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.
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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 http://www.us-gemini.noao.edu/sciops/data/dataAcknowIndex.html. 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.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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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). https://doi.org/10.1038/nature04276
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