PLANET ERIS

Planet Eris, also designated 136199 Eris, is the largest known dwarf planet in the solar system and the ninth largest body orbiting the Sun directly. It is a trans-Neptunian object (TNO), orbiting the Sun in a region of space known as the scattered disc, just beyond the Kuiper belt, and accompanied by at least one moon, Dysnomia.

Discovery of Planet Eris:
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, (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. outine 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 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 (2003 EL61) was announced by another group in Spain. Yet more observations released in October 2005 revealed that the object had a moon, later named Dysnomia. Scientists plan to use this information to determine the mass of Eris.

Classification of Planet Eris:
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. 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 IAU 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 of Planet Eris:
Eris is named after the goddess Eris, a personification of strife and discord. 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, which was granted automatically by the IAU under their naming protocols for minor planets.



Nicknames of Planet Eris:
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 eponymous heroine of the television series Xena: Warrior Princess. The discovery team had reportedly saved the nickname 'Xena' for the first body they discovered that was larger than Pluto. The nickname "Lila" has also been used.

Choosing an official name of Planet Eris:
The delay in assigning a name was due to uncertainty over whether the object was classified as a planet or a minor planet; different nomenclature procedures apply to these different classes of object. The decision on a name had to wait until after the August 24, 2006 IAU ruling defining the object as a dwarf planet.[13] 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). Since 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. The name in part reflects the discord in the astronomical community caused by the debate over the object's (and Pluto's) nature.

Orbit of Planet Eris:
The diagram illustrates the orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and Pluto (white/grey). The segments of orbits below the ecliptic are plotted in darker colours, and the red dot is the Sun. The diagram on the left is a polar view while the diagrams on the right are different views from the ecliptic.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 astronomical units. 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 2000 OO67 and Sedna), while currently closer to the Sun than Eris, have greater average orbital distances. 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 an 8" 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 until 1929, and will enter Pisces in 2036. Because the orbit of Eris is highly inclined, it only passes through a few constellations of the traditional Zodiac.

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Thermal measurement of Planet Eris:
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. 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 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-s 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.

Mass and Density of Planet Eris:
The mass of Eris is about 1.3 times that of Pluto (based on a rough value for Dysnomia's period). This implies Eris' bulk density is roughly 2 g/cm³ and that, like Pluto, Eris is made of a mixture of rock and ice.

Surface of Planet Eris:
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. 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 TNO, 2003 EL61, which reveal the presence of water ice but not methane.

Atmosphere of Planet Eris:
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, surface temperatures vary between about -232 and -248 degrees Celsius



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