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A single sub-kilometre Kuiper belt object from a stellar occultation in archival data


The Kuiper belt is a remnant of the primordial Solar System. Measurements of its size distribution constrain its accretion and collisional history, and the importance of material strength of Kuiper belt objects1,2,3,4. Small, sub-kilometre-sized, Kuiper belt objects elude direct detection, but the signature of their occultations of background stars should be detectable5,6,7,8,9. Observations at both optical10 and X-ray11 wavelengths claim to have detected such occultations, but their implied abundances are inconsistent with each other and far exceed theoretical expectations. Here we report an analysis of archival data that reveals an occultation by a body with an approximately 500-metre radius at a distance of 45 astronomical units. The probability of this event arising from random statistical fluctuations within our data set is about two per cent. Our survey yields a surface density of Kuiper belt objects with radii exceeding 250 metres of , ruling out inferred surface densities from previous claimed detections by more than 5σ. The detection of only one event reveals a deficit of sub-kilometre-sized Kuiper belt objects compared to a population extrapolated from objects with radii exceeding 50 kilometres. This implies that sub-kilometre-sized objects are undergoing collisional erosion, just like debris disks observed around other stars.

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Figure 1: Photon counts as a function of time of the candidate occultation event observed by FGS2.
Figure 2: Cumulative KBO size distribution as a function of KBO radius for objects located between 30 and 60  au.

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We thank H. K. Chang for comments that helped to improve this manuscript. Some of the numerical calculations presented here were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. Partial support for this research was provided by NASA through a grant from the Space Telescope Science Institute. R.S. acknowledges support from the ERC and the Packard Foundation. A.G.-Y. is supported by the Israeli Science Foundation, an EU Seventh Framework Programme Marie Curie IRG fellowship and the Benoziyo Center for Astrophysics, a research grant from the Peter and Patricia Gruber Awards, and the William Z. and Eda Bess Novick New Scientists Fund at the Weizmann Institute. S.Z. acknowledges support from the Israel Science Foundation–Adler Foundation for Space Research. E.O.O. is an Einstein Fellow.

Author Contributions H.E.S. wrote the detection algorithm, analysed the FGS data for occultation events, calculated the detection efficiency of the survey, preformed the bootstrap analysis and wrote the paper. E.O.O. calculated the stellar angular radii, the velocity information of the observations, the correlated noise and other statistical properties of the data. R.S. guided this work and helped with the scientific interpretation of the results. A.G.-Y. proposed using HST FGS data for occultation studies and helped to make the data available for analysis. M.W. extracted the FGS photometry streams and provided coordinates and magnitudes of the guide stars. M.L. helped in gaining access to the FGS data and provided insights into the operation and noise properties of the FGS. E.N. provided expert interpretation of the FGS photometric characteristics in the HST operational environment. S.Z. took part in the statistical analysis of the data. All authors discussed the results and commented on the manuscript.

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Correspondence to H. E. Schlichting or E. O. Ofek.

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Schlichting, H., Ofek, E., Wenz, M. et al. A single sub-kilometre Kuiper belt object from a stellar occultation in archival data. Nature 462, 895–897 (2009).

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