The large obliquity of Saturn explained by the fast migration of Titan

Abstract

The obliquity of a planet is the tilt between its equator and its orbital plane. Giant planets are expected to form with near-zero obliquities. After its formation, some dynamical mechanism must therefore have tilted Saturn up to its current obliquity of 26.7°. This event is traditionally thought to have happened more than 4 Gyrs ago during the late planetary migration because of the crossing of a resonance between the spin-axis precession of Saturn and the nodal orbital precession mode of Neptune. Here, we show that the fast tidal migration of Titan measured by Lainey et al. (2020) is incompatible with this scenario, and that it offers a new explanation for Saturn's current obliquity. A significant migration of Titan would prevent any early resonance, invalidating previous constraints on the late planetary migration set by the tilting of Saturn. We propose instead that the resonance was encountered recently, about 1 Gyr ago, forcing Saturn's obliquity to increase from a small value (possibly less than 3°), up to its current state. This scenario suggests that Saturn's normalised polar moment of inertia lies between 0.224 and 0.237. Our findings bring out a new paradigm for the spin-axis evolution of Saturn, Jupiter, and possibly giant exoplanets in multiple systems, whereby obliquities are not settled once for all, but continuously evolve as a result of the migration of their satellites.

arXiv commentAuthor's manuscript of the letter published in Nature Astronomy on 18-01-2021. The published version can be found at https://www.nature.com/articles/s41550-020-01284-x

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• Non-Trivial Oblique Spin Equilibria of Super-Earths in Multi-planetary Systems
• The past and future obliquity of Saturn as Titan migrates
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• Warm Jupiters Beyond the Tidal Synchronization Limit May Exhibit a Wide Range of Secondary Eclipse Depths
• Tilting Uranus via Secular Spin-Orbit Resonance with Planet 9
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• The Coupled Tidal Evolution of the Moons and Spins of Warm Exoplanets