Hubble Space Telescope Proves Tenth Planet Size

According to a statment issued by NASA, for the first time, NASA's Hubble Space Telescope has seen distinctly the "tenth planet," currently nicknamed "Xena," and has found that it is only slightly larger than Pluto.

Though previous ground-based observations suggested that Xena's
diameter was about 30 percent greater than Pluto, Hubble observations taken Dec. 9 and 10, 2005, showed Xena's diameter as 1,490 miles (with an uncertainty of 60 miles). Pluto's 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. Brown's 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 Xena's 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 Hubble's view. That's 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.

Xena's 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 Hubble's telescopic eye.

Xena's 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 Neptune's 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.

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