  A critical step toward the emergence of planets in a protoplanetary disk
consists in accretion of planetesimals, bodies 1-1000 km in size, from smaller
disk constituents. This process is poorly understood partly because we lack
good observational constraints on the complex physical processes that
contribute to planetesimal formation. In the outer solar system, the best place
to look for clues is the Kuiper belt, where icy planetesimals survived to this
day. Here we report evidence that Kuiper belt planetesimals formed by the
streaming instability, a process in which aerodynamically concentrated clumps
of pebbles gravitationally collapse into 100-km-class bodies. Gravitational
collapse was previously suggested to explain the ubiquity of equal-size
binaries in the Kuiper belt. We analyze new hydrodynamical simulations of the
streaming instability to determine the model expectations for the spatial
orientation of binary orbits. The predicted broad inclination distribution with
80% of prograde binary orbits matches the observations of trans-Neptunian
binaries. The formation models which imply predominantly retrograde binary
orbits can be ruled out. Given its applicability over a broad range of
protoplanetary disk conditions, it is expected that the streaming instability
seeded planetesimal formation also elsewhere in the solar system, and beyond.
