Hubble Space Telescope Obtains Best-Ever Size Measurement of Xena; Still Larger Than Pluto

PASADENA, Calif.—To paraphrase a certain young lady from literature, the tenth planet Xena is getting curiouser and curiouser. Data released today by the Space Telescope Science Institute reveals that Xena is about 5 percent larger than Pluto, which means that it must be the most reflective planet in the solar system.

According to Mike Brown, a California Institute of Technology planetary scientist who codiscovered Xena last year, the Hubble Space Telescope measurement shows that Xena is about 2,400 kilometers in diameter, give or take 100 kilometers. The planet's smaller-than-expected size, together with its distance from Earth and its brightness, mean that Xena must reflect 86 percent of all light.

"This makes it more reflective than the nine known planets, and more reflective than everything else other than Enceladus," explains Brown. Enceladus is a moon orbiting Saturn.

Brown and his colleagues, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University, publicly announced the discovery of Xena on July 29, 2005. Their argument from the beginning has been that Xena deserves to be formally declared the tenth planet because it is larger than Pluto. If the designation is approved by the International Astronomical Union, Xena will assume a formal new name that will presumably be taken from Greek or Roman mythology, and will become only the fourth planet to have been discovered in historical times.

Also designated as 2003 UB313, Xena was originally found by Brown, Trujillo, and Rabinowitz with the NASA-funded 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory. Like its nine siblings, Xena independently orbits the sun, and like Pluto, is a Kuiper-belt object.

Brown says the high reflectivity of Xena is surprising, but perhaps explainable. "I think what is going on, is that the temperature of Xena-which is about minus 400 degrees Fahrenheit-causes the atmosphere to be frozen to the surface. This frozen atmosphere would make a nice, bright layer a few inches thick.

"When Xena gets closer to the sun and heats up to a sultry 370 degrees below zero, the frozen atmosphere probably re-evaporates and the surface probably looks more like Pluto for a time. But Xena is currently 10 billion miles from the sun, and because of its distance is about as cold as it ever gets."

The situation will change, however, when Xena approaches closer to the sun on its 560-year elliptical orbit. Though we won't live to see it, Xena will eventually get within 38 astronomical units of Earth (in other words, 38 times the distance between the sun and Earth), and things will heat up significantly.

"Xena is about to undergo the worst case of global warming of any planet in the solar system," says Brown. "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."

Xena's closest planetary neighbor is Pluto, which Xena resembles in various ways. Pluto's own elliptical orbit takes it as far away as 50 astronomical units from the sun during its 250-year trip around the sun. This means that Xena is sometimes much closer to Earth than Pluto-although never closer than Neptune.

Since the discovery, Brown and his colleagues have intensely studied the planet with a variety of instruments, and have already learned many of its characteristics. In the course of their investigations, they have also discovered that the new planet has a small moon. This body, unofficially nicknamed "Gabrielle" after Xena's sidekick on the television show, will also be formally named at a later date. Gabrielle is about 250 kilometers in diameter and reflects only about 1 percent of the sunlight that its parent reflects. Because of its small size, Gabrielle may very well be oddly shaped.

Brown says that Gabrielle's orbit around Xena hasn't yet been fully determined. But once it is, the researchers will be able to derive the mass of the planet itself. That's because the entire mass of the system (planet and moon together) orbits a common center of gravity. Therefore, once the researchers figure out the distance between moon and planet, how fast the moon revolves around the planet, and how much the moon makes the planet wobble, then they'll know how much Xena weighs. And this information will lead to new insights on its composition.

Based on spectral data, 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. But the methane surface on Xena is somewhat more yellowish than the reddish-yellow surface of Pluto, perhaps because Xena is farther from the sun.

Brown and Trujillo first photographed the new planet with the 48-inch Samuel Oschin Telescope on October 31, 2003. However, the object was so far away that its motion was not detected until they reanalyzed the data in January of last year.

The search for new planets and other bodies in the Kuiper belt is funded by NASA. For more information on the program, see the Samuel Oschin Telescope's Website at http://www.astro.caltech.edu/palomarnew/sot.html

For more information on Mike Brown's research, see http://www.gps.caltech.edu/~mbrown.