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