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Comet-like mineralogy of olivine crystals in an extrasolar proto-Kuiper belt


Some planetary systems harbour debris disks containing planetesimals such as asteroids and comets1. Collisions between such bodies produce small dust particles2, the spectral features of which reveal their composition and, hence, that of their parent bodies. A measurement of the composition of olivine crystals (Mg2−2xFe2xSiO4) has been done for the protoplanetary disk HD 100546 (refs 3, 4) and for olivine crystals in the warm inner parts of planetary systems. The latter compares well with the iron-rich olivine in asteroids5,6 (x ≈ 0.29). In the cold outskirts of the β Pictoris system, an analogue to the young Solar System, olivine crystals were detected7 but their composition remained undetermined, leaving unknown how the composition of the bulk of Solar System cometary olivine grains compares with that of extrasolar comets8,9. Here we report the detection of the 69-micrometre-wavelength band of olivine crystals in the spectrum of β Pictoris. Because the disk is optically thin, we can associate the crystals with an extrasolar proto-Kuiper belt a distance of 15–45 astronomical units from the star (one astronomical unit is the Sun–Earth distance), determine their magnesium-rich composition (x = 0.01 ± 0.001) and show that they make up 3.6 ± 1.0 per cent of the total dust mass. These values are strikingly similar to those for the dust emitted by the most primitive comets in the Solar System8,9,10, even though β Pictoris is more massive and more luminous and has a different planetary system architecture.

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Figure 1: Photometric and spectral observations of the planetary system of β Pictoris.
Figure 2: Diagram demonstrating the dependence of the 69-μm band on grain temperature and composition.


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Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL and IMEC (Belgium); CEA and OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS and SISSA (Italy); and IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy) and CICT/MCT (Spain). B.L.d.V. is an Aspirant Fellow of the Fund for Scientific Research, Flanders.

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B.L.d.V. analysed the data, performed the modelling and wrote the manuscript; B.A. was heavily involved in designing the study, doing the analysis, applying the statistical methods and in writing the paper; J.A.D.L.B. did the data reduction and helped in writing the manuscript; C.W. was largely responsible for the study design and obtaining the observations, and commented on the manuscript; L.B.F.M.W. was heavily involved in designing the study and provided much input into the scientific discussion; B.V. was involved in the data reduction and commented on the manuscript; M.M. was involved in the modelling of the temperature structure and opacities and commented on the manuscript; G.L.P. was responsible for obtaining the observations and commented on the manuscript; C.D. was essential in the scientific discussions and helped in obtaining the observations; L.D. was essential in the scientific discussion and in writing the manuscript. The rest of the authors all contributed equally to obtaining the observations, having scientific discussions and in commenting on the manuscript.

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Correspondence to B. L. de Vries.

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de Vries, B., Acke, B., Blommaert, J. et al. Comet-like mineralogy of olivine crystals in an extrasolar proto-Kuiper belt. Nature 490, 74–76 (2012).

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