  We investigate the prospects for the capture of the proposed Planet 9 from
other stars in the Sun's birth cluster. Any capture scenario must satisfy three
conditions: the encounter must be more distant than ~150 au to avoid perturbing
the Kuiper belt; the other star must have a wide-orbit planet (a>~100au); the
planet must be captured onto an appropriate orbit to sculpt the orbital
distribution of wide-orbit Solar System bodies. Here we use N-body simulations
to show that these criteria may be simultaneously satisfied. In a few percent
of slow close encounters in a cluster, bodies are captured onto heliocentric,
Planet 9-like orbits. During the ~100 Myr cluster phase, many stars are likely
to host planets on highly-eccentric orbits with apastron distances beyond 100
au if Neptune-sized planets are common and susceptible to planet--planet
scattering. While the existence of Planet 9 remains unproven, we consider
capture from one of the Sun's young brethren a plausible route to explain such
an object's orbit. Capture appears to predict a large population of
Trans-Neptunian Objects (TNOs) whose orbits are aligned with the captured
planet, and we propose that different formation mechanisms will be
distinguishable based on their imprint on the distribution of TNOs.
