  The solar system's distant reaches exhibit a wealth of anomalous dynamical
structure, hinting at the presence of a yet-undetected, massive trans-Neptunian
body - Planet 9. Previous analyses have shown how orbital evolution induced by
this object can explain the origins of a broad assortment of exotic orbits,
ranging from those characterized by high perihelia to those with extreme
inclinations. In this work, we shift the focus toward a more conventional class
of TNOs, and consider the observed census of long-period, nearly planar,
Neptune-crossing objects as a hitherto-unexplored probe of the Planet 9
hypothesis. To this end, we carry out comprehensive $N-$body simulations that
self-consistently model gravitational perturbations from all giant planets, the
Galactic tide, as well as passing stars, stemming from initial conditions that
account for the primordial giant planet migration and sun's early evolution
within a star cluster. Accounting for observational biases, our results reveal
that the orbital architecture of this group of objects aligns closely with the
predictions of the P9-inclusive model. In stark contrast, the P9-free scenario
is statistically rejected at a $\sim5\,\sigma$ confidence-level. Accordingly,
this work introduces a new line of evidence supporting the existence of Planet
9 and further delineates a series of observational predictions poised for
near-term resolution.
