pebble accretion

Pebble accretion, the capture of pebble-sized solids by a solid planetary embryo, increasing its size, is considered a possibility as a stage in planet formation of solid planetary embryos which may end up as rocky planets or may accrete gas to become gas planets with solid cores. Regarding planet formation theories and the phrase pebble accretion, the term pebble is used not only for centimeter-scale solids (as per everyday use of pebble) but up to and including meter-scale.

Pebble accretion is currently investigated because it appears to have an advantage over planetesimal impacts as a means of embryos' further growth, due to the apparent probabilities that either would the growth indicated by existing planets. Such probabilities are worked out from the cross sections for an impact versus that for accreting pebbles, along with the expected number of pebbles and planetesimals that will be available, and the length of time they have to do it. Some planet-forming scenarios appear to allow insufficient time (the protoplanetary disk being likely to disperse before a planet is formed) and pebble accretion is potentially more efficient, offering a plausible means of building the planet within the time available. Its effectiveness depends upon timescales regarding bringing pebbles into the cross section versus its depletion of portion of the disk near enough for pebbles to drift into the cross section. 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 pebble-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, it is also possible that such a drift could assist in the planetesimal's accretion, bringing more material from further out in the disk.