Though previous ground-based observations suggested that Xena's diameter was
about 30 per cent greater than Pluto, Hubble observations of December 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 1422 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, California.
Brown's research team discovered Xena, officially catalogued as 2003 ub313,
and its results have been accepted for publication in the astrophysical journal.
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 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.
