NASA’s Hubble Space Telescope is hot on the trail of an intriguing
new class of solar system object that might be called a Pluto
“mini-me” — dim and fleeting objects that travel in pairs in the
frigid, mysterious outer realm of the solar system called the
Kuiper Belt.

In results published today in the journal Nature, a team of
astronomers led by Christian Veillet of the Canada-France-Hawaii
Telescope Corporation (CFHT) in Kamuela, Hawaii, is reporting the
most detailed observations yet of the Kuiper Belt object (KBO)
1998 WW31, which was discovered four years ago and found to be a
binary last year by the CFHT.

Pluto and its moon Charon and countless icy bodies known as KBOs
inhabit a vast region of space called the Kuiper Belt. This
‘junkyard’ of material left over from the solar system’s formation
extends from the orbit of Neptune out to 100 times as far as the
Earth is from the Sun (which is about 93 million miles) and is the
source of at least half of the short-period comets that whiz through
our solar system. Only recently have astronomers found that a small
percentage of KBOs are actually two objects orbiting around each
other, called binaries.

“More than one percent of the approximately 500 known KBOs are
indeed binary: a puzzling fact for which many explanations will be
proposed in what is going to be a very exciting and rapidly evolving
field of research in the coming years,” says Veillet.

Hubble was able to measure the total mass of the pair based on their
mutual 570-day orbit (a technique Isaac Newton used 400 years ago to
estimate the mass of our Moon). The ‘odd-couple’ 1998 WW31 together
are about 5,000 (0.0002) times less massive than Pluto and Charon.

Like a pair of waltzing skaters, the binary KBOs pivot around a
common center of gravity. The orbit of 1998 WW31 is the most
eccentric ever measured for any binary solar system object or
planetary satellite. Its orbital distance varies by a factor of ten,
from 2,500 to 25,000 miles (4,000 to 40,000 kilometers). It is
difficult to determine how KBOs wind up traveling in pairs. They
may have formed that way, born like twins, or may be produced by
collisions where a single body is split in two.

Ever since the first KBO was discovered in 1992, astronomers have
wondered how many KBOs may be binaries, but it was generally assumed
that the observations would be too difficult for most telescopes.
However, the insights to be gained from study of binary KBOs would be
significant: measuring binary orbits provide estimates of KBO masses,
and mutual eclipses of the binary allow astronomers to determine
individual sizes and densities. Assuming some fraction of KBOs should
be binary – just as has been discovered in the asteroid belt –
astronomers eventually began to search for gravitationally entwined
pairs of KBOs.

Then, finally, exactly a year ago on April 16, 2001, Veillet and
collaborators announced the first discovery of a binary KBO: 1998 WW31.
Since then, astronomers have reported the discoveries of six more
binary KBOs. “It’s amazing that something that seems so hard to do
and takes many years to accomplish can then trigger an avalanche of
discoveries,” says Veillet. Four of those discoveries were made with
the Hubble Space Telescope: two were discovered with a program led by
Michael Brown of the California Institute of Technology in Pasadena,
CA, and two more with a program led by Keith Noll of the Space
Telescope Science Institute in Baltimore, MD. The sensitivity and
resolution of Hubble is ideal for studying binary KBOs because the
objects are so faint and so close together.

The Kuiper Belt is one of the last big missing puzzle pieces to
understanding the origin and evolution of our solar system and
planetary systems around other stars. Dust disks seen around other
stars could be replenished by collisions among Kuiper Belt-type
objects, which seems to be common among stars. These collisions
offer fundamental clues to the birth of planetary systems.


Electronic image files, animation, illustrations, and additional
information are available on the Internet at:

The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).