Tilting Uranus via Spin–Orbit Resonance with Planet Nine

Abstract

Uranus’s startlingly large obliquity of 98° has yet to admit a satisfactory explanation. The most widely accepted hypothesis involving a giant impactor that tipped Uranus onto its side encounters several difficulties with regard to Uranus’s spin rate and prograde satellite system. An obliquity increase that was driven by capture of Uranus into a secular spin–orbit resonance remains a possible alternative hypothesis that avoids many of the issues associated with a giant impact. We propose that secular spin–orbit resonance could have excited Uranus’s obliquity to its present-day value if it was driven by the outward migration of an as-yet-undetected outer solar system body commonly known as Planet Nine. We draw support for our hypothesis from an analysis of 123 N-body simulations with varying parameters for Planet Nine and its migration. We find that in multiple instances, a simulated Planet Nine drives Uranus’s obliquity past 98°, with a significant number falling within 10% of this value. We note a significant caveat to our results in that a much faster than present-day spin axis precession rate for Uranus is required in all cases for it to reach high obliquities. We conclude that while it was, in principle, possible for Planet Nine (if it exists) to have been responsible for Uranus’s obliquity, the feasibility of such a result hinges on Uranus’s primordial precession rate.