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A pathway to the formation of planetesimals, and eventually giant planets,may occur in concentrations of dust grains trapped in pressure maxima. Dramaticcrescent-shaped dust concentrations have been seen in recent radio images atsub-mm wavelengths. These disk asymmetries could represent the initial phasesof planet formation in the dust trap scenario, provided that grain sizes arespatially segregated. A testable prediction of azimuthal dust trapping is thatprogressively larger grains should be more sharply confined and furthermore thetrapped grains should follow a distribution that is markedly different from thegas. However, gas tracers such as CO and the infrared emission from smallgrains are both very optically thick where the submm continuum originates, soobservations have been unable to test the trapping predictions or to identifycompact concentrations of larger grains required for planet formation bycore-accretion. Here we report multifrequency observations of HD142527, from34GHz to 700GHz, that reveal a compact concentration of ~cm-sized grains, witha few Earth masses, embedded in a large-scale crescent of ~mm-sized particles.The emission peaks at wavelengths shorter than ~1mm are optically thick andtrace the temperature structure resulting from shadows cast by the innerregions. Given this temperature structure, we infer that the largest dustgrains are concentrated in the 34 GHz clump. We conclude that dust trapping isefficient for approximately cm-sized grains and leads to enhancedconcentrations, while the smaller grains largely reflect the gas distribution.

A compact concentration of large grains in the HD142527 protoplanetary dust trap