Planet Xena - 2003 UB313

February 2, 2006

Scientists specify size of ‘10th planet’

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Meanwhile, 2003 UB313 is now known to be about 1,860 miles (3,000 kilometers) in diameter, give or take 190 miles (300 kilometers).

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Pluto is 1,430 miles (2,300 kilometers) wide.

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But 2003 UB313 is much farther away. Its elongated orbit takes it far out into the icy Kuiper Belt, twice as far from the sun as Pluto. Many astronomers now say Pluto is a Kuiper Belt Object and should never have been called a planet.

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So if 2003 UB313 is termed a planet, as some suggest, then a handful of other good-sized, round worlds known to exist — and perhaps hundreds yet to be found in the Kuiper Belt — would also have to be called planets. Among the other candidates: Sedna, which is about three-fourths as large as Pluto, 2004 DW and Quaoar.

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One suggestion is to call the outer worlds “dwarf planets.”

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Scott Sheppard of the Carnegie Institute of Washington takes this view:

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“Whichever way you care to count them, with the discovery and measurement of the size of 2003 UB313 there are no longer nine major planets in the solar system,” Sheppard writes in an analysis for Nature.