’10th planet’ has a partner

Whether or not astronomers decide to classify a recently discovered solar system body as the 10th planet, this icy object now has a feature common to most other planets. Researchers announced Sept. 30 that the body, officially dubbed 2003 UB313 but nicknamed Xena, has a moon.

The newfound moon has a diameter about one-tenth that of Xena, which the scientists estimate is about 2,700 kilometers wide (SN: 8/6/05, p. 83: Bigger than Pluto: Tenth planet or icy leftover?). Xena codiscoverer Mike Brown of the California Institute of Technology in Pasadena and his colleagues spotted the small moon using the Keck II telescope atop Hawaii’s Mauna Kea. They relied on a recently completed laser system on the telescope that removes the blurring caused by Earth’s atmosphere. The images were as clear as they would be if taken from space.

Brown’s team plans to use the Hubble Space Telescope later this fall to track the orbit of the moon, which they call Gabrielle, named for the fictional Xena’s sidekick on the television show Xena: Warrior Princess. The researchers intend to use the Gabrielle data to precisely calculate the weight of Xena, which now ranks as the most distant body known in the solar system.

Brown and his team also used the Keck laser-assisted system to identify a small moon around another outer-solar-system object that they had previously found, known both as 2003 EL61 and Santa. This object, Xena, and Pluto, appear to be part of a family of entities with similar features and histories, Brown says.