From Planetary Society and their Planetary News:
Hubble Measures Size of “10th Planet”
By Amir AlexanderMore so perhaps than any other object in the sky, for 2003UB313 size matters. When the giant space rock, nicknamed “Xena,” was first announced in July of 2005, it came with a startling assertion: it is larger than Pluto, the ninth and smallest solar planet. If this is the case, many argued that it should be recognized as a planet, the 10th orbiting our Sun, and the first discovered in 75 years.. It is precisely this possibility that has kept the International astronomical Union (IAU) debating the status and name of Xena for almost a year now, without reaching a definite conclusion. But if it is not as large as Pluto, then Xena would be relegated to the ranks of other giant Kuiper belt objects (KBO’s), such as Sedna and Quaoar, impressive and interesting in themselves, but not large enough to upset the nine-planet status quo.
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Studies of Xena’s spectrum currently being conducted at the Gemini observatory in Hawaii may help uncover some of the mysteries of Xena, or it may be years before planetary scientists learn more about this remarkable new member of our Solar System. But if the latest measurement holds, the day may not be far when the official count of planets around the Sun will move up by one, to 10.