Xena has a moon (called Gabrielle)

More on the solar system's 10th planet
By Lucy Sherriff

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.

http://www.theregister.co.uk/2005/10/03/xena_has_a_moon/