Tenth Planet Has A Moon

The moon, 100 times fainter than Xena and orbiting the planet onceevery couple of weeks, was spotted on September 10, 2005, with the10-meter Keck II telescope at the W.M. Keck Observatory in Hawaii byMichael E. Brown, professor of planetary astronomy, and his colleaguesat Caltech, the Keck Observatory, Yale University, and the GeminiObservatory in Hawaii. A paper about the discovery was submitted onOctober 3 to Astrophysical Journal Letters.

"Since the day we discovered Xena, the big question has been whetheror not it has a moon," says Brown. "Having a moon is just inherentlycool-and it is something that most self-respecting planets have, so itis good to see that this one does too."

Brown estimates that the moon, nicknamed "Gabrielle"-after thefictional Xena's fictional sidekick-is at least one-tenth of the sizeof Xena, which is thought to be about 2700 km in diameter (Pluto is2274 km), and may be around 250 km across.

To know Gabrielle's size more precisely, the researchers need toknow the moon's composition, which has not yet been determined. Mostobjects in the Kuiper Belt, the massive swath of miniplanets thatstretches from beyond Neptune out into the distant fringes of the solarsystem, are about half rock and half water ice. Since a half-rock,half-ice surface reflects a fairly predictable amount of sunlight, ageneral estimate of the size of an object with that composition can bemade. Very icy objects, however, reflect a lot more light, and so willappear 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 colleaguesto pin down Gabrielle's exact orbit around Xena. With that data, theywill be able to calculate Xena's mass, using a formula first devisedsome 300 years ago by Isaac Newton.

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

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

"With Laser Guide Star Adaptive Optics, observers not only get moreresolution, but the light from distant objects is concentrated over amuch smaller area of the sky, making faint detections possible," saysMarcos 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 asmall moon in January around 2003 EL61, code-named "Santa," anotherlarge new Kuiper Belt object. No moon was spotted around 2005 FY9-or"Easterbunny"-the third of the three big Kuiper Belt objects recentlydiscovered by Brown and his colleagues using the 48-inch Samuel OschinTelescope at Palomar Observatory. But the presence of moons aroundthree of the Kuiper Belt's four largest objects-Xena, Santa, andPluto-challenges conventional ideas about how worlds in this region ofthe solar system acquire satellites.

Previously, researchers believed that Kuiper Belt objects obtainedmoons through a process called gravitational capture, in which twoformerly separate objects moved too close to one another and becomeentrapped in each other's gravitational embrace. This was thought to betrue of the Kuiper Belt's small denizens-but not, however, of Pluto.Pluto's massive, closely orbiting moon, Charon, broke off the planetbillions of years ago, after it was smashed by another Kuiper Beltobject. Xena's and Santa's moons appear best explained by a similarorigin.

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

For more information on the discovery and on Xena, visit www.gps.caltech.edu/~mbrown/planetlila