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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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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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