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^1,2. 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 migration^3,4,5 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 Saturn^8,9,10. 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.

行星的倾角是它的赤道和它的轨道平面之间的倾斜。巨行星预计将以接近零的倾角^1,2形成。因此，土星形成后，一定是某种动力机制使土星倾斜到目前的 26.7° 倾角。^{由于土星的自旋轴进动与海王星6}的节点轨道进动模式之间的共振交叉，这一事件传统上被认为发生在 4 Gyr 之前的晚期行星迁移^3,4,5期间。在这里，我们展示了土卫六的快速潮汐迁移，参考文献中报告了测量结果。⁷与这种情况不相容，并且它为土星当前的倾角提供了新的解释。土卫六的大量迁移将阻止任何早期共振，这将使先前因土星^8、9、10倾斜而对晚期行星迁移的限制无效。相反，我们建议共振是最近遇到的，大约 1 Gyr 前，并迫使土星的倾角从一个小值（可能小于 3°）增加到其当前状态。这种情况表明土星的归一化极惯性矩介于 0.224 和 0.237 之间。^{我们的发现为土星木星11}的自旋轴演化提供了新范式 可能还有多个系统中的巨型系外行星，其中倾角不是一劳永逸的，而是由于其卫星的迁移而不断演变。