Trans-Neptunian Binaries as Evidence for Planetesimal Formation by the\n Streaming Instability

A critical step toward the emergence of planets in a protoplanetary disk\nconsists in accretion of planetesimals, bodies 1-1000 km in size, from smaller\ndisk constituents. This process is poorly understood partly because we lack\ngood observational constraints on the complex physical processes that\ncontribute to planetesimal formation. In the outer solar system, the best place\nto look for clues is the Kuiper belt, where icy planetesimals survived to this\nday. Here we report evidence that Kuiper belt planetesimals formed by the\nstreaming instability, a process in which aerodynamically concentrated clumps\nof pebbles gravitationally collapse into 100-km-class bodies. Gravitational\ncollapse was previously suggested to explain the ubiquity of equal-size\nbinaries in the Kuiper belt. We analyze new hydrodynamical simulations of the\nstreaming instability to determine the model expectations for the spatial\norientation of binary orbits. The predicted broad inclination distribution with\n80% of prograde binary orbits matches the observations of trans-Neptunian\nbinaries. The formation models which imply predominantly retrograde binary\norbits can be ruled out. Given its applicability over a broad range of\nprotoplanetary disk conditions, it is expected that the streaming instability\nseeded planetesimal formation also elsewhere in the solar system, and beyond.\n