Moon Discovered Orbiting Solar System's 10th Planet (2003 UB313)

http://www.spaceflightnow.com/news/n0510/02xenamoon/

Moon discovered orbiting solar system's 10th planet
CALIFORNIA INSTITUTE OF TECHNOLOGY NEWS RELEASE
October 2, 2005

The newly discovered 10th planet, 2003 UB313, is looking more and more
like one of the solar system's major players. It has the heft of a real
planet (latest estimates put it at about 20 percent larger than Pluto),
a catchy code name (Xena, after the TV warrior princess), and a
Guinness
Book-ish record of its own (at about 97 astronomical units-or 9 billion
miles from the sun-it is the solar system's farthest detected object).
And, astronomers from the California Institute of Technology and their
colleagues have now discovered, it has a moon.

[Image]
The discovery of the moon of the 10th planet from the W.M. Keck
Observatory. The planet appears in the center, while the moon is the
small dot at the 3 o'clock position. Credit: W.M. Keck Observatory

The moon, 100 times fainter than Xena and orbiting the planet once
every
couple of weeks, was spotted on September 10, 2005, with the 10-meter
Keck II telescope at the W.M. Keck Observatory in Hawaii by Michael E.
Brown, professor of planetary astronomy, and his colleagues at Caltech,
the Keck Observatory, Yale University, and the Gemini Observatory in
Hawaii. A paper about the discovery was submitted on October 3 to
Astrophysical Journal Letters.

"Since the day we discovered Xena, the big question has been whether or
not it has a moon," says Brown. "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."

Brown estimates that the moon, nicknamed "Gabrielle"-after the
fictional
Xena's fictional sidekick-is at least one-tenth of the size of Xena,
which is thought to be about 2700 km in diameter (Pluto is 2274 km),
and
may be around 250 km across.

To know Gabrielle's size more precisely, the researchers need to know
the moon's composition, which has not yet been determined. Most objects
in the Kuiper Belt, the massive swath of miniplanets that stretches
from
beyond Neptune out into the distant fringes of the solar system, are
about half rock and half water ice. Since a half-rock, half-ice surface
reflects a fairly predictable amount of sunlight, a general estimate of
the size of an object with that composition can be made. Very icy
objects, however, reflect a lot more light, and so will appear
brighter-and thus bigger-than similarly sized rocky objects.

Further observations of the moon with the Hubble Space Telescope,
planned for November and December, will allow Brown and his colleagues
to pin down Gabrielle's exact orbit around Xena. With that data, they
will be able to calculate Xena's mass, using a formula first devised
some 300 years ago by Isaac Newton.

"A combination of the distance of the moon from the planet and the
speed
it goes around the planet tells you very precisely what the mass of the
planet is," explains Brown. "If the planet is very massive, the moon
will go around very fast; if it is less massive, the moon will travel
more slowly. It is the only way we could ever measure the mass of
Xena-because it has a moon."

[Image]
Artist's concept of the 10th planet and its moon. The sun and other
planets appear in the distance. Credit: R. Hurt, IPAC

The researchers discovered Gabrielle using Keck II's recently
commissioned Laser Guide Star Adaptive Optics system. Adaptive optics
is
a technique that removes the blurring of atmospheric turbulence,
creating images as sharp as would be obtained from space-based
telescopes. The new laser guide star system allows researchers to
create
an artificial "star" by bouncing a laser beam off a layer of the
atmosphere about 75 miles above the ground. Bright stars located near
the object of interest are used as the reference point for the adaptive
optics corrections. Since no bright stars are naturally found near
Xena,
adaptive optics imaging would have been impossible without the laser
system.

"With Laser Guide Star Adaptive Optics, observers not only get more
resolution, but the light from distant objects is concentrated over a
much smaller area of the sky, making faint detections possible," says
Marcos van Dam, adaptive optics scientist at the W.M. Keck Observatory,
and second author on the new paper.

The new system also allowed Brown and his colleagues to observe a small
moon in January around 2003 EL61, code-named "Santa," another large new
Kuiper Belt object. No moon was spotted around 2005 FY9-or
"Easterbunny"-the third of the three big Kuiper Belt objects recently
discovered by Brown and his colleagues using the 48-inch Samuel Oschin
Telescope at Palomar Observatory. But the presence of moons around
three
of the Kuiper Belt's four largest objects-Xena, Santa, and
Pluto-challenges conventional ideas about how worlds in this region of
the solar system acquire satellites.

Previously, researchers believed that Kuiper Belt objects obtained
moons
through a process called gravitational capture, in which two formerly
separate objects moved too close to one another and become entrapped in
each other's gravitational embrace. This was thought to be true of the
Kuiper Belt's small denizens-but not, however, of Pluto. Pluto's
massive, closely orbiting moon, Charon, broke off the planet billions
of
years ago, after it was smashed by another Kuiper Belt object. Xena's
and Santa's moons appear best explained by a similar origin.

"Pluto once seemed a unique oddball at the fringe of the solar system,"
Brown says. "But we now see that Xena, Pluto, and the others are part
of
a diverse family of large objects with similar characteristics,
histories, and even moons, which together will teach us much more about
the solar system than any single oddball ever would."