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The planet Uranus is another solar system anomaly, where according to the NASA profile has an axial tilt of 97.8 degrees, also considered to be retrograde. This NASA summary "Uranus" suggests the current theory of a large planet-sized impact earlier in its history.

Does the planet-impact theory still hold true or have new accepted theories come to light?

Most of all, are there any results from any simulations available?

A note, this is posted as a separate question to my other question "What is the current accepted theory as to why Venus has a slow retrograde rotation?" as the axial tilt is significantly different.

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    $\begingroup$ Well, sorry to all responses but there is no accepted theory. A theory must be founded on axioms that make predictions that are testable against evidence. To that end we don't have any theories, just specilation. $\endgroup$ – AtmosphericPrisonEscape Mar 5 '16 at 0:53
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Nasa.gov speculates that the most likely possibility is that an object with a mass close to that of Earth's collided with Uranus, causing it to rotate on its side from then on.

A recent test by Space.com suggests that "Planet Uranus Got Sideways Tilt From Multiple Impacts". These findings suggest that two or more smaller collisions probably occurred, asserting that the early solar system was probably more turbulent than most would think.

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  • $\begingroup$ Note that all 5 of Uranus's major moons orbit "on their side" too...that is, their orbital inclination relative to Uranus's equator is very small. This is good evidence that something impacted (more accurately, flew thru) the proto-planet's accretion disk. The disk got re-oriented and the moons accreted afterwards. $\endgroup$ – DrZ214 Aug 26 '17 at 3:00
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There is not a single accepted theory as to what caused Uranus' tilt. Rather there are two families of explanations and we still need to collect more data to figure out which is correct.

As explained by @Timtech, Uranus' tilt might have been caused by a giant impact. This is supported by simulations showing that an impact might have been violent enough to tilt the planet and eject debris that agglomerated into moons. But this is not the only possibility.

Another explanation for Uranus' tilt is a spin-orbit resonance. As Uranus was a growing protoplanet, its disc generated a resonance, causing the disc in which it was forming to tilt.

Simulations have been done showing that the tilt might have happened during planetary migration without any giant impacts.

Three of the linked articles above are from 2020, showing that this is still a hot topic of research and there is no consensus on what happened to Uranus.

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To add to the existing answers and adapting from my answer to this question about the Uranian satellites:

The single giant impact idea appears to be unlikely. In this scenario, the circumplanetary disc would re-align with the Uranian equator after the collision, but would end up with a retrograde rotation and thus produce retrograde satellites rather than the observed prograde system (Morbidelli et al. 2012).

As noted by Rogoszinski & Hamilton (2020), the timescale for tilting Uranus via a resonances between the precession of the rotation axis and the orbits of the other giant planets appears to be too long compared to the expected timescale for giant planet migration in the Nice model: they estimate ~108 years to reach a tilt of 90°. Under ideal conditions, they estimate a tilt of ~40° could be reached in ~107 years.

From the abstract:

We find that two collisions totaling to 1 M is sufficient to explain the planet's current spin state. Finally, we investigate hybrid models and show that resonances must produce a tilt of ∼40° for any noticeable improvements to the collision model.

But as noted in usernumber's answer, this is an active area of research and our knowledge of the ice giant planets is woefully incomplete.

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