Pebble accretion
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.
In a protoplanetary disk, the probability of accretion of pebbles ranging in size from centimeters to a meter is ≤10% on planets up to about 20 Earth masses.
[1] A protoplanetary disk is made up of a mix of gas and solids including dust, pebbles, planetesimals, and protoplanets.
[2] Gas in a protoplanetary disk is pressure supported and as a result orbits at a velocity slower than large objects.
[10] The aerodynamic deflection of the gas around the planetesimal reduces the efficiency of pebble accretion resulting in a maximum growth timescale of 100 km.
[10] The gravitational influence of these massive bodies can create a partial gap in the gas disk altering the pressure gradient.
[13] The accretion of the cores of giant planets via the collision and mergers of planetesimals is slow and may be difficult to complete before the gas disk dissipates.
[16] The period of runaway growth is then extended and the largest objects are able to accrete a sizable fraction of the pebbles and grow into giant planet cores.
[18] As the cores grow larger some reach masses sufficient to create partial gaps in the gas disk, altering its pressure gradient and blocking the inward drift of pebbles.
Cores that do not grow massive enough to clear gaps in the pebble disk are only able to accrete small gas envelopes and instead become ice giants.
In simulations, cold gas giants like Jupiter and Saturn can form via pebble accretion if their initial embryos began growing beyond 20 AU.
[19][20] However, dedicated formation models indicate that it is difficult to reconcile growth via pebble accretion with the final mass and composition of the solar system ice giants Uranus and Neptune.
[24] Here growth is due to a mix of pebble and planetesimal accretion until an oligarchical configuration of isolated lunar-massed embryos forms.
[24][25] The cutoff of the inward drift of icy pebbles by the formation of Jupiter before the ice line moved into the terrestrial region would limit the water fraction of the planets formed from these embryos.
[27] As a result, the density of the gas and the aerodynamic drag felt by pebbles embedded in the disk would have decreased significantly with distance.
