Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Charon's radius and atmospheric constraints from observations of a stellar occultation


The physical characteristics of Pluto and its moon, Charon, provide insight into the evolution of the outer Solar System. Although previous measurements have constrained the masses of these bodies1,2, their radii and densities have remained uncertain. The observation of a stellar occultation by Charon in 1980 established a lower limit on its radius of 600 km (ref. 3) (later refined to 601.5 km; ref. 4) and suggested a possible atmosphere4. Subsequent, mutual event modelling yielded a range of 600–650 km (ref. 5), corresponding to a density of 1.56 ± 0.22 g cm-3 (refs 2, 5). Here we report multiple-station observations of a stellar occultation by Charon. From these data, we find a mean radius of 606 ± 8 km, a bulk density of 1.72 ± 0.15 g cm-3, and rock-mass fraction 0.63 ± 0.05. We do not detect a significant atmosphere and place 3σ upper limits on atmospheric number densities for candidate gases. These results seem to be consistent with collisional formation for the Pluto–Charon system in which the precursor objects may have been differentiated6, and they leave open the possibility of atmospheric retention by the largest objects in the outer Solar System.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Observed occultation chords.
Figure 2: Light curves showing the occultation of C313.2 by Charon.
Figure 3: Atmospheric model fits to the light-curve data.


  1. Null, G. W. & Owen, W. M. J. Charon/Pluto mass ratio obtained with HST CCD observations in 1991 and 1993. Astron. J. 111, 1368–1381 (1996)

    Article  ADS  Google Scholar 

  2. Olkin, C. B., Wasserman, L. H. & Franz, O. G. The mass ratio of Charon to Pluto from Hubble Space Telescope astrometry with Fine Guidance Sensors. Icarus 164, 254–259 (2003)

    Article  ADS  Google Scholar 

  3. Walker, A. R. An occultation by Charon. Mon. Not. R. Astron. Soc. 192, 47p–50p (1980)

    Article  ADS  Google Scholar 

  4. Elliot, J. L. & Young, L. A. Limits on the radius and a possible atmosphere of Charon from its 1980 stellar occultation. Icarus 89, 244–254 (1991)

    Article  ADS  Google Scholar 

  5. Tholen, D. J. & Buie, M. W. in Pluto and Charon (eds Stern, S. A. & Tholen, D. J.) 193–219 (Univ. Arizona Press, Tucson, 1997)

    Google Scholar 

  6. McKinnon, W. B. On the origin of the Pluto-Charon binary. Astrophys. J. 344, L41–L44 (1989)

    Article  ADS  Google Scholar 

  7. McDonald, S. W. & Elliot, J. L. Pluto-Charon stellar occultation candidates: 2000–2009. Astron. J. 119, 1999–2007 (2000); Erratum. 120, 1599 (2000)

    Article  ADS  Google Scholar 

  8. McKinnon, W. B., Simonelli, D. P. & Schubert, G. in Pluto and Charon (eds Stern, S. A. & Tholen, D. J.) 295–343 (Univ. Arizona Press, Tucson, 1997)

    Google Scholar 

  9. French, R. G. & Gierasch, P. J. Diffraction calculation of occultation light curves in the presence of an isothermal atmosphere. Astron. J. 81, 445–451 (1976)

    Article  ADS  Google Scholar 

  10. Bartholdi, P. & Owen, F. The occultation of Beta Scorpii by Jupiter and Io. II. Io. Astron. J. 77, 60–65 (1972)

    Article  ADS  Google Scholar 

  11. National Research Council (U.S.). International Critical Tables of Numerical Data, Physics, Chemistry, And Technology (eds West, C. J. & Hull, C.) (McGraw-Hill Book Co., New York, 1933)

    Google Scholar 

  12. Trafton, L., Stern, S. A. & Gladstone, G. R. The Pluto-Charon system: The escape of Charon's primordial atmosphere. Icarus 74, 108–120 (1988)

    Article  ADS  CAS  Google Scholar 

  13. Yelle, R. V. & Elliot, J. L. in Pluto and Charon (eds Stern, S. A. & Tholen, D. J.) 347–390 (Univ. Arizona Press, Tucson, 1997)

    Google Scholar 

  14. Elliot, J. L., Person, M. J. & Qu, S. Analysis of stellar occultation data. II. Inversion, with application to Pluto and Triton. Astron. J. 126, 1041–1079 (2003)

    Article  ADS  Google Scholar 

  15. Elliot, J. L. et al. The recent expansion of Pluto's atmosphere. Nature 424, 165–168 (2003)

    Article  ADS  CAS  Google Scholar 

  16. Pasachoff, J. M. et al. The structure of Pluto's atmosphere from the 2002 August 21 stellar occultation. Astron. J. 129, 1718–1723 (2005)

    Article  ADS  CAS  Google Scholar 

  17. Spencer, J. et al. Mid-infrared detection of large longitudinal asymmetries in Io's SO2 atmosphere. Icarus 176, 283–304 (2005)

    Article  ADS  CAS  Google Scholar 

  18. Broadfoot, A. L., Shemansky, D. E. & Kumar, S. Mariner 10: Mercury atmosphere. Geophys. Res. Lett. 3, 577–580 (1976)

    Article  ADS  CAS  Google Scholar 

  19. Stern, S. A. & Trafton, L. Constraints on bulk composition, seasonal variation, and global dynamics of Pluto's atmosphere. Icarus 57, 231–240 (1984)

    Article  ADS  CAS  Google Scholar 

  20. Yelle, R. V. & Lunine, J. I. Evidence for a molecule heavier than methane in the atmosphere of Pluto. Nature 339, 288–290 (1989)

    Article  ADS  CAS  Google Scholar 

  21. Marcialis, R. L., Rieke, G. H. & Lebofsky, L. A. The surface composition of Charon: Tentative identification of water ice. Science 237, 1349–1351 (1987)

    Article  ADS  CAS  Google Scholar 

  22. Buie, M. W., Cruikshank, D. P., Lebofsky, L. A. & Tedesco, E. F. Water frost on Charon. Nature 329, 522–523 (1987)

    Article  ADS  CAS  Google Scholar 

  23. Buie, M. & Grundy, W. M. The distribution and physical state of H2O on Charon. Icarus 148, 324–339 (2000)

    Article  ADS  CAS  Google Scholar 

  24. Roush, T. L. Charon: more than water ice? Icarus 108, 243–254 (1994)

    Article  ADS  Google Scholar 

  25. Canup, R. M. A giant impact origin of Pluto-Charon. Science 307, 546–550 (2005)

    Article  ADS  CAS  Google Scholar 

  26. Brown, M. E., Trujillo, C. A. & Rabinowitz, D. L. 2003 EL_61, 2003 UB_313, and 2005 FY_9. IAU Circ. 8577 (2005)

  27. Trujillo, C. A., Barkume, K. M., Brown, M. E., Schaller, E. L. & Rabinowitz, D. L. Near infrared spectra from Mauna Kea of the new brightest Kuiper Belt Object. Bull. Am. Astron. Soc. 37, 52.06 (2005)

    Google Scholar 

  28. USNO. The Astronomical Almanac for the Year 2005 (US Govt Printing Office, Washington DC, 2003)

    Google Scholar 

Download references


B.A.B., J.W.G. and T.T. are Guest Observers at Instituto de Astronomía, Universidad Católica del Norte, Antofagasta, Chile. We thank N. Vogt and M. Murphy of Universidad Católica del Norte for their expertise, assistance, and the use of their telescope at Cerro Armazones; G. Gutiérrez and F. Sánchez, telescope operators at Las Campanas; R. Carrasco, K. Volk (observers), and E. Wendroth (telescope operator) at Gemini South; M. Ottoboni and R. Campos at Pico Dos Dias; and C. Czelusniak, assistant observer from Universidade Estadual de Ponta Grossa. Full credits for Gemini Observatory are available at Support for this work was provided by NASA Planetary Astronomy. Additional support was provided by the Belgian Federal Office for Scientific, Technical and Cultural Affairs and the Flemish Ministry for Foreign Policy, European Affairs, Science and Technology.

Author information

Authors and Affiliations


Corresponding author

Correspondence to A. A. S. Gulbis.

Ethics declarations

Competing interests

Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Table

This lists the five sites from which we arranged to view the occultation of C313.2 by Charon and provides details about the observations. (DOC 41 kb)

Supplementary Discussion

This provides a complete description of how we established Charon’s radius and error bar (DOC 23 kb)

Rights and permissions

Reprints and Permissions

About this article

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing