Kuiper Belt:
In 1950, Dutch astronomer Jan Oort proposed that certain comets came from a vast spherical shell of icy bodies near the edge of the Solar System. This giant swarm of objects is now named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One astronomical unit, or AU, is the mean distance of Earth from the Sun: about 150 million kilometers or 93 million miles.)
The Oort Cloud contains billions of icy bodies in solar orbit. Occasionally, passing stars disturb the orbit of one of these bodies, causing it to come streaking into the inner solar system as a long-period comet. These comets have very large orbits and are observed in the inner solar system only once. In contrast, short-period comets take less than 200 years to orbit the Sun and they travel along the plane in which most of the planets orbit. They come from a region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper, who proposed its existence in 1951.
The Kuiper Belt, extending out to about 50 AU around the Sun, is populated with thousands of small icy bodies.
]In 1992, astronomers detected a reddish speck about 42 AU from the Sun-- the first time a Kuiper Belt object (or KBO for short) had been sighted. More than 1,000 KBOs have been identified since 1992. (They are sometimes called Edgeworth Kuiper Belt objects, acknowledging another astronomer who also is credited with the idea, or they are simply called Trans-Neptunian Objects (TNOs.)
The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.
The boundary between (major) planet and minor planet has never been defined and the recent discovery of other "Trans-Neptunian Objects" (TNOs), including some larger than Pluto, triggered the IAU to form a Working Group on "Definition of a Planet" from its Division III members.
Quaoar and Orcus
One of the largest KBOs is Quaoar (2002 LM60), named by its discoverers after the mythical creation-force figure of the Tongva tribe of the Los Angeles basin. Quaoar orbits the Sun every 288 years about a billion miles beyond the orbit of Pluto (somewhere around 42 AU). Quaoar was photographed in 1980, but was not recognized as a KBO until 2002, by Astronomer Mike Brown and his colleagues at Caltech in Pasadena, California.
Quaoar is about 1250 km in diameter, roughly the size of Pluto's moon Charon. Nothing larger has been found in our solar system since Pluto was discovered in 1930 (and Pluto's moon Charon in 1978). It's huge. In fact, if you took the 50,000 numbered asteroids and put them together, it would be about the same volume as Quaoar.
An even larger KBO (2004 DW, now officially named Orcus) was found at a distance of about 45 AU from the Sun.
2005 FY9, codenamed "Easterbunny," is a very large Kuiper belt object discovered on March 31, 2005 by the team led by Mike Brown at Caltech. Its discovery was announced on July 29, 2005 on the same day as two other very large trans-Neptunian objects (TNOs), 2003 EL61 and 2003 UB313, now officially known as Eris.
2005 FY9
2005 FY9 is still awaiting its official name by the IAU. Detected by the Spitzer Space Telescope, initial estimates gave 2005 FY9 a diameter of 50% to 75% that of Pluto. It is similar in size to 2003 EL61, although somewhat brighter. This makes it the largest known Kuiper belt object after 2003 UB313 and Pluto.
The object orbits the Sun every 308 years. Like Pluto's, its orbit is somewhat eccentric and inclined.
2003 EL61
2003 EL61 is yet another object in the Kuiper Belt, discovered by Mike Brown and his team at Caltech. EL61 is also located in the region of space beyond Neptune that includes Pluto and the large planetoids Quaoar and Orcus, 2005 FY9, and the planet 2003 UB313, among others. 2003 EL61 is currently the third brightest object in this region after Pluto and 2005 FY9. It is so bright that it can readily be seen by high-end amateur telescopes equipped with CCD cameras. Other than being extremely bright, 2003 EL61 appeared at first to be typical of a type of Kuiper belt objects that astronomers call "scattered Kuiper belt objects." They are called "scattered" because it is believed that they once had a close encounter with Neptune, which gravitationally "scattered" these objects onto more eccentric orbits. The mass of 2003 EL61 is about 32% that of Pluto.
Sedna
In March 2004, a team of astronomers announced the discovery of a planet-like object, or planetoid, orbiting the Sun at an extreme distance, in the coldest known region of our solar system. Mike Brown, along with Doctors Chad Trujillo of the Gemini Observatory in Hawaii and David Rabinowitz of Yale University, New Haven, Conn., originally found the "planetoid" on November 14, 2003, using the 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory near San Diego. Within days, the object was observed by telescopes in Chile, Spain, Arizona and Hawaii, and soon after, NASA's new Spitzer Space Telescope looked for it.
The planetoid (2003 VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the Sun only briefly during its 10,500-year solar orbit. Sedna is about one-quarter to three-eighths the size of the planet Pluto. At the farthest point in its long, elliptical orbit, Sedna is 130 billion kilometers (84 billion miles) from the Sun - that's about 86 AU, compared with the mean distances of Neptune (about 30 AU) and Pluto (about 39 AU).
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The discoverers of Sedna describe it as an inner Oort Cloud object, because it never enters the Kuiper Belt. Sedna never comes closer to the Sun than 76 AU. Sedna is quite an oddity: nobody expected to find an object like it in the largely empty space between the Kuiper Belt and the Oort Cloud. Possibly the Oort Cloud extends much farther in toward the Sun than previously thought, or perhaps Sedna is yet another type of object from the very early solar system, trapped between the Kuiper Belt and the Oort Cloud. Other notable features of Sedna include its size and reddish color; it is the second reddest object in the solar system, after Mars. At an estimated size of three-fourths the size of Pluto, it is likely the largest object found in the solar system since Pluto was discovered in 1930. Sedna lies extremely far from the Sun, in the coldest known region of our solar system, where the temperature never rises above minus 240 degrees Celsius (minus 400 Fahrenheit).
The KBO is usually even colder because it approaches the Sun this closely only briefly during its 10,500 year orbit around the Sun. At its most distant, "Sedna" is 130 billion kilometers (84 billion miles) from the Sun. That is 900 times Earth's distance from the Sun.
Scientists used the fact that even the Spitzer telescope was unable to detect the heat of the extremely distant, cold object to determine that it must be no more than 1,700 kilometers (about 1,000 miles) in diameter, smaller than Pluto. By combining all available data, Brown estimates the size at about halfway between that of Pluto and Quaoar, the planetoid discovered by the same team in 2002. Until "Sedna" was detected, Quaoar was the largest known body beyond Pluto.
Because KBOs are so distant, their sizes are difficult to measure. The given diameter of a KBO depends on assumptions about how its brightness relates to its size. To estimate size based on brightness, one assumes what percentage of sunlight the object's surface reflects; this percentage is known as the albedo. Thinking that the albedo of an average KBO is similar to that of comets, astronomers calculated the sizes of KBOs based on the reflectivity of comets, which is about 4 percent. An efficient way to calculate an object's albedo is to measure the heat it radiates in the infrared. In 2004, astronomers using the Spitzer Space Telescope did a survey of KBOs at infrared wavelengths and found that they averaged about 12 percent; thus, KBOs might be smaller objects than astronomers originally thought. However, new discoveries may alter this perception.
![Credit: NASA, ESA, and A. Feild [STScI]](https://webarchive.library.unt.edu/eot2008/20080917173121im_/http://solarsystem.nasa.gov/planets/images/inset-kbos4.jpg)
