The existence of Planet Nine has been suggested to explain the pericenter clustering of extreme trans-Neptunian objects (TNOs). However, the underlying dynamics involving Planet Nine, test particles, and Neptune is rich, and it remains unclear which dynamical processes lead to the alignment and how they depend on the properties of Planet Nine. Here we investigate the secular interactions between an eccentric outer perturber and TNOs starting in a near-coplanar configuration. We find that a large number of TNOs could survive outside of mean-motion resonances at 4 Gyr, which differs from previous results obtained in the exact coplanar case with Neptune being treated as a quadrupole potential. In addition, secular dynamics leads to the orbital clustering seen in N-body simulations. We find that a near-coplanar Planet Nine can flip TNO orbital planes, and when this happens, the geometrical longitudes of pericenter of the TNOs librate around 180° during the flip. Orbital precession caused by the inner giant planets can suppress the flips while keeping the longitude of pericenter librating when 30 au ≲ r p ≲ 80 au and a ≳ 250 au. This results in the alignment of the pericenter of the low-inclination TNOs (i ≲ 40°). We find that the anti-aligned population and flipped orbits could be produced by an eccentric (e 9 ≳ 0.4) outer planet of ∼10 M ⊕ in a wide a 9 ≳ 400 ∼ 800 au orbit. Future surveys of the high-inclination TNOs will help further constrain the properties of possible outer planets.
