‘10th planet’ Xena has moon

The discovery of "Gabrielle" from Mauna Kea adds fuel to a debate over celestial definitions

WAIMEA, Hawaii » Astronomers on Mauna Kea who found a possible 10th planet on the edge of our solar system have made another discovery -- it has a moon.

Using the W.M. Keck Observatory, a team of astronomers spotted a faint object trailing next to it on Sept. 10.

The new planet has been temporarily nicknamed Xena after a "warrior princess" character on a television show, so the moon is being called Gabrielle, after a sidekick of the television warrior, the Keck Observatory said.

The planet Xena was first detected in 2003 and publicly announced on July 29 by a team that included Michael Brown of the California Institute of Technology and Chad Trujillo of Gemini Observatory.

It normally circles the sun farther away than the ninth planet, Pluto, but it sometimes comes inside Pluto's orbit.

Brown and Trujillo were part of another team that discovered the moon Gabrielle on Sept. 10.

"It is tiny compared to the primary (Xena), and much fainter," said Brown. "We have never seen satellites like this before."

The newly discovered moon likely will not quell the debate over what exactly is a planet and whether Pluto should keep its status as a planet. The problem is there is no official definition for a planet.

Possessing a moon is not a criterion of planethood, since Mercury and Venus are moonless.

Besides Xena and Pluto, other objects on the outskirts of the solar system have been dubbed Santa and Easterbunny until the International Astronomical Union can pick permanent names for them, the observatory said.

Although it is not considered a planet, Santa also has a tiny moon, but none has been found circling Easterbunny.

Since the moons are substantially smaller than Pluto's moon, Charon, astronomers are considering a new theory about how they formed. They may have been produced by objects hitting Xena and Santa, Brown said.

The moon discovery is also important because it can help scientists determine the new planet's mass.

By determining the moon's distance and orbit around Xena, scientists can calculate how heavy Xena is. For example, the faster a moon goes around a planet, the more massive the planet is.

Scientists expect to learn more about the moon's composition during further observations with the Hubble Space Telescope in November.

The moon is about 155 miles wide and 60 times fainter than Xena . It is currently 9 billion miles away from the sun, or about three times Pluto's current distance from the sun.