Relative angle δ between the expected mean plane of the Kuiper belt and IP8 in the presence of a 10 m_⊕ Planet 9 with orbital elements a₉ (top panel: = 400 AU; bottom panel: = 600 AU), e₉ (=0.4−0.6), and i₉ (=20°−30°), as a function of the semimajor axis a (>50 AU). The curve denotes the prediction from secular theory and the dots indicate the results from numerical computations with no approximations. The figure indicates these two approaches show good agreement.