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A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt


The two newly discovered1 satellites of Pluto (P1 and P2) have masses that are small compared to both Pluto and Charon—that is, between 5 × 10-4 and 1 × 10-5 of Pluto's mass, and between 5 × 10-3 and 1 × 10-4 of Charon's mass. This discovery, combined with the constraints on the absence of more distant satellites of Pluto2, reveal that Pluto and its moons comprise an unusual, highly compact, quadruple system. These facts naturally raise the question of how this puzzling satellite system came to be. Here we show that P1 and P2's proximity to Pluto and Charon, the fact that P1 and P2 are on near-circular orbits in the same plane as Pluto's large satellite Charon1, along with their apparent locations in or near high-order mean-motion resonances, all probably result from their being constructed from collisional ejecta that originated from the Pluto–Charon formation event. We also argue that dust–ice rings of variable optical depths form sporadically in the Pluto system, and that rich satellite systems may be found—perhaps frequently—around other large Kuiper belt objects.

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Figure 1: The architecture of the Pluto system compared to other KBOs with known satellites and to the Earth–Moon system.

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We thank W. McKinnon, R. Canup and W. Ward for reading and commenting on this manuscript.

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Correspondence to S. A. Stern.

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Background Information

Pluto, the most distant planet in our Solar System, has been known for nearly 30 years to have a moon - Charon - about half as wide as the planet itself. Although some scientists suspected that the planet may have other, smaller moons, at such a great distance from the Earth they would be very hard to spot.

Now two such moons have been seen for the first time. Hal Weaver and co-workers report in Nature that they have found them in images taken by the Hubble Space Telescope. Compared with Charon, the two new moons, called P1 and P2, are tiny. Their exact size is hard to gauge, but reasonable assumptions about their reflectivity indicate that they are both between 48 and 165 kilometres across, compared with Charon's diameter of about 1,200 kilometres. The researchers estimate that P1 orbits Pluto once every 38 days, and P2 every 25 days.

Where did these moons come from? Charon is believed to have formed, like our own Moon, from the debris created when another object slammed into its parent planet. In a second, related paper, Alan Stern and colleagues suggest that a small amount of the material from this impact on Pluto gathered together under its own gravity to form P1 and P2.

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Stern, S., Weaver, H., Steffl, A. et al. A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt. Nature 439, 946–948 (2006).

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