The definition of what is and what is not a planet is being challenged by a newly announced discovery Friday. Astronomers from the Palomar Observatory near San Diego, California said they found a chunk of ice and rock in the solar system larger than Pluto.
The object, given the temporary generic name of 2003 UB313, is about 9 billion miles away from the sun. It was first photographed in 2003 using the Gemini Observatory in Hawaii. But it was an indistinguishable bright spot among a field of stars in an area of the sky where astronomers don't usually look at for planets or planet-type objects.
Solar system bodies found beyond the orbit of Neptune are called "trans-Neptunian objects". One of them, Pluto, is also classified as a planet. Up until now all other planet-like discoveries beyond Neptune, including Sedna, found by the same Palomar team in 2004, are called Kuiper Belt objects, or minor planets.
Using a photo taken on January 8, the Palomar team compared it with previous 2003 UB313 photos to triangulate the object's distance, brightness and orbit.
Using mathematical formulas, they found a range of sizes for 2003 UB313, with an estimated average showing the approximate diameter of the new body to be about 2,600 kilometers. But even at the smallest estimate scientists are certain 2003 UB313 is larger than Pluto's 2,250 kilometer diameter.
To determine size, Scientists deduce the relative size of a solar system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of 2003 UB313 is not yet known for sure. Scientists can not yet tell how much light from the sun is reflected away from its surface, but the amount of light the planet does reflect puts a lower limit on its size.
"Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto," Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology said in a NASA press release. "I'd say it's probably one and a half times the size of Pluto, but we're not sure yet of the final size."
"We are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system," Brown added.
The object is inclined by a whopping 45 degrees to the main plane of the solar system, where most of the other planets orbit. That's why it eluded discovery: nobody was looking there until now, Brown said.
Most objects in the solar system are just a few degrees from the main plane of the solar system, called the Ecliptic.  Pluto, which is considered substantially different from the rest of the planets, is only 17 degrees from this main plane.  Astronomers who study the sky for asteroids and comets usually look in the region of the sky that the rest of the planets occupy.
The brightness of this object is just barely fainter than the telescopes from earlier sky surveys were able to identify, so new advances in the making of telescopes has also played a part in this discovery.
Now that the discovery data has been released, the scientists will turn over their work to the International Astronomical Union (IAU), which will review it, classify the object and approve a new name to replace the temporary 2003 UB313. 
Sometimes the discoverers of a new planetary body get to choose its name, but not always. "We have a name we really like, and we want it to stick," Brown told reporters at a press conference. He said the team has submitted the name to the IAU but would not disclose their choice. In the 2004 naming of Sedna, Brown noted objects found in the Kuiper Belt are usually named after deities of the Underworld. 
The astronomers have been using a code name of sorts for the discovery. The tongue-in-cheek nickname  is "Xena" after the fictional main character of the television series of the same name. "Because we always wanted to name something Xena," Dr. Brown said in a New York Times report.
There has been controversy in the scientific community about what is and what is not a planet. Some purists insist the Solar system has only eight planets and that Kuiper Belt object including Pluto, Sedna and 2003 UB313 cannot be listed among the major planets.
A minor furor erupted when the IAU suggested removing Pluto from the pantheon of planets a few years ago. Because of that Pluto's status as a "planet" in addition to being a Kuiper Belt object was cemented. But with the discovery of 2003 UB313 the "planet or not" debate may rekindle.
"Pluto has been a planet for so long that the world is comfortable with that," Brown said in the teleconference. "It seems to me a logical extension that anything bigger than Pluto and farther out is a planet."
Offering additional justification, Brown said 2003 UB313 appears to be surfaced with methane ice, as is Pluto. That's not the case with other large Kuiper Belt objects, however.
"This object is in a class very much like Pluto," he said.
The new "10th planet" was not the only new discovery in the Kuiper Belt this past week. José-Luis Ortiz at the Sierra Nevada Observatory in Spain announced the discovery of an object called 2003 EL61. This object, thought to be larger than Sedna, is the brightest trans-Neptune object next to Pluto.
Although some news media confused the discovery of 2003 EL61 with 2003 UB313, they are different objects.
2003 EL61 is so bright, it can be seen with high-end amateur-grade telescopes equipped with CCD cameras. Although this second discovery may not be as large as 2003 UB313, scientists have confirmed that 2003 EL61 has its own moon.
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Bigger than Pluto, possible 10th planet found