It is weird.
Something knocked it over on its axis. The north/south pole line of Earth is tipped over by 23 degrees from the plane of its orbit. That has never been explained.
Its big moons are a bit odd. The five of them all circle the planet in the same direction as it rotates, as well as in the plane of its equator. The innermost one, Miranda, is the least massive, the next two, Umbriel and Ariel, are more massive, and the last two, Titania and Oberon, are the most massive. The densities of the moons are the same. It's suspicious that their densities are related to how far they are from the planet.
It is a little odd that they exist. The formation of Jupiter and Saturn may have helped the formation of their moons because they were large enough to draw in a lot of gas. Runaway gas accretion is a phase in which the moons formed, but they are less massive and probably didn't go through it. It's hard to explain how the moons are half rock and half ice.
It has been thought for a while that there was an impact on Uranus, which could explain its tilt and other features as well. Could it explain the moons?
A new paper shows that it is possible. Updating previous research, they used computer models to create simulations of giant planet-on-planet impacts, assuming different sized impactors. The material from the impact falls into a flattened disk made of dust from the impactor and the planet. They follow the evolution of the disk to see how the moons could form.
They track what happens to the disk due to its thermal properties and how thick it is.
If the impactor was a planet-sized rocky beast with about three times Earth's mass, hits at an oblique angle, and is moving at the terrifying speed of 65,500 kilometers per hour, they can reproduce a lot of the properties seen.
It is expected that the planet will be knocked over. They found that the collision stirs up the outermost layers of the planet, which keeps its internal heat inside. Jupiter, Saturn, and Neptune all emit more heat than the Sun does, but Uranus emits less heat than the Sun does, indicating that its heat is trapped inside and can't leak out. That's a plus for the sims as well.
It also reproduces the moons. An impact of that size makes a large amount of heat as it spreads out and cools. The impactor needs to be three times Earth's mass to hit the planet because 99% of the material falls back onto Uranus. The disk is able to stick around long enough to form moons because of the extra mass.
The ice particles in the disk are relatively small. What happens next is dictated by the interaction of gas and solid materials. Ice particles grew much larger than rock particles in the sims. The larger the particle, the more gas it can interact with, and this separates the ice and rock a bit.
Rock is denser than ice and the moon's density gets bigger the farther out it is. The ratio of rock to ice on a moon is roughly half rock and half ice, but it gets higher the farther out a moon was. The moons in the sims got bigger farther out in the disk as well, reproducing what you see in real life.
It's pretty nifty. They say they made some changes to their models, but most don't change the physics much. These simulations are meant to show that they can reproduce a lot of what is seen in the system, and not to close the book on anything. There are more steps to be taken, but it seems like scientists are zeroing in on why Uranus is a mess.
I won't go into how it pinched off a bit of gas and thenlched, one of many apt metaphors.