Pluto-Charon is the only known double planet in the solar system, orbiting about 40 times as far away from the Sun as Earth. It is embedded in the Kuiper Belt of planetesimals, comets, and miniature icy worlds that surround our planetary system in a thick disk. The Kuiper Belt is a larger and more populous, icy-rich analog to its better known cousin, the Asteroid Belt of rocky debris orbiting between Mars and Jupiter.

Astronomers have suspected for more than a decade that Pluto and its 1200 km-wide satellite, Charon, formed as a pair during a giant collision in the ancient past between proto-Pluto and another Kuiper Belt object. Evidence for this collision includes the orbital configuration, the relative masses, and the angular momentum of the Pluto-Charon system.

Now, SwRI astronomers Drs. Alan Stern, Robin Canup, and Daniel Durda have found clues that some KBOs in neighboring orbits to Pluto may, in fact, be debris created in the Pluto-Charon forming event. Their results are being presented Tuesday, October 12, at the American Astronomical Society's (AAS) Division for Planetary Sciences meeting in Padua, Italy.

The evidence found by the SwRI team linking some KBOs called "Plutinos" to Pluto-Charon comes in three forms. First, there is a close orbital similarity between some KBOs and Pluto that is consistent with the expected distribution of debris from the Pluto-Charon formation event. Second, the colors of Pluto and some KBOs, and Charon and other KBOs, suggest similar surface compositions. Third, the apparent size distribution of the objects that suggest themselves as potential shards of the Pluto-Charon forming collision is similar to both laboratory results from studies of catastrophic collisions and asteroid belt families known to result from collisions.

Future research will be required to prove this new hypothesis, dubbed "Pluto's Family." If borne out by future tests, it would constitute the first discovery of a genetically related, parent-daughter family of objects in the Kuiper Belt. Further, because the KBO region surrounding Pluto has been known for some time to be delivering some comets to Earth's vicinity, the new work suggests that a small, but nonetheless important, fraction of the comets observed by astronomers may actually consist of samples of Pluto and Charon.

SwRI is an independent, nonprofit, applied engineering and physical sciences research and development organization with a staff of 2,700 and an annual research volume of $304 million.

WAY OUT THERE AT SPACEDAILY
Raining Diamonds On Uranus
Berkeley - October 1, 1999 - Future explorers of our solar system may well find diamonds hailing down through the atmospheres of Neptune and Uranus say Berkeley scientists. These planets contain a high proportion of methane, which UC Berkeley researchers have now shown can turn into diamond at the high temperatures and pressures found inside these planets.