Pluto's New Moons Likely Born with Charon; Pluto May Even Have Rings(Forwarded)

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Southwest Research Institute
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Dr. Alan Stern, (303) 324-5269

February 22, 2006

Pluto's New Moons Likely Born with Charon; Pluto May Even Have Rings

Boulder, Colorado -- In a paper published today in Nature, a team of
U.S. scientists led by Dr. S. Alan Stern of Southwest Research
Institute® (SwRI®), concludes that two newly discovered small moons of
Pluto were very likely born in the same giant impact that gave birth to
Pluto's much larger moon, Charon. The team also argues that other, large
binary Kuiper Belt Objects (KBOs) may also frequently harbor small
moons, and that the small moons orbiting Pluto may generate debris rings
around Pluto.

The team making these findings included Drs. Bill Merline, John Spencer,
Andrew Steffl, Eliot Young and Leslie Young of SwRI; Dr. Hal Weaver of
the Johns Hopkins University Applied Physics Laboratory; Max Mutchler of
the Space Telescope Science Institute; and Dr. Marc Buie of the Lowell
Observatory. This team discovered Pluto's two small moons in 2005 using
sensitive images obtained by the Hubble Space Telescope, as reported by
Weaver et al. in an accompanying paper in the February 23 issue of Nature.

"The evidence for the small satellites being born in the Charon-forming
collision is strong; it is based around the facts that the small moons
are in circular orbits in the same orbital plane as Charon, and that
they are also in, or very near, orbital resonance with Charon," says
lead author Stern, executive director of the SwRI Space Science and
Engineering Division.

"Tests of this scenario will come from refined orbital data, from
measuring the rotational periods of these moons, and from determinations
of their densities and surface compositions," says co-author Weaver.

Collisions, both large and small, are major processes that shaped many
aspects of our solar system. Scientists use computer simulations to
study the origin of planetary systems formed by impact events of a scale
much larger than could be simulated in a laboratory. Another large
collision, like the one thought to have created Charon and Pluto's small
moons, is believed responsible for the formation of the Earth-moon pair.

"The idea that Pluto's small moons and Charon resulted from a giant
impact now seems compelling. Future simulations to determine the
characteristics of the impact required to produce all three satellites
should provide improved constraints on the early dynamical history of
the Kuiper Belt," adds Dr. Robin Canup, director of SwRI's Space Studies
Department, who in 2005 produced the most comprehensive models to date
of the Charon-forming impact.

Based on the growing realization that binary "ice dwarf" pairs like
Pluto-Charon are common in the Kuiper Belt, the Pluto satellite
discovery team concludes that numerous triple, quadruple and even
higher-order systems may be discovered across the Kuiper Belt in years
to come.

"Finding small satellites around KBOs is difficult because their large
distance from the Sun makes them appear very faint. As a result, we
don't really know how common it is for KBOs to have multiple
satellites," adds co-author Steffl. "One good way to test this is to
search around objects that have been ejected from the Kuiper Belt into
orbits that bring them much closer to the Sun. So far, about 160 of
these objects, called Centaurs, have been discovered. We hope to use
Hubble to search for faint moons around some of them."

Co-author Merline adds, "If Pluto's small moons generate debris rings
from impacts on their surfaces, as we predict, it would open up a whole
new class of study because it would constitute the first ring system
seen around a solid body rather than a gas giant planet."

"The Pluto system never fails to reward us when we look at it in new
ways," concludes Stern. "What a bonanza and an illustration of the
richness of nature Pluto has consistently proved to be. Our discovery of
its two new moons reinforces that lesson yet again."

The paper, "A Giant Impact Origin for Pluto's Small Moons and Satellite
Multiplicity in the Kuiper Belt," by Stern et al. is available in the
February 23 issue of Nature. NASA funded this work.

Editors: An image to accompany this story is available at
http://www.swri.org/press/2006/plutoimpact.htm