Pluto’s status as our solar system’s ninth planet may be safe if a recently
discovered Kuiper Belt Object is a typical “KBO” and not just an oddball.

Astronomers have new evidence that KBOs (Kuiper Belt Objects) are smaller
than previously thought.

KBOs – icy cousins to asteroids and the source of some comets – are the
leftover building blocks of the outer planets. Astronomers using the
world’s most powerful telescopes have discovered about 1,000 of these
objects orbiting beyond Neptune since discovering the first one in 1992.
These discoveries fueled debate on whether Pluto is a planet or a large
(1,400-mile diameter) closer-in KBO.

Researchers estimate that the total mass of the Kuiper Belt is about a
tenth of Earth’s mass. Most theorize that there are more than 10,000 KBOs
with diameters greater than 100 kilometers (62 miles), compared to 200
asteroids known to be that large in the main asteroid belt between Mars
and Jupiter.

“People were finding all these KBOs that were huge – literally half the
size of Pluto or larger,” University of Arizona astronomer John Stansberry
said. “But those supposed sizes were based on assumptions that KBOs have
very low albedos, similar to comets.”

Albedo is a measure of how much light an object reflects. The more light an
object reflects, the higher its albedo. Actual data on Kuiper Belt Object
albedos have been hard to come by because the objects are so distant, dim
and cold. Many astronomers have assumed that KBO albedos – like comet
albedos – are around four percent and have used that number to calculate
KBO diameters.

However, in early results from their Spitzer Space Telescope survey of 30
Kuiper Belt Objects, Stansberry and colleagues found that a distant KBO
designated 2002 AW197 reflects 18 percent of its incident light and is
about 700 kilometers (435 miles) in diameter. That’s considerably smaller
and more reflective than expected, Stansberry said.

“2002 AW197 is believed to be one of the largest KBOs thus far discovered,”
he said. “These results indicate that this object is larger than all but
one main-belt asteroid (Ceres), about half the size of Pluto’s moon,
Charon, and about 30 percent as large and a tenth as massive as Pluto.”

Stansberry and his colleagues took the data with Spitzer’s Multiband
Imaging Photometer (MIPS) on April 13, 2004. George Rieke’s team at the
University of Arizona developed and built the extremely heat-sensitive
MIPS. It detects heat from very cold objects by taking images at
far-infrared wavelengths.

In this case, MIPS detected heat from a Kuiper Belt Object with a surface
temperature of around minus 370 degrees Fahrenheit at an astonishing
distance of 4.4 billion miles (7 billion kilometers), or one-and-a-half
times farther away frm the sun than Pluto.

Without MIPS, astronomers operating under the assumption that 2002 AW197
reflects four percent of its incident light would calculate that it is 1500
kilometers (932 miles) in diameter, or two-thirds as large as Pluto,
Stansberry said.

“We’re finally starting to get data on the basic physical parameters of
KBOs,” Stansberry said. “That will help us determine what their
compositions are, how they evolve, how massive they are, what their real
size distributions and dynamics are and how Pluto fits into the whole
picture,” he said.

Such data will also offer insight on how comets are processed on their
successive journeys around the sun, he added.

“It’s not surprising that comets are darker than KBOs,” Stansberry
said.”When something in the Kuiper Belt chips off a piece of a Kuiper Belt
Object, presumably that piece would have a higher albedo on its first swing
through the inner solar system. But it doesn’t take long before it loses
its high albedo surface and builds up a lot of very dark materials, at
least in its outermost surface.”

Others with Stansberry in this Spitzer study are Dale Cruikshank and Josh
Emery of NASA Ames Research Center, Yan Fernandez of the University of
Hawaii, George Rieke of the University of Arizona and Michael Werner of
NASA’s Jet Propulsion Laboratory.

Stansberry said the team will finish collecting their KBO data with Spitzer
soon. “We’ll know a lot more about how big and bright these things are by
this time next year,” he said.

Stansberry is presenting the research today at the 86th annual meeting of
the American Astronomical Society Division of Planetary Science in
Louisville, Ky.

More information about this and other new results from the Spitzer Space
Telescope is on the Web at http://www.spitzer.caltech.edu/Media/index.shtml
The Spitzer Space Telescope is managed for NASA by the Jet Propulsion
Laboratory in Pasadena, Calif.