Uranus and Pluto have their axes of rotation almost 90 degrees towards the ecliptic. But why do their moons tilt the same way?

Does it mean that their tilts were caused by passing near some external mass that affected the planet and its moons equally, rather than some planet specific process such as a comet impact or tectonics?

Mars' rotational axis tilts over time, but its moons today are spot on over its temporary equator. Earth's moon is aligned with the ecliptic and doesn't seem to care about our planetary tilt, although (or because of?) oceanic masses tugging on its tides. Exoplanets have been found that don't orbit in alignment with their star's rotation, and why would they do so billions of years after formation?


2 Answers 2


I have managed to find two hypotheses for Uranus's tilt that explain why its moons are also orbiting on a tilted plane.

The multiple impacts hypotheses lies on the fact that, if Uranus had become tilted by the force of a single large impact (as was commonly believed), then the moons should have stayed on their original plane. A corrected version of this hypothesis said that, if the accretion disc from which Uranus was forming was still around, an impact would have destroyed it and recreated it on the same place as the new equator. The moons would then have accreted from the disc on that plane. This, however, would have produced moons with retrograde motion (which is not the case for most). So the latest idea is that Uranus suffered multiple impacts, which, according to the simulations, works to produce the planetary system as we see it today. (There's another problem with this hypothesis.)

The collision-free theory (or collissionless scenario as described in the paper) proposes that Uranus could have become tilted progressively as it formed without the need to postulate an impact (or series of impacts), if it had an additional satellite and started with a large inclination (which is allowed by current models); the extra moon could have been ejected later. The simulations worked for an initial inclination of more 17 degrees and a moon of 0.01 Uranian mass and at 50 Uranian radii.


Just as tidal forces can push a moon outwards and over time, circularize the orbit, the planet's equatorial bulge has a similar force that draws the Moon into an orbit over the equator. This is true given sufficient time and enough rotation speed for the planet to have a sufficient equatorial bulge.

For a formation moon, there should be some similarity in the angular momentum during formation naturally. Similar to an impact moon like our own where the impact sets the planet rotating.

A captured moon can orbit in either direction with or against the planet's rotation at any angle to the planet's equatorial planet, at least, initially but the equatorial bulge has a tidal effect that over time, draws the moon over it.

Earth's tidal bulge, that moves the moon a few cm away every year is only a couple meters high over the oceans and a few inches over land. Earth's equatorial bulge is 42.77 km. Some 4 orders of magnitude more significant than the tidal bulge. It's not hard to see how a bulge of that size, over many orbits, would influence the moon into an orbit over the equator. The same is true for ring systems, which form over a planet's equator. "Ask an Astronomer" says basically the same thing.

Exceptions would be if the moon was recently captured, or if the planet rotated very slowly (Venus for example), or if the moon was close to the unstable region of the hill sphere where the object it orbits (planet) and the object that limits the planet's hill sphere (sun) both have an effect on the moon. I suspect (but I don't know how to do the math), that our moon's orbit being 5 and change degrees off Earth's equator is due to the proximity to the sun and Moon being no too far from the unstable part of the Hill Sphere.

The other moons you mention are much further inside their planet's Hill Spheres and much more governed by the planet's equatorial bulge. see table and chart below.

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    $\begingroup$ @pablodf76 suggests that the orbital inclination of the moons was caused during their and their planet's formation, one way or another. You suggest that tidal forces could change their inclination along with the changing rotational inclination (obliquity) of the planet's axis. Mars is know to tilt quite a bit, but its small moons don't seem to care about the equator, nor does Earth's Moon. Did the moons of Uranus form tilted (maybe during some impact) or did they follow Uranus' equator tilting here and there? $\endgroup$
    – LocalFluff
    Feb 6, 2017 at 19:10
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    $\begingroup$ On formation moons I agree with him. Captured moons, like Mars 2 moons are both just about 1% off Mars' equator so I don't think you can say that they don't care about Mars' equator.. As to the rest, I'll try to update later. $\endgroup$
    – userLTK
    Feb 6, 2017 at 23:54
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    $\begingroup$ I thought they were a couple of degrees from the ecliptic, like our Moon, but you'r of course right. They follow Mars' equator. $\endgroup$
    – LocalFluff
    Feb 7, 2017 at 8:40

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