Evidence for the Streaming Instability from Non-Keplerian Effects in Cold Classical TNO Binaries

Recent results from planet formation simulations show that the Streaming Instability model for planetesimal formation is a good match to the orientation of Trans-Neptunian Object (TNO) binaries and the properties of New Horizons target Arrokoth. Indeed, constraints from the near-primoridal Cold Classical TNOs (CCTNO) are some of the most powerful observational evidence for this potentially general planet formation mechanism. Further justification and characterization of the Streaming Instability model can come from additional studies of CCTNO binaries. In particular, we investigate whether the components of CCTNO binaries have contact-binary-like shapes expected from collapse of a cloud of particles with too much angular momentum. We have developed the publicly-available tool MultiMoon which investigates non-Keplerian effects within small body binaries due to shapes and unresolved inner components. Applying MultiMoon to all known CCTNO binaries, we find several that are consistent with components with the shape of contact binaries and/or hierarchical triples. By rigorously investigating upper limits, we show that a significant fraction of known CCTNO binaries are consistent with the hypothesis that the components have contact-binary-like shapes. We consider this additional evidence supporting the Streaming Instability model for the formation of these systems.