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Xena Poses A Bright Mystery

Gif animation of Xena based on the original tracking images. More images and captions at Mike Brown's page at Caltech.
by Phil Berardelli
SpaceDaily US Editor
Pasadena CA (SPX) Apr 13, 2006
Xena, the newly found and unofficial tenth planet, poses quite a mystery for astronomers. The latest images taken by the Hubble Space Telescope and interpreted by astronomers show Xena - whose name also remains unofficial - is about 5 percent larger in diameter than Pluto - which is smaller than expected based on earlier images.

Mike Brown, the planetary scientist at California Institute of Technology who co-discovered Xena last year, said because the Hubble measurement shows it is about 2,400 kilometers (1,500 miles) in diameter, its size, distance from Earth and apparent brightness mean Xena must reflect 86 percent of the sunlight reaching it.

"This makes it more reflective than the nine known planets, and more reflective than everything else other than (Saturn's moon) Enceladus," Brown explained.

Brown and colleagues Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University, reported their discovery of Xena last July 29. Brown and Trujillo first photographed Xena with the 48-inch Samuel Oschin Telescope on Oct. 31, 2003, but the object was so far away they could not detect its motion until they re-analyzed the data in January of last year.

Since then, the researchers have argued that Xena deserves to be formally declared the tenth planet because it is larger than Pluto and, like Pluto, it orbits the Sun independently and it most likely originated in the Kuiper belt, the debris belt beyond Neptune's orbit.

If the International Astronomical Union approves the designation, Xena will be given a new formal name, probably taken from Greek or Roman mythology, and making it the fourth planet - along with Uranus, Neptune and Pluto - to have been discovered by modern astronomy.

Regarding Xena's high reflectivity, Brown said he thinks the phenomenon is due to the combination of the body's temperature - around minus 400 degrees Fahrenheit (minus 240 degrees Celsius) - and the nature of its orbit.

Before Xena was ejected from the Kuiper belt, it probably harbored a thin atmosphere of water vapor and possibly some methane. After the ejection event, Xena's atmosphere froze and fell to the surface, leaving a bright layer a few inches thick.

"When Xena gets closer to the Sun and heats up to a sultry 370 degrees below zero (minus 223 degrees Celsius), the frozen atmosphere probably re-evaporates and the surface probably looks more like Pluto for a time," Brown said. "But Xena is currently 10 billion miles from the Sun (about three times farther out than Pluto), and because of its distance is about as cold as it ever gets."

Later in its 560 year elliptical orbit, however, Xena will move much closer to the Sun, reaching a distance of about 3.5 billion miles, or 38 astronomical units (Earth's distance from the Sun), so its surface will warm significantly.

"Xena is about to undergo the worst case of global warming of any planet in the solar system," Brown said. "The change will be equivalent to Earth's heating up to an average temperature of 400 degrees - but then the cycle will repeat and Xena will get cold again."

Meanwhile, Pluto's own 250 year elliptical orbit will take it as far away as 50 astronomical units (4.65 billion miles) from the Sun - actually placing Xena closer to Earth than Pluto, although never closer than Neptune.

Brown said his team has been studying Xena with a variety of instruments, and they have learned more of its characteristics. For example, they have also discovered the new planet, like Pluto, has a small moon. This body, unofficially nicknamed Gabrielle - after Xena's sidekick on the syndicated television show "Xena: Warrior Princess," also will be formally named by the IAU at a later date.

Gabrielle is about 250 kilometers (150 miles) in diameter and its surface is only about one-hundredth as reflective as Xena's. It is not yet known, but Brown said Gabrielle may be oddly shaped.

He also said the team has not yet been able to determine Gabrielle's orbit around Xena, but as soon as they do, they will be able to determine the planet's mass, which should lead to more insight about its composition.

Based on spectral data so far, the researchers think the planet is covered with a layer of methane that has seeped from the interior. As in the case of Pluto, the methane has undergone chemical transformations, probably due to the faint solar radiation that has caused the methane layer to redden. On Xena, however, the methane surface is somewhat more yellowish than the reddish-yellow surface of Pluto.

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Tenth Planet Only Slightly Bigger Than Pluto
Baltimore MD (SPX) Apr 12, 2006
Scientists using the Hubble Space Telescope have resolved the size of the so-called tenth planet, nicknamed Xena, and found it is only a little larger than Pluto.

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