Xena
Xena information Average distance from Sun: 1012000000 km (6.7647 AU) Perihelion (closest): 565000000 km Perihelion (farthest): 1460000000 km Equatorial radius: 1200 km (Diameter: 2400 km) Siderial rotation period (length of day): > 8 earth hours Siderial orbit period (length of year): 203500 earth days Mean orbit velocity: 3436 m/s Orbital eccentricity: 0.44177 Orbital inclination to ecliptic: 44.187 degrees Equatorial inclination to orbit: 197.63427 degrees Surface temperature (min/max): 30 K		Xena, Sun
Xena 2003 UB313 is a trans-Neptunian object (TNO) larger than Pluto. Its size, and the prospect of discovering other similarly-sized objects in the future stimulated an attempt by the International Astronomical Union (IAU) to define the term 'planet' more precisely. Under a new definition approved on August 24, 2006, 2003 UB313 was designated a 'dwarf planet'. Before this clarification, it had been described as our solar system's 'tenth planet' by its Mount Palomar-based discovery team and NASA. However, one of the discoverers, Michael Brown, has stated his approval of the new 'dwarf planet' label. 2003 UB313 orbits the Sun in a region of space known as the scattered disc accompanied by at least one moon; the pair are currently the most distant known objects in the solar system. Mike Brown announced in April 2006 that the Hubble Telescope has measured the diameter to be 2400 km. Brown also believes that it is coated in methane. These observations suggest that 2003 UB313 is more similar to Pluto than any previously known body. A ruling on what to name 2003 UB313 is currently being delayed pending a decision on whether it qualifies as a planet. The discoverers have confidentially submitted potential names to the IAU, who oversee astronomical naming conventions. Claims that 2003 UB313 has been officially named 'Xena' or 'Lila' are incorrect; 'Xena' is an informal codename used by its discoverers among themselves, and 'Lila' is a name in the address of the website where the object was announced, after the newly-born daughter of one of the discoverers. Neither name was submitted to the IAU. 'Xena' has caught on as a popular nickname however, and the moon has also been nicknamed 'Gabrielle' after another character on the TV series Xena: Warrior Princess. 2003 UB313 was discovered by the team of Michael 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, 2003 EL61 and 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 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/h, 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 2003 UB313's 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. 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, S/2005 (2003 UB313) 1, nicknamed 'Gabrielle.' Scientists plan to use this information to determine the mass of 2003 UB313. 2003 UB313 is classified as a dwarf planet and a scattered disk object (SDO). The latter is a category of TNO which are believed to have been 'scattered' from the Kuiper belt and 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 current known SDOs, theoretical models suggest that objects which 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 2003 UB313 in high-inclination orbits. As 2003 UB313 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 due to continuing debate over whether Pluto should be classified as a planet, the IAU charged 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. 2003 UB313 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. The object currently has the provisional designation 2003 UB313, granted automatically according to the IAU's naming protocols for minor planets. The next step in the object's identification will be the external verification of its orbit and assignment of a permanent designation number. Should 2003 UB313 be treated as any other minor planet, its discoverers will then have the exclusive right to propose a name during a ten year window that begins with its permanent numbering, subject to the approval of the Committee on Small Bodies Nomenclature of the IAU's Division III. According to the IAU rules, TNOs must be named after deities of creation, with the exception of plutinos, which are named after underworld deities. The potential for the object's classification as a major planet, however, forced a delay from adhering to the same steps, timelines and approval procedures as those that apply to garden-variety asteroids and comets. The IAU ultimately released a short statement regarding the naming of 2003 UB313, indicating the object would not be named until it has been decided if it is a planet or not. Now that the body's status has been clarified, it seems likely that the nomenclature process will continue through the Committee on Small Bodies Nomenclature's conventional avenues. The discoverers have already submitted their name proposal for 2003 UB313, which under IAU rules cannot be publicly disclosed. Brown's team had violated this rule in 2003 when they announced the name 'Sedna' for that planetoid before it had officially been approved, prompting some criticism within the astronomical community; the IAU later relaxed its rules and permitted an expedited process for major new discoveries. The team refers to the object informally by the nickname Xena, after the television series Xena: Warrior Princess. They had saved this name for the first body they found that was larger than Pluto. The X of Xena is a reference to Percival Lowell's Planet X; they had wanted more female names for astronomical bodies, and this was the closest they could come to a mythological female name beginning with X. On the other hand, the team has also claimed that they chose the name because 'We have always wanted to name something Xena', seemingly implying that the name was chosen prior to the association with Planet X. The name Xena has been used by news media such as CNN, but it has not been proposed to the IAU. The second reported name, Lila, is not a nickname for 2003 UB313, but rather comes from 'Planet Lila', the name of the discovery web page URL, which was named after Michael Brown's newborn daughter, Lilah. Two days after announcing the discovery, Brown discussed his team's ideas about naming the objects on his website: 'If the object falls under the rules for other Kuiper belt objects, however, it must be named after some figure in a creation mythology. We have decided to attempt to follow that ruling scheme. One such particularly apt name would have been Persephone. In Greek mythology Persephone is the (forcibly abducted) wife of Hades (Roman Pluto) who spends six months each year underground. The mourning of her mother Demeter causes the dead of winter. The new planet is on an orbit that could be described in similar terms; half of the time in the vicinity of Pluto and half of the time much further away. Sadly, the name Persephone was used in 1895 as a name for the 399th known asteroid. The same story can be told for almost any other Greek or Roman god of any consequence. Luckily, the world is full of mythological and spiritual traditions. In the past we have named Kuiper belt objects after native American, Inuit, and [minor] Roman gods. Our new proposed name expands to different traditions, still.' He added later on his website that IAU lacks consensus on what the object is, and even which committee should be charged with approving its name. The committee which oversees major planets has suggested that if this object is classified as a major planet, the naming should continue the Greco-Roman tradition for planets. Brown indicated in a recent article that he would propose the name Persephone if this tradition were to be upheld, despite the fact that this name has been assigned to the 399th known asteroid. Persephone has often been used in science fiction as the name of the tenth planet: see tenth planets in fiction. It should be noted that the practice of not repeating names for celestial bodies is only a guideline (several exceptions exist), and is not strictly enforced when dealing with celestial bodies of different categories; therefore, it is reasonable to expect that a new planet would have sufficient importance to overrule the exclusivity of nomenclature reserved for an asteroid. It is possible that the IAU might adopt the Roman version of the name Persephone, 'Proserpina,' conforming to exclusively Roman naming tradition for the planets. When thinking of a name, co-discoverer Chad Trujillo went on the record as saying, 'The name may turn out not to be Greek or Roman - there are so many asteroids named now that there are very few Greek/Roman names left!' Their previous record of name giving has been inclusive of other cultures, using names from Native American and Irish folklore. 2003 UB313 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 2003 UB313, 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, 2003 UB313's 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 2003 UB313 in dark skies. 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. The diameter of 2003 UB313 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. This revised estimate of the diameter makes 2003 UB313 only 4% larger than Pluto (According to Hubble, 2003 UB313's 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 2003 UB313's eccentric orbit takes it closer and farther from the Sun. Previous observations of the thermal emission of 2003 UB313 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. 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 the difference between the results would appear consistent with 1.1-? error margin. Another possible explanation, in a language accessible to wide public, is offered by Max-Planck Institut für Radioastronomie behind the IRAM results in a recent press release. 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 depending on many factors. By itself, this uncertainty could bridge the gap between the two measures. The discovery team followed up their initial identification of 2003 UB313 with spectroscopic observations made at the 8 m Gemini North Telescope in Hawai?i on January 25, 2005. Infrared light from the object revealed the presence of methane ice, indicating that the surface of 2003 UB313 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, 2003 UB313 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, 2003 UB313 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 2003 UB313 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. During 2005, the adaptive optics team at the Keck telescopes in Hawai?i carried out observations of the four brightest Kuiper belt objects (Pluto, 2005 FY9, 2003 EL61, and 2003 UB313), using the newly commissioned laser guide star adaptive optics system. Observations taken on 10 September revealed a moon in orbit around 2003 UB313, provisionally designated S/2005 (2003 UB313) 1. In keeping with the 'Xena' nickname already in use for 2003 UB313, the moon was nicknamed Gabrielle by its discoverers, after the television warrior princess's sidekick. As with the 'Xena' name, this is purely informal and it is expected another official name will be chosen for the moon, depending upon the final decided status of 2003 UB313.
More Discovered by M.E. Brown, C.A. Trujillo, D.L. Rabinowitz in October 21, 2003
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