Vol. 674
10. Planets and planetary systems

Spin of protoplanets generated by pebble accretion: Influences of protoplanet-induced gas flow

A new mechanism to generate the spin of protoplanets’ pebble accretion consists of the accretion of centimeter- to meter-sized particles into planetesimals in a protoplanetary disk, aided by aerodynamic drag from the gas present in the disk. This mechanism has been studied for over a decade, as it is efficient enough to build protoplanets of 10 Earth masses. The present study, by Takaoka et al., addresses the issue of protoplanetary spin. To do so, the authors performed two types of numerical simulations. The first one involved hydrodynamical simulations aimed at modeling the gas flow around the planet. They then used this gas flow as input for simulations of the three-dimensional orbits of the pebbles accreting onto the protoplanet. These simulations show that the gas envelope surrounding the protoplanet drags the accreting pebbles, resulting in an acceleration of spin regardless of planetary mass, pebble size, and the headwind speed of the gas. However, spin increases with a thicker envelope and an initial prograde spin of the protoplanet. These results suggest that the formation of the Earth by pebble accretion could have facilitated the formation of the Moon through a giant impact on a fast-spinning proto-Earth.