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Pebble accretion
In pebble accretion the accretion of objects ranging from cm's up to meters in diameter onto planetesimals in a protoplanetary disk is enhanced by aerodynamic drag. 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. In the inner Solar System the varying rates of growth due to a declining density of gas yield a small mass for Mars and a low mass asteroid belt.
==Description==
Pebbles ranging in size from cm's up to a meter in size are accreted at an enhanced rate in a protoplanetary disk. A protoplanetary disk is made up of a mix of gas and solids including dust, pebbles, planetesimals, and protoplanets.〔 Gas in a protoplanetary disk is pressure supported and as a result orbits at a velocity slower than large objects.〔 The gas affects the motions of the solids in varying ways depending on their size, with dust moving with the gas and the largest planetesimals orbiting largely unaffected by the gas.〔 Pebbles are an intermediate case, aerodynamic drag causes them to settle toward the central plane of the disk and to orbit at a sub-keplerian velocity resulting in radial drift toward the central star. The pebbles frequently encounter planetesimals as a result of their lower velocities and inward drift. If their motions were unaffected by the gas only a small fraction, determined by gravitational focusing and the cross section of the planetesimals, would be accreted by the planetesimals. The remainder would follow hyperbolic paths, accelerating toward the planetesimal on their approach and decelerating as they recede. However, the drag the pebbles experience grows as their velocities increase, slowing some enough that they collide with the planetesimal. Others become gravitationally bound, and are eventually accreted after spiraling toward the planetesimal.〔 Pebble accretion primarily affects the accretion rates of the largest planetesimals. These objects are massive enough that the duration of encounters is sufficient for gas drag to significantly reduce the relative velocities of the pebbles.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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