Abstract
The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto1. It resides at present at 95.7 astronomical units (1 au is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 ut. The event is consistent with a spherical shape for Eris, with radius 1,163 ± 6 kilometres, density 2.52 ± 0.05 grams per cm3 and a high visible geometric albedo, 
. No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ∼1 nanobar, about 10,000 times more tenuous than Pluto's present atmosphere2,3,4,5. As Pluto's radius is estimated3,4,5,6,7,8 to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.
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Acknowledgements
We thank W. M. Owen and S. Preston for providing astrometric updates on the occulted star just before the observations, and I. Belskaya and M. E. Brown for discussions when writing the paper. We acknowledge support from the French grant ‘Beyond Neptune’ and from the Institut Universitaire de France. J.L.O., A.J.C.-T., L.C. and M.T.E. acknowledge funding from Spanish AYA grants and FEDER funds. TRAPPIST is a project funded by the Belgian Fund for Scientific Research (FRS-FNRS) with the participation of the Swiss National Science Foundation (SNF). J.I.B.C. acknowledges CNPq and FAPERJ grants. F.B.-R. acknowledges the support of CDFB/CAPES, Brazil. W.J.B.C., W.R. and F.P.S. thank the Brazilian Agency FAPEMIG.
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B.S. helped plan the campaign, centralized the stellar occultation predictions, participated in the observations, analysed data, wrote and ran the diffraction, limb-fitting and ray-tracing codes, and wrote part of the paper. J.L.O. helped plan the campaign, analysed data for the prediction, participated in the observations, obtained and analysed data, and wrote part of the paper. E.L. analysed the implications of the results for the Eris thermal model, albedo constraints and putative atmospheric structure, and wrote part of the paper. M.A., F.B.-R., A.H.A., J.I.B.C., R.V.M., D.N.d.S.N. and R.B. discovered the star candidate and analysed data for the predictions. E.J. and A.M. obtained and analysed the positive occultation detection at La Silla/TRAPPIST and San Pedro/Harlingten telescopes, respectively. F.B.-R., F.C., M.G. and J.M. analysed data, D.H. calculated Dysnomia’s position at the moment of occultation and wrote part of the paper. All other authors participated in the planning of the campaign and/or the observations, and the authors listed in Supplementary Table 2 were responsible for the observations. All authors were given the opportunity to review the results and comment on the manuscript.
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Supplementary Information
This file contains Supplementary Text and Data 1-7, Supplementary Figures 1- 5 with legends, Supplementary Tables 1-5 and additional references. This file was replaced on 20 December to reinstate the type missing from the top of page 5. (PDF 489 kb)
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Sicardy, B., Ortiz, J., Assafin, M. et al. A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation. Nature 478, 493–496 (2011). https://doi.org/10.1038/nature10550
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