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Discovery of two new satellites of Pluto


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.


  1. Christy, J. W. & Harrington, R. S. The satellite of Pluto. Astron. J. 83, 1005–1008 (1978)

    ADS  Article  Google Scholar 

  2. Buie, M. W., Tholen, D. J. & Horne, K. Albedo maps of Pluto and Charon—Initial mutual event results. Icarus 97, 211–227 (1992)

    ADS  Article  Google Scholar 

  3. Young, L. A. Bulk Properties and Atmospheric Structure of Pluto and Charon. PhD thesis, MIT (1994)

    Google Scholar 

  4. Sicardy, B. et al. Charon's size and an upper limit on its atmosphere from a stellar occultation. Nature 439, 52–54 (2006)

    ADS  CAS  Article  Google Scholar 

  5. Stern, S. A., Parker, J. W., Fesen, R. A., Barker, E. S. & Trafton, L. M. A search for distant satellites of Pluto. Icarus 94, 246–249 (1991)

    ADS  Article  Google Scholar 

  6. Stern, S. A., Parker, J. W., Duncan, M. J., Snowdall, J. C. J. & Levison, H. F. Dynamical and observational constraints on satellites in the inner Pluto-Charon system. Icarus 108, 234–242 (1994)

    ADS  Article  Google Scholar 

  7. Stern, S. A. Does the Pluto system contain additional satellites? Lunar Planet. Inst. Conf. XXXIV, abst. 1106 (2003)

    Google Scholar 

  8. Hartig, G. F., Krist, J. E., Martel, A. R., Ford, H. C., Illingworth, G. D. . Proc. SPIE 4854, 532–543 (2003)

    ADS  Article  Google Scholar 

  9. Bernstein, G. M. et al. The size distribution of trans-Neptunian bodies. Astron. J. 128, 1364–1390 (2004)

    ADS  Article  Google Scholar 

  10. Russell, H. N. On the albedo of the planets and their satellites. Astrophys. J. 43, 173–195 (1916)

    ADS  Article  Google Scholar 

  11. Lamy, P. L., Toth, I., Fernandez, Y. R. & Weaver, H. A. in Comets II (eds Festou, M. C., Keller, H. U. & Weaver, H. A.) 223–264 (Univ. Arizona Press, Tucson, 2004)

    Google Scholar 

  12. Marcialis, R. L. et al. The albedos of Pluto and Charon—Wavelength dependence. Astron. J. 103, 1389–1394 (1992)

    ADS  Article  Google Scholar 

  13. Steffl, A. J. et al. New constraints on additional satellites of the Pluto system. Astron. J. (submitted); preprint at

  14. Stern, S. A. et al. A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt. Nature doi:10.1038/nature04548 (this issue)

  15. Sirianni, M. et al. The photometric performance and calibration of the Hubble Space Telescope Advanced Camera for Surveys. Publ. Astron. Soc. Pacif. 117, 1049–1112 (2005)

    ADS  Article  Google Scholar 

  16. Allen, C. W. Astrophysical Quantities 3rd edn (Atholone, London, 1976)

    Google Scholar 

  17. Gulbis, A. A. S. et al. Charon's radius and atmospheric constraints from observations of a stellar occultation. Nature 439, 48–51 (2006)

    ADS  CAS  Article  Google Scholar 

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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).

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