Eris (planet)

Eris is the largest known dwarf planet in the Solar System and the ninth largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto.

Eris was first spotted in YOLD 3169 by a Palomar Observatory-based team led by Mike Brown, but she was not identified until YOLD 3171. She 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, Dysnomia; recent observations have found no evidence of further satellites. The current distance from the Sun is 96.7 AU, roughly three times that of Pluto. With the exception of some comets the pair are the most distant known natural objects in the Solar System.

Because Eris is larger than Pluto, its discoverers and NASA called 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; after all, it is right and good for Eris to shake things up! Under a new definition approved on Sweetmorn, Bureaucracy 17, YOLD 3172, Eris is a "dwarf planet" along with Pluto, Ceres, and Makemake.

Discovery
Eris was discovered by the team of Mike Brown, Chad Trujillo, and David L. Rabinowitz on Setting Orange, Chaos 5, YOLD 3171 (Mungday), from images taken on Prickle-Prickle, The Aftermath 2, YOLD 3169. The discovery was announced on Boomtime, Confusion 56, YOLD 3171. 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.

Routine observations were taken by the team, using the 1200 mm Samuel Oschin reflecting telescope at Mount Palomar Observatory, California, but the image of Eris 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. Eventually, 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 to be estimated. The team had planned to delay announcing their discovery until further observations allowed more accurate calculations of Eris' orbit, but brought their announcement forward when the discovery of another large TNO they had been tracking was announced by a different team in Spain.

More observations revealed that Eris had a moon, later named Dysnomia. Observations of Dysnomia's orbit permitted scientists to determine the mass of Eris, which they calculated to be 27% greater than Pluto.

Classification
Eris is classified as a dwarf planet and trans-Neptunian object (TNO); the intersection of these categorizations makes it a "plutoid". Eris' orbital characteristics can be used to 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. 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.

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 ongoing debate over whether Pluto should be classified as a planet, the Greyface IAU agents 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. At this time both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of planet. Brown has since stated his approval of the "dwarf planet" label. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.

Orbit
Eris has an orbital period of 557 years, and as of 3174 lies at 96.7 astronomical units from the Sun, almost its maximum possible distance. (Its aphelion is 97.5 AU.) Eris came to perihelion between YOLD 2864-2865, aphelion around 3143, and will return to perihelion around 3422 to 3424. Eris and its moon are currently the most distant known objects in the Solar system apart from long-period comets and space probes. However, approximately forty known TNOs, most notably Sedna, while currently closer to the Sun than Eris, have greater average orbital distances than Eris' semimajor axis of 67.7 AU.

The Eridian Erisian orbit is highly eccentric, like Eris herself, and brings Her to within 37.9 AU of the Sun, a typical perihelion for scattered objects. This is within the orbit of Pluto, but still safe from direct interaction with Neptune (29.8-30.4 AU). Pluto, on the other hand, like other plutinos, follows a less inclined and less eccentric orbit and, protected by orbital resonance, 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 highly inclined: it is tilted at an angle of about 44 degrees to the ecliptic.

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 Charge-Coupled Device 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, where most bodies are found.

Eris is now in the constellation Cetus. It was in Sculptor from YOLD 3042 until 3095 and Phoenix from roughly 3006 until 3041. In 3202 it will enter Pisces and stay there until 3231, when it will enter Aries.[2] It will then move into the northern sky, entering Perseus in 3294 and Camelopardalis (where it will reach its northernmost declination) in 3339. Because the orbit of Eris is highly inclined, it only passes through a few constellations of the traditional Zodiac.

Size, mass, and density
(also see The Mass of Planet Eris)

The diameter of Eris has been measured to be 2397 km, give or take 100 km, using images from the Hubble Space Telescope.

(The brightness of an object depends both on its size and its albedo, the amount of light it reflects). At a distance of 97 AU, an object with a radius of 3000 km would have an angular size of 40 milliarcseconds, which is directly measurable with the 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.

This makes Eris only 0-8% larger than Pluto, which is about 2306 km across. It also indicates an albedo of 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.

In YOLD 3173, 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.[6] The Spitzer and Hubble estimates overlap in the range of 2400-2500 km, 4-8% larger than Pluto.

The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period, 15.774 days, Eris is 27 percent more massive than Pluto.

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 ± 400 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 (-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 Institut de radioastronomie millimétrique 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 discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 m Gemini North Telescope in Hawaii on Setting Orange, Chaos 25, YOLD 3171. Infrared light from the object revealed the presence of methane ice, indicating that the surface may be similar to that of Pluto, which at the time was the only TNO known to have surface methane, and of Neptune's moon Triton, which also has methane on its surface.[9]

Due to its distant eccentric orbit, Eridian Erisian surface temperatures are estimated to vary between about 30 and 56 kelvins (−243 and −217 degrees Celsius).[10]

Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey.[10] Pluto's reddish colour is believed to be due to deposits of Tholians 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. Even though Eris can be up to three times further from the Sun than Pluto, it approaches close enough that some of the ices on the surface might 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, 2003 EL, which reveal the presence of water ice but not methane.

The Mass of Planet Eris
The Mass of Planet Eris is a Discordian holyday or whollyday celebrated on 37 Bureaucracy (Septembber 18), the anniversary of its discovery. This holyday is also know as the Mass of Eristotle.

Moon
Later, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, and Eris), using the newly commissioned laser guide star adaptive optics system.

Images taken on 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's sidekick. When Eris received its official name from the IAU, the moon received the name Dysnomia, after the Greek demon of lawlessness who is Eris' daughter. The Discordian name for the moon is Shamlicht.