http://arxiv.org/abs/1908.10969
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 $70^{\circ}$. We find tilts greater than $70^{\circ}$ to be very rare and tilts beyond $90^{\circ}$ to be impossible, but a subsequent collision with an object about $0.5\,M_{\oplus}$ could tilt Uranus from $70^{\circ}$ to $98^{\circ}$. Neptune, on the other hand, needs a less massive disk to explain its $30^{\circ}$ 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.
Z. Rogoszinski and D. Hamilton
Fri, 30 Aug 19
33/58
Comments: 18 pages, 8 figures, submitted to ApJ
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