Science 340, 1199–1202 (2013)

Credit: ESO/L. CALÇADA

Many stars have orbiting planets. Such satellites form within a young star's circumstellar disk of dust and gas — the left-over debris from star formation — which lingers for about ten million years. But the details of how planets grow from submillimetre particles are not clear. Assuming that collisions lead to the formation of ever larger clumps of material, what stops planetesimals from fragmenting or crashing into the central star? Observations presented by Nienke van der Marel and co-workers now support a model that proposes a dust trap as a nursery for planets.

Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the authors studied the local star Oph-IRS 48 (pictured in this artistic impression), which has an inner and outer disk separated by a gap (inner cavity). They resolved an asymmetric arc containing a high density of millimetre-scale particles, up to 4 mm, in the outer cavity. Smaller dust grains are uniformly distributed. Their numerical simulations support the idea that a gas-pressure 'bump' created by an anticyclone can concentrate dust grains and stop them spiralling into the star.