2003 UB313

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The correct title of this article is 2003 UB₃₁₃. It features superscript or subscript characters that are substituted or omitted due to technical limitations.

2003 UB₃₁₃

2003 UB313 (centre) and moon (right of centre). Keck Observatory.
Discovery A
Discoverer	M. E. Brown, C. A. Trujillo, D. L. Rabinowitz
Discovery date	October 21, 2003
Alternate designations B	officially none; informally Xena
Category	Trans-Neptunian object (Scattered disc object)
Orbital elements C
Epoch March 6, 2006 (JD 2453800.5)
Eccentricity (e)	0.44177
Semi-major axis (a)	67.6681 AU (10.12 Tm)
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
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
Dimensions	2400 km ± 100 km
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

2003 UB₃₁₃ is a trans-Neptunian object (TNO) larger than the planet Pluto.^[1] It has been described as our solar system's "tenth planet" by its Mount Palomar-based discovery team, NASA, and some media outlets. Its discovery has stimulated an attempt by the International Astronomical Union (IAU) to define the term 'planet' more precisely. Under a draft definition, to be submitted to vote by the General Assembly of the IAU on August 24, 2006, 2003 UB₃₁₃ would qualify as a planet.^[2]. Technically, if the motion is ratified, 2003 UB₃₁₃ would then be considered the twelfth planet, since Ceres and Charon would also be promoted to planetary status.

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. 2003 UB₃₁₃'s 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.

Discovery

Time-lapse images showing the movement of 2003 UB₃₁₃ (circled) against the stars.

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 trans-Neptunian objects, 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. However, when 90377 Sedna was discovered it was moving at 1.75 arcsec/h, and in light of that the team decided to reanalyze their old data with a lower limit on the angular motion, sorting through the false positives by eye. In January 2005, this 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.

Classification

This article or section contains information about scheduled or expected future events. It may contain information of a speculative nature and the content may change dramatically as the event approaches and more information becomes available.

Animation showing the movement of 2003 UB₃₁₃ on the images used to discover it. 2003 UB₃₁₃ is located on the left side, slightly above the middle of the image. The three frames were taken over a period of three hours.

At present, 2003 UB₃₁₃ is classified as a scattered disk object (SDO), 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. On August 16, 2006, the IAU announced its proposal to redefine the term planet to mean "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” Per this definition, 2003 UB₃₁₃ qualifies as a planet and further qualifies as a pluton, which includes planets whose orbit takes more than 200 Julian years to complete and whose orbit is more tilted and elliptical than a traditional planetary orbit. The IAU General Assembly is scheduled to vote on this proposed definition by August 24, 2006.^[3]

Name

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^[4] 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, may well force a deviation from adhering to the same steps, timelines and approval procedures as those that apply to garden-variety asteroids and comets. The IAU has released a short statement regarding the naming of 2003 UB₃₁₃, indicating the object will not be named until it has been decided if it is a planet or not.^[5]

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.^[6]

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.^{[citation needed]} On the other hand, the team has also claimed that they chose the name because "We have always wanted to name something Xena",^[7] 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."^[8]

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.^[9] 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.

Orbit

Position of 2003 UB₃₁₃ on 29 July 2005. On the left is the view from "above" the plane of the solar system, while on the right is the view from "in front". Darker blue indicates the part of the orbit below the ecliptic plane. Also shown are Saturn, Uranus, Neptune, and Pluto.

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 67.669 astronomical units, perihelion distance 37.78 AU, aphelion distance 97.56 AU although about forty known TNOs (most notably 2000 OO₆₇ and Sedna), while currently closer to the Sun than 2003 UB₃₁₃, have greater average orbital distances.^[10]

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 new 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.^[11] 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.

Distribution of trans-Neptunian Objects.

Size

Optical measurement from HST pictures

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,^[12] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.^[13]

2003 UB₃₁₃ compared to Pluto, 2005 FY₉, 2003 EL₆₁, Sedna, Quaoar, and Earth

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.

Thermal measures

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.^[14] 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 magnitude that 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^[15] the difference between the results would appear consistent with 1.1-σ 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.^[14]

Surface

The infrared spectrum of 2003 UB₃₁₃, 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 2003 UB₃₁₃ 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 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.

Moon

S/2005 (2003 UB₃₁₃) 1

2003 UB313 and satellite: artist's impression
Discovery
Discovered by	Michael E. Brown (using the Keck Observatory)
Discovered in	10 September 2005
Orbital characteristics
Semi-major axis (a)	30,000 - 36,000 km
Eccentricity
Orbital period (P)	~14 d
Inclination
Parent body	2003 UB313
Physical characteristics
Mean diameter	300 - 400 km

During 2005, the adaptive optics team at the Keck telescopes in Hawaiʻi 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.

The satellite is about 60 times fainter than 2003 UB₃₁₃, and its diameter is estimated to be approximately eight times smaller. Its orbital period is currently crudely estimated to be about two weeks (14 days), but further observations to be made with the Keck telescopes in August 2006 will allow a much better measurement on the period. Once astronomers refine the period and the semimajor axis of the satellite's orbit (currently estimated at 36,000 km^[16]), they will be able to determine the mass of the system.

Astronomers now know that three of the four brightest Kuiper belt objects (KBOs) have satellites, while among the fainter members of the belt only about 10% are known to have satellites. This is believed to imply that collisions between large KBOs have been frequent in the past. Impacts between bodies of the order of 1000 km across would throw off large amounts of material which would coalesce into a moon. A similar mechanism is believed to have led to the formation of Earth's own Moon when the Earth was struck by a giant impactor early in the history of the solar system.

See also

• Astronomical naming conventions
• Definition of planet (for the major points of debate on the definition of a "planet")
• Dwarf planet
• Titius-Bode Law
• International Astronomical Union (International body responsible for naming planets)
• Kuiper belt
• Minor planet
• Neptune
• Oort cloud
• Planet (for the generally accepted definition)
• Planet X
• Pluto
• Scattered disc (solar system region)
• Solar System
• Tenth planet
• Trans-Neptunian objects

Notes and references

• M. E. Brown, C. A. Trujillo, D. L. Rabinowitz Discovery of a Planetary-sized Object in the Scattered Kuiper Belt, The Astrophysical Journal, 635 (Dec. 2005), Issue 1, pp. L97-L100. Preprint on arXiv. DOI:10.1086/499336
• M. E. Brown, E.L. Schaller, H.G. Roe, D. L. Rabinowitz, C. A. Trujillo Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope To appear in The Astronomical Journal. Preprint (pdf) on Brown's site. DOI:10.1086/504843
• 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 Satellites of the largest Kuiper belt objects, The Astrophysical Journal (2006), 639, Issue 1, pp. L43-L46. Preprint on arXiv. DOI:10.1086/501524
• Gomes R. S., Gallardo T., Fernández J. A., Brunini A. (2005), On the origin of the High-Perihelion Scattered Disk: the role of the Kozai mechanism and mean motion resonances, Celestial Mechanics and Dynamical Astronomy, Vol. 91, pp. 109-129 DOI:10.1007/s10569-004-4623-y
• Bertoldi F., Altenhoff W., Weiss A., Menten K. M., Thum C. (2006). The trans-neptunian object UB₃₁₃ is larger than Pluto, Nature, 2 February 2006;439(7076):563-4 DOI:10.1038/nature04494
• M. E. Brown and C. A. Trujillo Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar The Astronomical Journal, 127 (2004), pages 2413–2417 DOI:10.1086/382513 Reprint (pdf) on Brown's site. Describing in detail the method applied to the recent measure of 2003 UB₃₁₃
• SpaceDaily Report on IAU Draft Definition of Planets and Plutons

External links

• Michael Brown's webpage about 2003 UB₃₁₃
• Brown's webpage about 2003 UB₃₁₃'s satellite
• compiled list of data
• MPEC listing for 2003 UB₃₁₃
• Java 3D orbit visualization
• Spaceflight Now article about 2003 UB₃₁₃, 2003 EL₆₁, and 2005 FY₉
• Slacker Astronomy Interview With Co-Discoverer Trujillo
• NASA - new planet discovered
• Trans-Neptunian Object 2003 UB₃₁₃ — IAU statement regarding the planetary status of 2003 UB₃₁₃
• Simulation of 2003 UB₃₁₃'s orbit — 2003 UB₃₁₃ is in a 17:5 resonance with Neptune
• Keck observatory page about the discovery of 2003 UB₃₁₃'s moon
• Parade Magazine on the spacecraft New Horizons
• M. E. Brown et al., Satellites of the largest Kuiper belt objects, Astrophysics Journal Letters, December 25, 2005 preprint

Observatories involved

• Keck Observatory, Hawaiʻi, US
• Palomar
• Gemini
• IRAM (Institut de Radio Astronomie Millimétrique Institut for Milimetric Radio Astronomy): French-German (Max-Planck Institut fur Radioastronomy, Bonn) and Spanish (where the 30m telescope is situated) collaborative programme.
• HST

Press releases

• Gemini Observatory Shows That "10th Planet" Has a Pluto-Like Surface
• Caltech Press Release, 7/29/2005 "Planetary Scientists Discover Tenth Planet".
• Jet Propulsion Laboratory News Release
• NASA – Press release on discovery of tenth planet
• Press release on the Spitzer Space Telescope trying to image 2003 UB₃₁₃ again
• Press release of redefined size from Max Planck Institute for Radio Astronomy.

News stories

Wikinews has news related to:

Bigger than Pluto, possible 10th planet found

• 'Tenth Planet' found to be a whopper - Nature (registration required)
• Tenth planet discovered in outer solar system - NewScientistSpace.com
• Astronomers Discover "10th Planet" - Sky & Telescope article
• Tenth Planet Discovered - Astronomy Magazine article
• LARGER THAN PLUTO! 10th Planet Discovered? - Planetary Society
• Tenth planet found! - spaceflightnow.com
• Object Bigger than Pluto Discovered, Called 10th Planet - space.com
• Astronomers detect '10th planet' - BBC News
• You Call That a Planet? How astronomers decide whether a celestial body measures up. - Slate.com
• Sky and Telescope article on the moon of 2003 UB₃₁₃ and the diameter estimate
• Keck Observatory – Gabrielle: a moon of Xena (2003 UB₃₁₃)
• VOA News - Scientists Find '10th Planet' Bigger Than Pluto)
• Downsizing the "Tenth Planet" -- Irion 2006 (127): 1 -- ScienceNOW
• Amateur astronomers make first sighting of '10th planet' through McDonald Observatory telescope
• Hubblesite:Revised diameter (April 2006)
• 10th Planet Slightly Larger than Pluto-Yahoo! News (April 11, 2006

The Solar System  v·d·e
Planets: Mercury - Venus - Earth - Mars - Jupiter - Saturn - Uranus - Neptune - Pluto
Other: Sun - The Moon - Asteroid belt - Comets - Kuiper belt - Scattered disc - Oort cloud
2006 redefinition of planet candidates: Ceres - Charon - 2003 UB313
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.