Astronomers at the WM Keck Observatory have identified a moon orbiting Xena, a body they argue is the 10th planet in our solar system. They have called the moon Gabrielle, after Xena's sidekick in the TV series Xena: Warrior Princess.

Regular readers may remember that Xena's (formally 2003 UB313) discovery reignited an old debate about what exactly ought to be classified as a planet. There is also some debate over who found the planet first, but we will not revisit that here because for astronomers, the important question has always been whether or not Xena has a moon.

The California Institute of Technology's Michael Brown explains that although observations can show that Xena is physically larger than Pluto, without a companion body, it would be impossible to tell whether or not it is more massive.

"Finding a moon...allows us to precisely measure the mass of the planet. A more massive planet will pull on the moon tightly and it will circle the planet more quickly," he writes.

"A less massive planet will allow the moon to have a slow lazy orbit around the planet. We don't yet know the speed of the moon, but when we do we will suddenly have new insight into the size and even composition of the 10th planet."

The discovery could also shed new light on the history of the solar system. Several of the larger Kuiper belt objects, of which Xena is one, have moons, but how they acquired them is an open question.

The astronomical community has also yet to settle the question of whether Xena is a planet, and indeed, whether or not Pluto should be given that status. Many astronomers feel that Pluto ought more properly to be classed as a minor planet, or even just as a large Kuiper belt object.

Xena's discovery has prompted the International Astronomical Union to reconsider its definition of a planet. Currently it considers both Pluto and Xena to be trans-Neptunian objects, and says that until it has drawn up its new definition, that is what they will remain. ®