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

Press Release From: Southwest Research Institute
Posted: Wednesday, February 22, 2006


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

SwRI is an independent, nonprofit, applied research and development organization based in San Antonio, Texas, with more than 3,000 employees and an annual research volume of more than $435 million.

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