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

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.”

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.

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