pole-on position of the object[16] the difference between the results would appear consistent with 1.1-σ error margin.

Another possible explanation for the IRAM results is offered by the Max-Planck Institut für Radioastronomie. The ratio between the bolometric albedo (representing the total reflected energy and used in the thermal method) and the geometric albedo (representing the reflection in some visual wavelength and used to calculate the diameter from HST pictures) is not known with high precision and depends on many factors. By itself, this uncertainty could bridge the gap between the two measures.[15]

Surface

The infrared spectrum of Eris, compared to that of Pluto, shows the marked similarities between the two bodies. Arrows denote methane absorption lines.

The discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 m Gemini North Telescope in Hawaiʻi on January 25, 2005. Infrared light from the object revealed the presence of methane ice, indicating that the surface of Eris is rather similar to Pluto, which was the only TNO already known to show the presence of methane. Neptune's moon Triton is probably related to Kuiper Belt objects, and also has methane on its surface.

Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey. 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.

Methane is very volatile and its presence shows either that Eris has always resided in the distant reaches of the solar system where it is cold enough for methane ice to persist, or that it has an internal source of methane to replenish gas that escapes from its atmosphere. This contrasts with observations of another recently discovered Kuiper Belt object, 2003 EL61, which reveal the presence of water ice but not methane.

Atmosphere

Even though Eris can be up to three times further from the Sun than Pluto, it approaches close enough that some of the various ices that exist on the surface might become warm enough to sublimate and form a fine atmosphere; however, it is unclear whether this actually happens on Eris.

Due to its orbit, temperatures vary between about −232 and −248 degrees Celsius.

Eris is so far away from the Sun that in its skies the Sun only appears as a very bright star shining between a magnitude of -19 when nearest and -17 farthest away. The full Moon on Earth, by comparison, shines at a magnitude of -12.

Moon

Main article: Dysnomia (moon)

During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest Kuiper belt objects (Pluto, 2005 FY9, 2003 EL61, and Eris, using the newly commissioned laser guide star adaptive optics system. Observations taken on September 10 revealed a moon in orbit around Eris, which received the name Dysnomia at the same time as its primary. In keeping with the "Xena" nickname already in use for Eris, the moon was previously nicknamed Gabrielle by its discoverers, after the television warrior princess's sidekick. The name Dysnomia is taken from a mythological demon of lawlessness who was Eris' daughter; this is also a nod toward the former nicknames, as the character of Xena was played by Lucy Lawless.

See also

References

  • M. E. Brown, C. A. Trujillo, D. L. Rabinowitz Discovery of a Planetary-sized Object in the Scattered Kuiper Belt, The Astrophysical Journal, 635 (Dec. 2005), Issue 1, pp. L97-L100. Preprint on arXiv. doi:10.1086/499336
  • M. E. Brown, E.L. Schaller, H.G. Roe, D. L. Rabinowitz, C. A. Trujillo Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope To appear in The Astronomical Journal. Preprint (pdf) on Brown's site. doi:10.1086/504843
  • J. Licandro, W.M. Grundy, N. Pinilla-Alonso, J. de Leon Visible spectroscopy of 2003 UB313: Evidence for N2 ice on the surface of the largest TNO?. Accepted in A&A. Preprint on arXiv.
  • M. E. Brown, M. A. van Dam, A. H. Bouchez, D. LeMignant, C. A. Trujillo, R. Campbell, J. Chin, Conrad A, .S. Hartman, E. Johansson, R. Lafon, D. L. Rabinowitz, P. Stomski, D. Summers, P. L. Wizinowich Satellites of the largest Kuiper belt objects, The Astrophysical Journal (2006), 639, Issue 1, pp. L43-L46. Preprint on arXiv. doi:10.1086/501524
  • 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, Vol. 91, pp. 109-129 doi:10.1007/s10569-004-4623-y
  • Bertoldi F., Altenhoff W., Weiss A., Menten K. M., Thum C. (2006). The trans-neptunian object UB313 is larger than Pluto, Nature, 2 February 2006;439(7076):563-4 doi:10.1038/nature04494
  • M. E. Brown and C. A. Trujillo Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar The Astronomical Journal, 127 (2004), pages 2413 – 2417 doi:10.1086/382513 Reprint (pdf) on Brown's site. Describing in detail the method applied to the recent measure of 2003 UB313
  • SpaceDaily Report on IAU Draft Definition of Planets and Plutons
  • IAU Circular 8747 - Official publication of the IAU reporting the naming of Eris and Dysnomia (PDF file)

Notes

  1. ^ http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/16/
  2. ^ "The IAU draft definition of "planet" and "plutons"" (Press release). IAU. 2006-08-16. Retrieved 2006-08-16.
  3. ^ Robert Roy Britt, 'BREAKING NEWS: Pluto Demoted, No Longer a Planet'. Space.com. 24 August 2006.
  4. ^ http://www.iau.org/IAU/FAQ/2003_UB313.html
  5. ^ (134340) PLUTO, (136199) ERIS, AND (136199) ERIS I (DYSNOMIA)
  6. ^ "Largest known dwarf planet named Eris".
  7. ^ http://observer.guardian.co.uk/international/story/0,6903,1539933,00.html
  8. ^ "The discovery of 2003 UB313, the 10th planet". Retrieved 2006-08-01. Page now renamed to "The discovery of Eris, the largest known dwarf planet".
  9. ^ http://www.pasadenastarnews.com/Stories/0,1413,206~22097~3009692,00.html
  10. ^ Robert Roy Britt, [http://news.yahoo.com/s/space/20060824/sc_space/newplanetdefinitionleavesscientificlooseends ' SPACE.com New Planet Definition Leaves Scientific Loose Ends']. Space.com. 24 August 2006
  11. ^ http://cfa-www.harvard.edu/iau/lists/Centaurs.html
  12. ^ for an example of an amateur image of Eris, see [1]
  13. ^ The Resolution of the High Resolution Channel of the ACS is 40 marcsec (milliarcseconds) and the size of 1 pixel is ~25 marcsec i.e. ~1875 km at the distance of Eris
  14. ^ The reference to ‘direct’ measure by HST should not mislead into thinking that this method is as ‘direct’ and model-independent as measuring say Neptune’s size. Basically, the method consists in finding the statistically best fit to a smeared image of the size of less than 2 pixels by comparing it with smeared images of the background stars, using a given computer model of the optics (PSF). A non technical description of the method is given on Brown’s page, a detailed description of this approach and its limitations are in the paper on Quaoar (see references)
  15. ^ a b http://www.astro.uni-bonn.de/~bertoldi/ub313/
  16. ^ If the object is in pole-on position the side facing the Sun (and the observer) gets hotter producing stronger emissions thus resulting in overestimation of the diameter using the thermal method.

Observatories involved

  • Keck Observatory, Hawaiʻi, US
  • Palomar
  • Gemini
  • IRAM (Institut de Radio Astronomie Millimétrique Institut for Milimetric Radio Astronomy): French-German (Max-Planck Institut fur Radioastronomy, Bonn) and Spanish (where the 30m telescope is situated) collaborative programme.
  • HST

Press releases

News stories

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