  'Oumuamua, the first bona-fide interstellar planetesimal, was discovered
passing through our Solar System on a hyperbolic orbit. This object was likely
dynamically ejected from an extrasolar planetary system after a series of close
encounters with gas giant planets. To account for 'Oumuamua's detection, simple
arguments suggest that ~1 Earth mass of planetesimals are ejected per Solar
mass of Galactic stars. However, that value assumes mono-sized planetesimals.
If the planetesimal mass distribution is instead top-heavy the inferred mass in
interstellar planetesimals increases to an implausibly high value. The tension
between theoretical expectations for the planetesimal mass function and the
observation of 'Oumuamua can be relieved if a small fraction (~0.1-1%) of
planetesimals are tidally disrupted on the pathway to ejection into
'Oumuamua-sized fragments. Using a large suite of simulations of giant planet
dynamics including planetesimals, we confirm that 0.1-1% of planetesimals pass
within the tidal disruption radius of a gas giant on their pathway to ejection.
'Oumuamua may thus represent a surviving fragment of a disrupted planetesimal.
Finally, we argue that an asteroidal composition is dynamically disfavoured for
'Oumuamua, as asteroidal planetesimals are both less abundant and ejected at a
lower efficiency than cometary planetesimals.
