Pebble accretion, the capture of pebble-sized solids by a solid planetary embryo, effectively increasing its size, is thought to occur as a stage in planet formations of solid planetary embryo which may end up as rocky planets or accrete gas to become gas planets with solid cores. Regarding planet formation theories and in pebble accretion, the term pebble is used not only for centimeter-scale solids (as per everyday use of pebble) but up to meter-scale.
Pebble accretion is currently under much investigation because it appears to have an advantage over planetesimal impacts, due to the cross sections necessary to produce each. Some planet-forming scenarios appear to run out of time (when the disk is gone) and pebble accretion can be efficient, offering a realistic alternative. The effectiveness of such accretion in enlarging planetesimals depends upon timescales regarding bringing pebbles into the cross section versus depleting the portion of the disk from which pebbles migrate. The gas in the disk has various effects on the pebbles, both throughout the disk, and in particular ways for a pebble near a planetesimal, offering many possible scenarios and consequences for investigation. Objects of this size within a disk are subject to an inward drift which could arrest planet formation (radial-drift barrier), but if a planetesimal already exists in the drift path, such a drift could assist in the planetesimal's accretion, bringing more material from further out in the disk.