We study the dynamics of outer solar system bodies under the influence of the modified gravity theory MOND. We find that tidal forces due to the Galaxy (which are much stronger in MOND than in Newtonian gravity) tend to align the major axes of the orbits of outer Kuiper belt objects with the direction towards the center of the galaxy. Such an alignment is seen in the orbits of the handful of outer Kuiper belt objects that are known at present. The observed alignment has previously led to the Planet Nine hypothesis: that there is a large undiscovered planet in the outer solar system. It is expected that in the near future surveys dedicated to the outer solar system as well as Dark Energy Survey, the Vera Rubin Telescope and CMB-S4 will discover many more outer Kuiper belt objects and settle the question of whether the observed alignment is due to Planet Nine, modified gravity or merely due to an observational bias of the existing surveys. MOND gravity deviates from Newtonian gravity when the field is weak. It was originally developed to account for galaxy rotation curves without invoking dark matter. Thus the Kuiper belt, which is a subject of great intrinsic interest, may also prove a laboratory for studying fundamental physics.

Reference - Kate Brown and Harsh Mathur, arXiv:2304.00576; to be published in the Astronomical Journal