Tilting Uranus via Secular Spin-Orbit Resonance with Planet 9

Abstract

Uranus' startlingly large obliquity of 98 degrees 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 regards to the Uranus' spin rate and its 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' 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' obliquity past 98 degrees, with a significant number falling within 10 percent 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' obliquity, the feasibility of such a result hinges on Uranus' primordial precession rate.

17 pages, 12 figures, Accepted by the Planetary Science Journal

References in corpus (12)

• A Circumplanetary Disk Around PDS70c
• The Planet Nine Hypothesis
• Excitation of Planetary Obliquities Through Planet-Disk Interactions
• Tatooine's Future: The Eccentric Response of Kepler's Circumbinary Planets to Common-Envelope Evolution of their Host Stars
• The large obliquity of Saturn explained by the fast migration of Titan
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• The future large obliquity of Jupiter
• The Stability Boundary of the Distant Scattered Disk
• Injection of Inner Oort Cloud Objects Into the Distant Kuiper Belt by Planet Nine
• The past and future obliquity of Saturn as Titan migrates
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Cited by in corpus (3)

• Tilting Uranus via the migration of an ancient satellite
• Leaning Sideways: VHS 1256-1257 b is a Super-Jupiter with a Uranus-like Obliquity
• A search for Planet Nine with small spacecraft:Three-body, post-Newtonian, non-gravitational, planetary and Kuiper Belt effects