And they found that a moon with a minimum mass of around half that of Earth's Moon could tilt Uranus towards 90 degrees if it migrated by more than 10 times the radius of Uranus at a rate higher than 6 centimeters per year.
However, a larger moon with a size comparable to Ganymede was more likely, in the simulations, to produce the tilt and spin we see in Uranus today.However, the minimum mass – about half an Earth Moon – is about four times the combined mass of the current known Uranian moons.
At a tilt of about 80 degrees, the moon became destabilized, triggering a chaotic phase for the spin axis that ended when the moon ultimately collided with the planet, effectively "fossilizing" Uranus' axial tilt and spin.
"To our knowledge, this is the first time that a single mechanism is able to both tilt Uranus and fossilize its spin axis in its final state without invoking a giant impact or other external phenomena.It's not clear whether Uranus could have hosted a moon large enough and at a high enough migration rate to produce this scenario, and it will, the researchers say, be challenging to show with observations.However, a better understanding of the current rate of migration for Uranus's moons would go a significant way towards resolving these questions.