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Discovery of two new satellites of Pluto

Abstract

Pluto's first known satellite, Charon, was discovered1 in 1978. It has a diameter (1,200 km) about half that of Pluto2,3,4,17, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful5,6,7, but they were not sensitive to objects 150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites6. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of 38 days (P1) and 25 days (P2).

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Figure 1: Images of the Pluto–Charon system, showing the new satellites, S/2005 P 1 (P1) and S/2005 P 2 (P2).
Figure 2: Preliminary orbits for P1 and P2, assuming that they are circular and in the same plane as Charon's orbit.

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Acknowledgements

We thank G. Hartig for discussions of the ACS optical performance and for examining the images discussed here. We thank the Directors and staff at the Keck, Very Large Telescope, and Gemini observatories for their efforts in attempting ground-based recoveries of these new satellites under non-optimal conditions. We thank the Director and staff of the Space Telescope Science Institute (STScI) for their support of the Hubble Space Telescope observations. Support for this work was provided by NASA through a grant from the STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract.

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Correspondence to H. A. Weaver.

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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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Weaver, H., Stern, S., Mutchler, M. et al. Discovery of two new satellites of Pluto. Nature 439, 943–945 (2006). https://doi.org/10.1038/nature04547

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