(Possible) Planet Xena Has Moon Named "Gabrielle"

I kid you not.

WASHINGTON (Reuters) - Xena, the possible 10th planet in our solar system, has its own moon, a dim little satellite called Gabrielle, its discoverers reported.

Astronomers who reported Xena's discovery in July said they detected Xena's sidekick on September 10 using the Keck Observatory on Mauna Kea in Hawaii. Their findings will be submitted to the Astrophysical Journal Letters on Monday.

"Since the day we discovered Xena, the big question has been whether or not it has a moon," Michael Brown, of the California Institute of Technology, said in a statement. "Having a moon is just inherently cool -- and it is something that most self-respecting planets have, so it is good to see that this one does too."

Xena, known formally as 2003 UB313 but nicknamed for the warrior princess of television fame, and Gabrielle orbit the sun out beyond Pluto in a band known as the Kuiper Belt, a swath that is home to comets, asteroids and other space rocks.

The possible 10th planet moves in a highly eccentric orbit, tilted some 45 degrees above the orbital plane of the other planets. Its orbit is also elliptical, zooming in as close as 3.5 billion miles (5.6 billion km) from the sun and moving out to as far as 9 billion miles (14.5 billion km) away.

Earth orbits rather consistently at 93 million miles (150 million km) from the sun.
It takes Xena 560 Earth years to complete one trip around the Sun, compared to Pluto's 250 years.

Xena is one of three big planet-like bodies recently found in this region. The others have equally playful nicknames: Santa and Easterbunny.

A PLANET - OR NOT?



Size is important when it comes to making the grade as a planet. Astronomers know that Xena is bigger than Pluto but since they don't know what it is made of, they can't be sure that it is more massive. The discovery of the moon Gabrielle means Xena has at least enough mass to keep a satellite.

Gabrielle is estimated to orbit close to Xena, making a circuit perhaps every 14 days. Named for the TV princess's travelling companion, Gabrielle is about 60 times fainter than Xena.

The International Astronomical Union, which makes the decision on what is a planet, considers Xena a trans-Neptunian object, meaning its orbit crosses that of Neptune, just as Pluto's does. Many astronomers, including Brown, question Pluto's planetary status, too.

But Xena's discovery, and its size, have prompted the union to rethink the definition of planet.
On the union's Web site, it said: "The very rapid pace of discovery of bodies within the solar system over the last decade, and so our understanding of the Trans-Neptunian Region is therefore still evolving very rapidly. This is in serious contrast to the situation when Pluto was discovered."

A working group of the union is considering a new definition. Until the group finishes its work, the Web site statement said, all objects discovered at a distance of 40 times Earth's distance from the sun, "will continue to be regarded as part of the Trans-Neptunian population."

More information and images are available at http://www.gps.caltech.edu/(tilde)mbrown/planetlila/moon/index.html.