In pebble accretion the accretion of objects ranging from centimeters up to meters in diameter onto planetesimals in a protoplanetary disk is enhanced by aerodynamic drag from the gas present in the disc. This drag reduces the relative velocity of pebbles as they pass by larger bodies, preventing some from escaping the body's gravity. These pebbles are then accreted by the body after spiraling or settling toward its surface. This process increases the cross section over which the large bodies can accrete material, accelerating their growth. The rapid growth of the planetesimals via pebble accretion allows for the formation of giant planet cores in the outer Solar System before the dispersal of the gas disk. A reduction in the size of pebbles as they lose water ice after crossing the ice line and a declining density of gas with distance from the sun slow the rates of pebble accretion in the inner Solar System resulting in smaller terrestrial planets, a small mass of Mars and a low mass asteroid belt.
https://en.wikipedia.org/wiki/Pebble_accretion
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