Hubble Finds -ËœTenth Planet- Slightly Larger Than Pluto

Size of Kuiper Belt Objects: This illustration of the largest known Kuiper Belt Objects shows Xena slightly larger than Pluto. Photo courtesy: NASA, ESA, and A. Feild (STScI) For the first time, NASAs Hubble Space Telescope has seen distinctly the tenth planet, currently nicknamed Xena, and found that its only slightly larger than Pluto. Though previous ground-based observations suggested that Xenas diameter was about 30 percent greater than Pluto, Hubble observations taken Dec. 9 and 10, 2005, showed Xenas diameter as 1,490 miles (with an uncertainty of 60 miles). Plutos diameter, as measured by Hubble, is 1,422 miles. Hubble is the only telescope capable of getting a clean visible-light measurement of the actual diameter of Xena, said Mike Brown, planetary scientist at the California Institute of Technology in Pasadena, Calif. Browns research team discovered Xena, officially cataloged as 2003 UB313, and its results have been accepted for publication in the Astrophysical Journal. Only a handful of images were required to determine Xenas diameter. Located 10 billion miles from Earth with a diameter a little more than half the width of the United States, the object is 1.5 pixels across in Hubbles view. Thats enough to make a precise size measurement. Because Xena is smaller than previously thought, but comparatively bright, it must be one of the most reflective objects in the solar system. The only object more reflective is Enceladus, a geologically active moon of Saturn whose surface is continuously recoated with highly reflective ice by active geysers. Xenas bright reflectivity is possibly due to fresh methane frost on its surface. The object may have had an atmosphere when it was closer to the sun, but as it moved to its current location farther away this atmosphere would have frozen out, settling on the surface as frost. Another possibility is that Xena leaks methane gas continuously from its warmer interior. When this methane reaches the cold surface, it immediately freezes solid, covering craters and other features to make it uniformly bright to Hubbles telescopic eye. Xenas takes about 560 years to orbit the sun, and it is now very close to aphelion (the point on its orbit that is farthest from the sun). Brown next plans to use Hubble and other telescopes to study other recently discovered Kuiper Belt objects that are almost as large as Pluto and Xena. The Kuiper Belt is a vast ring of primordial icy comets and larger bodies encircling Neptunes orbit. Finding that the largest known Kuiper Belt object is a virtual twin to Pluto may only further complicate the debate about whether to categorize the large icy worlds that populate the belt as planets. If Pluto were considered to be the minimum size for a planet, then Xena would fulfill this criterion, too. In time, the International Astronomical Union will designate the official name.