October 2, 2005
Reported 10th planet Xena has moon named Gabrielle
By Deborah Zabarenko
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 from the sun and moving out to as far as 9
billion miles away.
Earth orbits rather consistently at 93 million miles from
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
traveling companion, Gabrielle is about 60 times fainter than
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