2003 UB313 is a trans-Neptunian object (TNO) discovered by astronomers at the Mount Palomar observatory in California, who have described it 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.
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 our solar system. Recent thermal observations by astrophysicists at IRAM have determined the object's diameter to be in the range of 2500 to 3500 km with a certainty of 68%. , while the discovery team has detected methane ice on the object's surface. Such observations make 2003 UB313 more similar to Pluto than any previously-known
A ruling on what to name 2003 UB313 is currently being delayed pending a decision on whether to promulgate a
formal definition of the term "planet" and the status of this object under such a definition. 2003 UB313's discoverers have confidentially submitted
potential names to the International Astronomical Union (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.
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, 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 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.
Animation showing the movement of 2003 UB313
on the images used to discover it. 2003 UB313
located on the left side, slightly above the middle of the image. The three frames were taken over a period of three hours.
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 because no consensus was reached.  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 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.
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 names has been to be inclusive of other cultures, including Native American and Irish folklore.
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, see ). 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 diameter of 2003 UB313 has only been inferred, not measured directly. Although estimates have in some cases varied greatly,
a consensus has emerged amongst various independent analyses that the object is indeed larger than Pluto.
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 the maximum albedo of 1.0 or 100%), it would still have to be about as large as Pluto (2306
km). In fact, its albedo is certainly 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 due to a technical glitch,  but new observations were made on August 23 and August 25, 2005 and are currently being analyzed.  Preliminary analyses indicated a size of 2,700 km (which is about 20% bigger than Pluto's 2,274 km diameter). Michael Brown
is quoted as saying "if it's not larger than Pluto, then I'll eat my telescope". 
To better determine 2003 UB313's radius, the discovery team was 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.
Initial HST observations, taken to measure diameter directly, are still being analysed. Preliminary results revealed by
Brown at a public talk at Foothill College in Los Altos Hills, CA suggested that 2003 UB313 has a high albedo of 92% and is thus only 1% larger than Pluto in diamenter (2300-2400
km). However, Brown has now stated that that measurement was wrong, citing that it was a "preliminary estimate." . Brown emphasizes that analysis of the Hubble data is still ongoing, with results expected by the end of February 2006. 
In a new development, an article in the February 2, 2006 issue of Nature by Bertoldi, Altenhoff, Weiss, Menten and Thum states that observations of the thermal emission of 2003 UB313
show it to have a diameter of about 3000 km ±300km ±100km. The first error margin relates to the measurements, the second
accounts for the unknown object orientation and rotation speed. The measurements were conducted in 1.2 mm wavelength where
the object brilliance depends only on its temperature and surface (diameter). .
The infrared spectrum of 2003 UB313
, compared to that of Pluto, shows the marked similarities between the
two bodies. Arrows denote methane
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 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.
Unofficial reports from the recent Hubble analysis indicate a high albedo, suggesting a surface made up of fresh ice. 
During 2005, the adaptive optics team at the Keck telescopes in Hawaii 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
are planned which 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 Mm ), they will be able to determine the mass of the system.
Astronomers now know that three of the four brightest Kuiper belt objects 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.