Modified Newtonian Dynamics as an Alternative to the Planet Nine Hypothesis

A new class of Kuiper Belt objects (KBOs) that lie beyond Neptune with semimajor axes greater than 250 astronomical units show orbital anomalies that have been interpreted as evidence for an undiscovered ninth planet. We show that a modified gravity theory known as modified Newtonian dynamics (MOND) provides an alternative explanation for the anomalies using the well-established secular approximation. We predict that the major axes of the orbits will be aligned with the direction toward the Galactic center and that the orbits cluster in phase space, in agreement with observations of KBOs from the new class. Thus, MOND, which can explain galactic rotation without invoking dark matter, might also be observable in the outer solar system.