Eris (dwarf planet)

Template:Minor Planet 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.

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. At this time, a ruling on what to name 2003UB₃₁₃ is being withheld pending decisions on the definition of planet and the status of this object under that definition. This is unlikely to happen before 2006.

The diameter of 2003UB₃₁₃ is undetermined, but recent observations by the Spitzer space telescope should be able to set an upper bound. Estimates now 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 2003UB₃₁₃ more similar to Pluto than previously discovered large outer solar system planetoids.

Discovery

File:2003 UB313 discovery image.jpg

Time-lapse images showing the movement of 2003 UB₃₁₃ 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 Kuiper Belt objects, 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 31, 2003 using the 48-inch Samuel Oschin reflecting telescope at Mount Palomar Observatory, California, but the object captured on the images was not discovered until January 2005, when further images of the same field revealed its slow motion against the background stars. Follow-up observations then allowed 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.

Classification

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.

2003 UB₃₁₃ is classified as a scattered disk object (SDO), a category of TNOs which are believed to have been "scattered" from the main portions of the Kuiper belt and into more distant and unusual orbits following gravitational interactions with Neptune. It is currently the most distant known solar system object at a distance of 97 astronomical units from the Sun, shortly after its aphelion. Approximately forty known TNOs (most notably 2000 OO₆₇ and Sedna), while currently closer to the sun than 2003 UB₃₁₃, have greater average orbital distances. See the "Orbit" section below for more details.

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 Kuiper Belt objects 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 [1], 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 [2].

The president of the IAU's working group to define the term planet has proposed that Pluto keeps its present classification for historical reasons, and that nothing else be named a planet [3]. This view is shared by at least one other member of the group [4]

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 of the IAU's Division III. According to the IAU rules, Kuiper belt objects 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 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 dropped hints on his website on what their name proposal had been:

"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 399^th 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." [7]

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 [8] 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.

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 557 years, and currently lies at almost its maximum possible distance from the Sun (aphelion), about 97 astronomical units away from the Earth. 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. This means 2003 UB₃₁₃ travels almost as great a "vertical" distance as it does relative to what has come to be defined as "horizontal."

The new object is bright enough with an apparent magnitude of about 19 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 would be able to 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.

Size

File:2003 UB313 artist's impression.jpg

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 probably less than 1.0, and so the new object is likely to be somewhat 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 [9], so estimates of an upper bound of around 5000 km have not been ruled out. The observations were rescheduled for August 25 and August 26 2005 soon after the failure was recognized.

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.

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 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 Kuiper Belt object 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, 2003 UB₃₁₃ appears almost grey. It is not yet known why the colours are so different.

Methane is very volatile and its presence shows that 2003 UB₃₁₃ has always resided in the distant reaches of the solar system where it is cold enough for methane ice to persist. This contrasts with observations of another recently discovered Kuiper Belt object, 2003 EL61, which reveal the presence of water ice but not methane.

External links

• MPEC listing for 2003 UB₃₁₃
• Java 3D orbit visualization
• Michael Brown's webpage about 2003 UB₃₁₃
• 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

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

News stories

• Tenth planet discovered in outer solar system — NewScientistSpace.com
• Astronomers Discover "10th Planet" - Sky & Telescope 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
• Astronomers to decide what makes a planet - Nature magazine web site (nature.com)
• You Call That a Planet? How astronomers decide whether a celestial body measures up. - Slate.com
• No rules for naming a planet - Pasadena Star News

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