(original) (raw)

TY - JOUR AU - Saillenfest, Melaine AU - Lari, Giacomo AU - Boué, Gwenaël PY - 2021 DA - 2021/04/01 TI - The large obliquity of Saturn explained by the fast migration of Titan JO - Nature Astronomy SP - 345 EP - 349 VL - 5 IS - 4 AB - The obliquity of a planet is the tilt between its equator and its orbital plane. Giant planets are expected to form with near-zero obliquities1,2. After the formation of Saturn, 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 Gyr ago during the late planetary migration3–5 because of the crossing of a resonance between the spin-axis precession of Saturn and the nodal orbital precession mode of Neptune6. Here, we show that the fast tidal migration of Titan for which the measurement is reported in ref. 7 is incompatible with this scenario, and that it offers a new explanation for Saturn’s current obliquity. A substantial migration of Titan would prevent any early resonance, which would invalidate previous constraints on the late planetary migration that were set by the tilting of Saturn8–10. We propose instead that the resonance was encountered more recently, about 1 Gyr ago, and forced Saturn’s obliquity to increase from a small value (possibly less than 3°) to its current state. This scenario suggests that Saturn’s normalized 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, Jupiter11 and possibly giant exoplanets in multiple systems, whereby obliquities are not settled once for all but evolve continuously as a result of the migration of their satellites. SN - 2397-3366 UR - https://doi.org/10.1038/s41550-020-01284-x DO - 10.1038/s41550-020-01284-x ID - Saillenfest2021 ER -