The effect of semicollisional accretion on planetary spins

Planetesimal accretion during planet formation is usually treated as collisionless. Such accretion from a uniform and dynamically cold disk predicts protoplanets with slow retrograde rotation. However, if the building blocks of protoplanets, planetesimals, are small, of the order of a meter in size, then they are likely to collide within the protoplanet's sphere of gravitational influence, creating a prograde accretion disk around the protoplanet. The accretion of such a disk results in the formation of protoplanets spinning in the prograde sense with the maximal spin rate allowed before centrifugal forces break them apart. As a result of semicollisional accretion, the final spin of a planet after giant impacts is not completely random, but is biased toward prograde rotation. The eventual accretion of the remaining planetesimals in the post-giant-impact phase might again be in the semicollisional regime and delivers a significant amount of additional prograde angular momentum to the terrestrial planets. We suggest that in our solar system, semicollisional accretion gave rise to the preference for prograde rotation observed in the terrestrial planets and perhaps the largest asteroids.