Direct contact binary planetesimal formation from gravitational collapse

TL;DR

This paper demonstrates through simulations that bilobate contact binary planetesimals like Arrokoth can form directly from the gravitational collapse of pebble clouds, supporting a specific formation hypothesis.

Contribution

The study introduces a novel application of the soft-sphere discrete element method to simulate planetesimal formation, showing that contact binaries can form directly from gravitational collapse.

Findings

Contact binary planetesimals can form directly from pebble cloud collapse.

Simulations produce a variety of shapes, including bilobate contact binaries.

Supports the hypothesis that Arrokoth formed via direct gravitational collapse.

Abstract

Bilobate contact binaries comprise a significant fraction of the relict Kuiper Belt, which includes the exemplary contact binary (486958) Arrokoth. The surfaces of its lobes contain similar amounts of highly volatile chemical species and few craters, indicating formation in a homogeneous and gentle environment. Arrokoth's bilobate shape was initially hypothesized to have formed via the direct gravitational collapse of a pebble cloud in the solar system's protoplanetary disk. However, alternative hypotheses have proposed that Arrokoth may be the result of binary planetesimal formation and the subsequent dynamical evolution of the binary components into contact through external perturbations over long timescales. Here, we show that contact binary planetesimals like Arrokoth can form directly from the gravitational collapse of pebble clouds. We used a soft-sphere discrete element method (SSDEM) to discover that planetesimals form a wide variety of shapes, including bilobate contact binaries. This method creates planetesimals as particle-aggregates with particles resting upon each other's surfaces via mutual surface penetration. The formation of contact binaries in our simulations strengthens the hypothesis that Arrokoth, and perhaps many other contact binaries in the Kuiper Belt, formed directly as bilobate objects from gravitational collapse, and so their shapes and surfaces record the era of planet formation.