ERIS (DWARF PLANET)

'Eris' (IPA: ), formally designated '136199 Eris' and formerly '' (see ''minor planet names''), is the largest known dwarf planet in the solar system and the ninth largest body orbiting the Sun directly. It is between 2,400 and 3,000 kilometres in diameter and 27% more massive than Pluto.^[5]
Eris was discovered in 2005 by a Mount Palomar-based team led by Mike Brown. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, named ''Dysnomia''; recent observations found no evidence of further satellites. Their current distance from the Sun is some 97 AU, or roughly three times that of Pluto. With the exception of some comets, the pair are currently the most distant known bodies in the Solar System.^[6]
Eris’ 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, motivated the International Astronomical Union (IAU) to define the term "planet" for the first time. Under a new definition approved on August 24, 2006, Eris was designated a "dwarf planet" along with Pluto and Ceres.^[7]

Discovery

Eris was discovered by the team of Mike 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, and . 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 1200 mm 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 Sedna was discovered, it was moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris' slow motion against the background stars.

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 () was announced by another group in Spain.
Yet more observations released in October 2005 revealed that the object had a moon, later named Dysnomia. Observations of the orbit permitted scientists to determine the mass of Eris in June 2007 to be 1.66 x 10²² kg ± 0.02 x 10²² kg.

Classification

Distribution of trans-Neptunian Objects.

Eris is classified as a dwarf planet and a scattered disk object (SDO). The latter is a category of the TNOs 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.^[8] 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.^[9] 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 IAU definition of a planet, adopted on 24 August 2006. Eris has been termed a dwarf planet (a term distinct from "planet") by the IAU.^[10] Brown has since stated his approval of the new "dwarf planet" label.^[11] The IAU subsequently placed Eris into its Minor Planet Catalogue, giving it the designation (136199) Eris.^[12]

Name

Eris (Athenian painting, circa 550 BCE)

Eris is named after the goddess Eris (Greek Έρις), a personification of strife and discord. 2003 UB 313 named Eris Blue, Jennifer The name was assigned on September 13, 2006 following an unusually long period in which it was best known by the provisional designation '', which was granted automatically by the IAU under their naming protocols for minor planets.

Nicknames

Before the name Eris was granted, two nicknames were used for the planet by the popular media.
"Xena" was an informal name used by the discovery team. It was inspired by the eponymous heroine of the television series ''. The discovery team had reportedly saved the nickname '' 'Xena' '' for the first body they discovered that was larger than Pluto. According to Brown, '' "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?), 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!" ''^[13]
The nickname '' "Lila" '' has also been used, but this is a misunderstanding of ''planetlila'', part of the URL of the discovery web page;^[14] the web page's name was derived from the name of Mike Brown's then-newborn daughter, Lilah.

Choosing an official name

The delay in assigning a name was due to uncertainty over whether the object would be classified as a planet or a minor planet; different nomenclature procedures apply to these different classes of object.^[15] The decision on a name had to wait until after the August 24, 2006 IAU ruling defining the object as a dwarf planet.^[16]
Brown had previously speculated that Persephone would be a good name for the object. 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). Because IAU regulations demand a name from a creation mythology for objects with orbital stability beyond Neptune's orbit, the team had also been considering such possibilities.
The discovery team proposed 'Eris' on 6 September, 2006. On 13 September, 2006 it was accepted as the official name by the IAU.^[17]
The name in part reflects the discord in the astronomical community caused by the debate over the object's (and Pluto's) nature.

Orbit

Eris has an orbital period of 556.7 years, and currently lies at almost its maximum possible distance from the Sun (aphelion). It will next reach perihelion, according to the best available data, on March 29, 2257.^[18] Its last perihelion was in late July of 1700. At a distance of roughly 97 astronomical units, it and its moon are currently the most distant known objects from the Sun in the solar system apart from long-period comets. Its semimajor axis is 67.669 AU, its perihelion distance is 37.78 AU, and its aphelion distance is 97.56 AU. Approximately forty known TNOs (most notably and ), while currently closer to the Sun than Eris, have greater ''average'' orbital distances.^[19]
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 plutinos, 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 in some amateur telescopes. A telescope with a 200 mm lens or mirror and a CCD can detect Eris under favorable conditions. 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.
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 to stay until 2065 when it enters Aries.^[20] After this it will move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173. Because the orbit of Eris is highly inclined, it only passes through a few constellations of the traditional Zodiac.

Size, mass and density

The diameter of Eris has been measured to be 2400 km using images from the Hubble Space Telescope.^[21] The brightness of an object depends both on its size and the amount of light it reflects (its albedo). At a distance of 97 AU, an object with a radius of 3000 km would have an angular size of 40 milliarcseconds,^[22] which is directly measurable with HST; although resolving such small objects is at the very limit of Hubble's capabilities, sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.

Image:EightTNOs.png|thumb|250px|left|Eris compared to Pluto, (136472) 2005 FY₉, (136108) 2003 EL₆₁, Sedna, Orcus, Quaoar, Varuna, and Earth.
#Earth
rect 646 1714 2142 1994 The Earth
#Eris and Dysnomia
circle 226 412 16 Dysnomia
circle 350 626 197 (136199) Eris
#Pluto and Charon
circle 1252 684 86 Charon
circle 1038 632 188 (134340) Pluto
#2005 FY9
circle 1786 614 142 (136472) 2005 FY9
#2003 EL61
circle 2438 616 155 (136108) 2003 EL61
#Sedna
circle 342 1305 137 (90377) Sedna
#Orcus
circle 1088 1305 114 (90482) Orcus
#Quaoar
circle 1784 1305 97 (50000) Quaoar
#Varuna
circle 2420 1305 58 (20000) Varuna
desc none
# - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
#Notes:
#Details on the new coding for clickable images is here:
#While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
#Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.

This revised estimate of the diameter makes Eris only 4% larger than Pluto. According to Hubble, Eris' diameter measures 2397 km, give or take 100 km. Pluto is about 2306 km 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.^[23]
In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave an estimate of Eris's diameter of 2600 +400 −200 km.^[24] According to these data, Hubble's estimate is the lowest possible figure, and Eris's diameter is more likely to be greater than Pluto's by 13%, and perhaps as much as 30%.
The mass of Eris is about 27 percent greater than Pluto (based on the current value for Dysnomia's period, 15.774 days).^[25]
^[26]

Thermal measurement

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 km, about a third larger than Pluto.^[27] If the object rotates quickly, resulting in a more even heat distribution and a temperature of 23 to 24 kelvin (-250 to -249 degrees Celsius), 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, or -246 degrees Celsius) 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.
The apparent inconsistency of the HST PSF results (2400 ± 100 km) with the above IRAM results (3000 ± 370 km) will certainly be studied at more length. Brown explains it by a slightly lower absolute 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 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.

Surface and atmosphere

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 Hawaii 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.^[28]
Due to its orbit, surface temperatures vary between about 30 and 56 kelvins (−243 and −217 degrees Celsius).
Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey. Pluto's reddish colour 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.^[29]
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. Methane is highly 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 TNO, , which reveal the presence of water ice but not methane.^[30]

Moon

Eris and Dysnomia: artist's impression. Eris is the main object, Dysnomia the small non-reddish dot just above it.

Main articles: Dysnomia (moon)

During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, , , and Eris), using the newly commissioned laser guide star adaptive optics system.^[31] Observations taken on September 10 revealed a moon in orbit around Eris, which received its name (Dysnomia) at the same time as its primary. In keeping with the "" 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. Brown's group has pointed out the link with the former nicknames, as the character of Xena was played by Lucy Lawless.

Notes

It should be noted at this point that Pluto is not, in fact, named after a cartoon character. Rather, the cartoon character was named after ''it'', though Venetia Phair, Pluto's christener, has had to counter accusations her whole life that she named the planet after a cartoon dog.^[32]

For an example of an amateur image of Eris, see Fred Bruenjes' Astronomy

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.

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 discussed in a paper on ^[33]

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.

References

1. Discovery Circumstances: Numbered Minor Planets Staff
2. Minor Planet Designations Staff
3. Asteroid Observing Services
4. The Mass of Dwarf Planet Eris, M.E. Brown and E.L. Schaller, , , Science, 2007
5. Dwarf Planet Outweighs Pluto
6. The discovery of 2003 UB313 Eris, the largest known dwarf planet Mike Brown
7.
8. On the origin of the High-Perihelion Scattered Disk: the role of the Kozai mechanism and mean motion resonances, Gomes R. S., Gallardo T., Fernández J. A., Brunini A., , , Celestial Mechanics and Dynamical Astronomy, 2005
9. NASA-Funded Scientists Discover Tenth Planet
10. IAU 2006 General Assembly: Resolutions 5 and 6
11. Pluto Demoted: No Longer a Planet in Highly Controversial Definition Robert Roy Britt
12. IAU Circular 8747 - Official publication of the IAU reporting the naming of Eris and Dysnomia (PDF file)
13. Xena and Gabrielle
14. The Discovery of Eris, the Largest Known Dwarf Planet
15. International Astronomical Association homepage
16. (134340) PLUTO, (136199) ERIS, AND (136199) ERIS I (DYSNOMIA) Green, Daniel W.E.
17. IAU0605: IAU Names Dwarf Planet Eris
18. (136199) Eris and Dysnomia
19. List of Centaurs and Scattered-Disk Objects
20. Horizon Online Ephemeris System
21. Comment on the recent Hubble Space Telescope size measurement of 2003 UB313 by Brown et al.
22.
23. Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope, M. E. Brown, E.L. Schaller, H.G. Roe, D. L. Rabinowitz, C. A. Trujillo, , , The Astronomical Journal, 2006
24. Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope John Stansberry, Will Grundy, Mike Brown, John Spencer, David Trilling, Dale Cruikshank, Jean-Luc Margot
25. Dysnomia, the moon of Eris Mike Brown
26. The Mass of Dwarf Planet Eris, M.E. Brown and E.L. Schaller, , , Science, 2007

27. Comment on the Recent Hubble Space Telescope size measurement of 2003 UB313 by Brown et al.
28. Gemini Observatory Shows That "10th Planet" Has a Pluto-Like Surface
29. Discovery of a Planetary-sized Object in the Scattered Kuiper Belt, M. E. Brown, C. A. Trujillo, D. L. Rabinowitz, , , The Astrophysical Journal, 2005
30.
31. Satellites of the largest Kuiper belt objects, 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, , , The Astrophysical Journal, 2006
32. The girl who named a planet
33. Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar, M. E. Brown and C. A. Trujillo, , , The Astronomical Journal, 2004 Describing in detail the method applied to the recent measure of

External links

★ Michael Brown's webpage about

★ Brown's webpage about 's satellite

★ compiled list of data

★ MPEC listing for

★ Java 3D orbit visualization

★ Spaceflight Now article about , , and

★ Slacker Astronomy Interview With Co-Discoverer Trujillo

★ Trans-Neptunian Object — IAU statement regarding the planetary status of

★ Simulation of 's orbit — is in a 17:5 resonance with Neptune

★ Keck observatory page about the discovery of 's moon

★ Caltech Press Release, 7/29/2005 "Planetary Scientists Discover Tenth Planet".

★ Press release on the Spitzer Space Telescope trying to image again

Observatories involved

★ Keck Observatory, Hawaii, US

★ Palomar

★ Gemini

★ IRAM (''Institut de Radio Astronomie Millimétrique'' (Institute for Millimetric Radio Astronomy)): French-German (Max-Planck Institut fur Radioastronomy, Bonn) and Spanish (where the 30 m telescope is situated) collaborative programme.

★ HST

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