2003 UB313 Xena

2003 UB313 (also written 2003 UB313) is a trans-Neptunian object (TNO) which California astronomers at Mount Palomar observatory describe as "definitely bigger" than the planet Pluto. The object has already been dubbed the tenth planet by the discoverers, NASA, and some media outlets, but it is not yet clear whether it will be widely accepted as a new planet or not. It has at least one moon.

No official name for the object has yet been approved, although its discoverers have submitted a potential name to the International Astronomical Union (IAU), who oversee astronomical naming conventions. Claims that 2003 UB313 has been named 'Xena' or 'Lila' are incorrect; both have been used informally by its discoverers but neither is the name submitted to the IAU.

A ruling on what to name 2003 UB313 is currently being delayed pending decisions on whether to promulgate a formal definition of the term 'planet' and the status of this object under such a definition.The diameter of 2003 UB313 is undetermined, but recent observations by the Spitzer space telescope should be able to set an upper bound soon.

Presently, estimates range from 2,390 km to 5,000 km or more. Initial observations show that methane ice is present on the object's surface. This makes 2003 UB313 more similar to Pluto than previously discovered large outer solar system planetoids.


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, and 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/hour, and in light of that the team decided to re-analyse 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 apparently forced to bring forward the announcement when they learned that word of the discovery had leaked out and might be announced by someone else.


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 appears to be larger than Pluto, it might come to be considered as the tenth planet in the Solar System, and was initially described as such by NASA and in media reports of its discovery. However, this is not a given, since the status of Pluto as a planet has been subject to debate for some time. Some astronomers believe that there are large numbers of undiscovered TNOs as large as or larger than Pluto. Classifying all of them as planets is seen as unwieldy by many.

The IAU has been reviewing the definition of the term 'planet' because of the increasing expectation that something bigger than Pluto would be found. The IAU was expected to move quickly to promulgate a definition, but this is now uncertain. Until this definition is available, the IAU will continue to regard all objects discovered at a distance greater than 40 AU as part of the general Trans-Neptunian population.

The president of the IAU's working group to define the term planet has proposed that Pluto keep its present classification for historical reasons, and that nothing else be named a planet. This view is shared by at least one other member of the group.

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, 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.

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 discovery web page URL uses the name "Planet Lila" (named after Michael Brown's newborn daughter, Lilah), and the team have also been informally referring to the object by the codename "Xena", after the television series Xena: Warrior Princess, but neither is the name put forward to the IAU.

Two days after announcing the discovery, Brown discussed his team's ideas about naming the objects on his website:


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 557 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 97 astronomical units, 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.

Like Pluto, its orbit is highly eccentric, and brings it to within 35 AU of the Sun at its perihelion (Pluto's distance from the Sun varies between 29 and 49.5 AU, while Neptune orbits at just over 30 AU).

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 (for an example of an amateur image of 2003 UB313, se).

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.


An artist's impression of the view towards the Sun from near 2003 UB313.The brightness of a solar system object depends both on its size and the amount of light it reflects (its albedo). If the distance to an object and its albedo are both known, its radius can easily be determined from its apparent magnitude, with a higher albedo implying a smaller radius.

Currently, the albedo of 2003 UB313 is unknown, and so its true size cannot yet be determined. However, astronomers have calculated that even if it reflected all the light it receives (corresponding to a maximum albedo of 1.0), it would still have to be about as large as Pluto (2390 km).

In fact, its albedo is less than 1.0, so the new object is likely to be larger than Pluto.Spitzer space telescope observations should provide an upper bound on the size of 2003 UB313.

A first round of observations failed to detect the new object, a result which was initially reported as indicating an upper size limit of about 3500 km, but was later found to be due to a technical glitch, so estimates of an upper bound of around 5000 km have not been ruled out. New observations were made on August 23 and August 25, 2005 and are currently being analyzed.

To better determine 2003 UB313's radius, the discovery team have been awarded observing time on the Hubble Space Telescope. At a distance of 97 AU, an object with a radius of about 3000 km would have an angular size of about 40 milliarcseconds, which is directly measurable with HST: although resolving such small objects is right at the limit of Hubble's capabilities, sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately. The team previously applied this technique to 50000 Quaoar, using the Advanced Camera for Surveys to directly measure its radius.


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 Hawaii 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 colour is believe 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.

References and Links Wikipedia

Articles in the News

Pluto is now 134340 - Xena is now Eris National Geographic - September 16, 2006 Hubble Finds the 'Tenth Planet' is Slightly Larger than Pluto PhysOrg - April 11, 2006

Distant world tops Pluto for size BBC - February 1, 2006

'Planet Xena' has a sidekick: Gabrielle, its moon BBC - October 3, 2005