The Role of Gravitational Collapse in Planetesimal Formation

The radial drift, fragmentation, and bouncing barriers limit pairwise growth to cm-sized pebbles, inhibiting planetesimal formation. Continued growth to 100-km planetesimal sizes may occur due to the gravitational collapse of clouds of pebbles gathered by interactions with nebular gas as in the streaming instability (Youdin & Goodman 2005; Johansen et al. 2007). The abundance of Kuiper Belt binaries is taken as evidence of this formation process because excess angular momentum of the pebble cloud would prevent coalescence into a single body (Nesvorný et al. 2010, 2019, 2020; Robinson et al. 2020). However, previous work has only explored a limited space of initial cloud conditions. Here, we examine how a collapsing cloud’s initial velocity and mass distributions affect the formation of planetesimal systems. In particular, we focus on the dynamics and multiplicity of the collapsed planetesimals as well as individual planetesimal morphology and spin dynamics.

We use PKDGRAV (Stadel 2001; Richardson et al. 2000)—a parallel tree gravitational N-body integrator—to model a cloud undergoing gravitational collapse and make use of PKDGRAV’s soft-sphere discrete element method to ensure that colliding particles may stick and rest upon one another rather than merging and forming a single larger spherical particle (Schwartz et al. 2012). With this model, we accurately track the evolution, rotational dynamics, and shapes of individual planetesimals. Because we do not use an artificial inflation factor to enhance the collision rate, our particles maintain realistic densities throughout the simulation. The use of inflation factors may induce overly vigorous planetesimal growth, which prevents the formation of tightly orbiting systems and biases final system architectures towards binarity rather than higher number multiplicity.

Our results show that over a wide range of initial angular velocity values, gravitational collapse can form planetesimal systems composed of: a single large primary planetesimal; a bound secondary planetesimal with multibody systems progressively more-common at higher angular velocities; and a region of aggregates/debris bound to each planetesimal system, with some debris forming their own binary/multibody systems. Clouds with increasingly even mass distributions create planetesimals with wider orbital separations. Further, evenly-distributed clouds are not as efficient at retaining mass from the initial cloud as clouds with particles concentrated toward their centers. Of note is that all binaries created in our simulations maintain noticeably tighter orbits and retain a considerable fraction of the initial cloud mass than binaries formed in simulations performed by Nesvorný et al. 2010 and Robinson et al. 2020.