  The cold classical Kuiper belt objects have low inclinations and
eccentricities and are the only Kuiper belt population suspected to have formed
in situ. Compared with the dynamically excited populations, which exhibit a
broad range of colours and a low binary fraction of ~10% cold classical Kuiper
belt objects typically have red optical colours with ~30% of the population
found in binary pairs; the origin of these differences remains unclear. We
report the detection of a population of blue-coloured, tenuously bound binaries
residing among the cold classical Kuiper belt objects. Here we show that widely
separated binaries could have survived push-out into the cold classical region
during the early phases of Neptune's migration. The blue binaries may be
contaminants, originating at ~38 au, and could provide a unique probe of the
formative conditions in a region now nearly devoid of objects. The idea that
the blue objects, which are predominantly binary, are the products of push-out
requires that the planetesimals formed entirely as multiples. Plausible
formation routes include planetesimal formation via pebble accretion and
subsequent binary production through dynamic friction and binary formation
during the collapse of a cloud of solids.
