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Eris (dwarf planet)

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: Eris

   Eris.Keck Observatory.
   Dwarf Planet Eris and the Sun.
                       Discovery ^A
       Discoverers     M. E. Brown,
                       C. A. Trujillo,
                       D. L. Rabinowitz
     Discovery date    October 21, 2003
        Alternate
   designations ^B     2003 UB[313]
        Category       Trans-Neptunian object
                       ( Scattered disc object)
                   Orbital elements ^C

                     Epoch March 6, 2006 ( JD 2453800.5)

   Semi-major axis (a) 67.6681 AU (10.12 T m)
    Eccentricity (e)   0.44177
     Perihelion (q)    37.77 AU (5.65 Tm)
      Aphelion (Q)     97.56 AU (14.60 Tm)
   Orbital period (P)  203,500 d (557 a)
   Mean orbital speed  3.436 km/ s
   Max. Orbital Speed  4.126 km/ s
   Min. Orbital Speed  2.567 km/ s
     Inclination (i)   44.187 °
    Longitude of the
   ascending node (Ω)  35.8696°
       Argument of
   perihelion (ω)      151.4305°
    Mean anomaly (M)   197.63427°
               Physical characteristics ^D
        Diameter       2400 km ± 100 km
                       (1500 mi ± 60 mi,
                       or 19% of Earth)
          Mass          ?×10^? kg
         Density        ? g/ cm³
     Surface gravity    ? m/s²
     Escape velocity    ? km/s
     Rotation period   > 8 h?
     Spectral class     ?
   Absolute magnitude  −1.12 ± 0.01
   Albedo ( geometric) 0.86 ± 0.07
      Mean surface
   temperature         ~30 K

   Eris ( IPA pronunciation /ˈɛɹɪs/) or /ˈiɹɪs/), also designated (136199)
   Eris or 136199 Eris (See Minor planet names), is the largest known
   dwarf planet in the solar system. 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.
   Mike Brown, who led the Mount Palomar-based discovery team, announced
   in April 2006 that the Hubble Telescope has measured Eris's diameter to
   be 2400 km, slightly larger than that of Pluto.

   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, 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" along with Pluto and Ceres. Brown has since
   stated his approval of the new "dwarf planet" label.

Discovery

   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.
   Enlarge
   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 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 EL[61] and (136472) 2005 FY[9]. 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 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 EL[61]) was announced by another group in
   Spain.

   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

   Distribution of trans-Neptunian Objects.
   Enlarge
   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.
   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

   Eris (Athenian painting, circa 550 BCE)
   Enlarge
   Eris (Athenian painting, circa 550 BCE)

   136199 Eris is named after the goddess Eris ( Greek Ἔρις), 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 UB[313], 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 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. Their only stated reason was that "We have always
       wanted to name something Xena" (apparently implying that the name
       was chosen without any reference to Planet X).
     * The nickname "Lila" has also been used, but this is a
       misunderstanding of planetlila, part of the URL of the discovery
       web page; 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 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.

   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, and 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 nature.

Orbit

   The diagram illustrates the orbit of Eris (blue) compared to those of
   Pluto and the three outermost planets (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.
   Enlarge
   The diagram illustrates the orbit of Eris (blue) compared to those of
   Pluto and the three outermost planets (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 OO[67] 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.

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, sophisticated image processing techniques such
   as deconvolution can be used to measure such angular sizes fairly
   accurately.
   Eris compared to Pluto, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar,
   Varuna, and Earth.
   Enlarge
   Eris compared to Pluto, 2005 FY[9], 2003 EL[61], 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 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^+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 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 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

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

   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 EL[61], 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, surface temperatures vary between about −232 and −248
   degrees Celsius.

Moon

   During 2005, the adaptive optics team at the Keck telescopes in Hawaii
   carried out observations of the four brightest TNOs (Pluto, 2005 FY[9],
   2003 EL[61], 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 its 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 an acknowledgement of the former
   nicknames, as the character of Xena was played by Lucy Lawless .

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