Scientists have discovered the 10th planet, 2003 UB313 (codenamed Xena), even has a moon.
The moon, 100 times fainter than Xena and orbiting the planet once every couple of weeks, was spotted on September 10, 2005, with the 10-meter Keck II telescope at the W.M. Keck Observatory in Hawaii by Michael E. Brown, professor of planetary astronomy, and his colleagues at Caltech, the Keck Observatory, Yale University, and the Gemini Observatory in Hawaii. A paper about the discovery was submitted on October 3 to Astrophysical Journal Letters.
Currently the moon is nicknamed Gabrielle and researchers estimate the planet is about one-tenth the size of Xena, which is believed to be about 2700km in diameter, and may be 250km across. The 9th planet, Pluto, has a diameter of 2274km. Read on over here.