Giant collisions can account for Uranus' and Neptune's large obliquities, yet generating two planets with widely different tilts and strikingly similar spin rates is a low probability event. Trapping into a secular spin-orbit resonance, a coupling between spin and orbit precession frequencies, is a promising alternative as it can tilt the planet without altering its spin period. We show with numerical integrations that if Uranus harbored a massive circumplanetary disk of at most 40 times the mass of its satellite system while it was accreting its gaseous atmosphere, then its spin precession rate will increase enough to resonate with its own orbit, potentially driving the planet's obliquity to 70°. We find tilts greater than 70°to be very rare and tilts beyond 90°to be impossible, but a subsequent collision with an object about 0.5 M ⊕ could tilt Uranus from 70°to 98°. Neptune, on the other hand, needs a less massive disk to explain its 30°tilt, eliminating the need for giant collisions all together. Minimizing the masses and number of giant impactors from three or more to just one increases the likelihood of producing the ice giants' spin-states by about an order of magnitude.

In this paper, we investigate whether Uranus’s 98° obliquity was a by-product of a secular spin–orbit resonance assuming that the planet originated closer to the Sun. In this position, Uranus’s spin precession frequency is fast enough to resonate with another planet located beyond Saturn. Using numerical integration, we show that resonance capture is possible in a variety of past solar system configurations but that the timescale required to tilt the planet to 90° is of the order ∼108 yr—a time span that is uncomfortably long. A resonance kick could tilt the planet to a significant 40° in ∼107 yr only if conditions were ideal. We also revisit the collisional hypothesis for the origin of Uranus’s large obliquity. We consider multiple impacts with a new collisional code that builds up a planet by summing the angular momentum imparted from impactors. Because gas accretion imparts an unknown but likely large part of the planet’s spin angular momentum, we compare different collisional models for tilted, untilted, spinning, and nonspinning planets. We find that a 1 M ⊕ strike is sufficient to explain the planet’s current spin state, but that two 0.5 M ⊕ collisions produce better likelihoods. Finally, we investigate hybrid models and show that resonances must produce a tilt of at least ∼40° for any noticeable improvements to the collision model. Because it is difficult for spin–orbit resonances to drive Uranus’s obliquity to 98° even under these ideal conditions, giant impacts seem inescapable.

Context. The 98 • -obliquity of Uranus is commonly attributed to giant impacts that occurred at the end of the planetary formation. This picture, however, is not devoid of weaknesses. Aims. On a billion-year timescale, the tidal migration of the satellites of Jupiter and Saturn has been shown to strongly affect their spin-axis dynamics. We aim to revisit the scenario of tilting Uranus in light of this mechanism. Methods. We analyse the precession spectrum of Uranus and identify the candidate secular spin-orbit resonances that could be responsible for the tilting. We determine the properties of the hypothetical ancient satellite required for a capture and explore the dynamics numerically.Results. If it migrates over 10 Uranus' radii, a single satellite with minimum mass 4 × 10 −4 Uranus' mass is able to tilt Uranus from a small obliquity and make it converge towards 90 • . In order to achieve the tilting in less than the age of the Solar System, the mean drift rate of the satellite must be comparable to the Moon's current orbital expansion. Under these conditions, simulations show that Uranus is readily tilted over 80 • . Beyond this point, the satellite is strongly destabilised and triggers a phase of chaotic motion for the planet's spin axis. The chaotic phase ends when the satellite collides into the planet, ultimately freezing the planet's obliquity in either a prograde, or plainly retrograde state (as Uranus today). Spin states resembling that of Uranus can be obtained with probabilities as large as 80%, but a bigger satellite is favoured, with mass 1.7 × 10 −3 Uranus' mass or more. Yet, a smaller ancient satellite is not categorically ruled out, and there is room for improving this basic scenario in future studies. Interactions among several pre-existing satellites is a promising possibility. Conclusions. The conditions required for the tilting seem broadly realistic, but it remains to be determined whether Uranus could have hosted a big primordial satellite subject to substantial tidal migration. The efficiency of tidal energy dissipation within Uranus is required to be much higher than traditionally assumed, more in line with that measured for the migration of Titan. Hints about these issues would be given by a measure of the expansion rate of Uranus' main satellites.

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