  We perform $N$-body simulations of the early phases of open cluster evolution
including a large population of planetesimals, initially arranged in
Kuiper-belt like discs around each star. Using a new, 4th-order and
time-reversible $N$-body code on Graphics Processing Units (GPUs), we evolve
the whole system under the stellar gravity, i.e. treating planetesimals as test
particles, and consider two types of initial cluster models, similar to IC348
and the Hyades, respectively. In both cases, planetesimals can be dynamically
excited, transferred between stars or liberated to become free-floating (such
as A/2017 U1 or 'Oumuamua) during the early cluster evolution. We find that
planetesimals captured from another star are not necessarily dynamically
distinct from those native to a star. After an encounter both native and
captured planetesimals can exhibit aligned periastrons, qualitatively similar
to that seen in the Solar system and commonly thought to be the signature of
Planet 9. We discuss the implications of our results for both our Solar system
and exoplanetary systems.
