A Collision With An Ice Planet May Have Given Uranus Its Tilt And Its Moons

Infrared image of Uranus showing its rings. LAWRENCE SROMOVSKY, UNIVERSITY OF WISCONSIN-MADISON/W.W. KECK OBSERVATORY/NASA

Unlike every other planet in the Solar System, Uranus orbits on its side at an unusual 98-degree tilt. Models have shown that a catastrophic collision may have caused this feature. Those same models, though, can’t recreate the distribution and composition of the moons and rings surrounding Uranus, long puzzling astronomers.

A new paper in Nature Astronomy tackles these questions by having the impactor to be a planetoid between one and three times the mass of Earth, mostly made of water ice. Astronomers believe a collision with a small icy planet is key to explaining the distribution of moons and rings around the planet, as well as its peculiar tilt.

In the new simulation, the icy impactor hit Uranus, dramatically shifting its orbital inclination, throwing material into orbit around the planet. Most of this material fell back onto the planet but a fraction of it stayed in orbit. Over time, the material coalesced in the current configuration of moons and rings.

The main difference in this new model is ice. A model involving a rocky impactor suggests it would have formed denser moons than what we see around Uranus. This is due to how quickly materials would condense under those conditions. Rocks and metals solidify much faster than “volatile” substances such as water or ammonia.

The researchers compared Uranus and its rings and moons against the formation of our Moon, believed to have been caused when a Mars-sized body hit primordial Earth, throwing material into orbit. In Earth's case, the impactor’s material, being rocky, solidified quickly to become our satellite, the Moon. In Uranus's case, it is likely a small icy planet hit the ice giant, tilting it, and the material from the collision remained gaseous for longer due to being further from the Sun, which is why Uranus's moons are small. The ratio of Uranus's mass to its moons' mass is in fact 100 times greater than that of Earth and the Moon.

“This model is the first to explain the configuration of Uranus' moon system, and it may help explain the configurations of other icy planets in our Solar System such as Neptune,” lead author Professor Shigeru Ida from the Tokyo Institute of Technology, said in a statement.  

“Beyond this, astronomers have now discovered thousands of planets around other stars, so-called exoplanets, and observations suggest that many of the newly discovered planets known as super-Earths in exoplanetary systems may consist largely of water ice and this model can also be applied to these planets.”

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