Massive Protoplanet Hit Young Uranus, Caused It To Tilt, Study Says


According to a Durham University-led study, young Uranus was hit by a protoplanet about twice the size of Earth that caused the ice giant to tilt and could explain its extremely cold temperatures.

Uranus. Image credit: NASA.

Uranus. Image credit: NASA.

“Uranus spins on its side, with its axis pointing almost at right angles to those of all the other planets in the Solar System,” said lead author Jacob Kegerreis, a Ph.D. student in the Institute for Computational Cosmology at Durham University.

“This was almost certainly caused by a giant impact, but we know very little about how this actually happened and how else such a violent event affected the planet.”

Kegerreis and colleagues ran the first high-resolution computer simulations of different massive collisions with young Uranus to try to work out how the planet evolved.

The research confirms an earlier study which said that Uranus’ tilted position was caused by a collision with a massive object — most likely a young protoplanet made of rock and ice — during the formation of the Solar System about 4 billion years ago.

The simulations also suggested that debris from the impactor could form a thin shell near the edge of the planet’s ice layer and trap the heat emanating from Uranus’ core.

The trapping of this internal heat could in part help explain Uranus’ very cold temperature of the planet’s outer atmosphere (minus 357 degrees Fahrenheit, or minus 216 degrees Celsius).

“We ran more than 50 different impact scenarios using a high-powered super computer to see if we could recreate the conditions that shaped the planet’s evolution,” Kegerreis said.

“Our findings confirm that the most likely outcome was that the young Uranus was involved in a cataclysmic collision with an object twice the mass of Earth, if not larger, knocking it on to its side and setting in process the events that helped create the planet we see today.”

There has been a question mark over how Uranus managed to retain its atmosphere when a violent collision might have been expected to send it hurtling into space.

According to the new study, this can most likely be explained by the impact object striking a grazing blow on the planet.

The collision was strong enough to affect Uranus’ tilt, but the planet was able to retain the majority of its atmosphere.

The research could also help explain the formation of Uranus’ rings and moons, with the simulations suggesting the impact could jettison rock and ice into orbit around the planet.

This rock and ice could have then clumped together to form the planet’s inner satellites and perhaps altered the rotation of any pre-existing moons already orbiting Uranus.

The simulations show that the impact could have created molten ice and lopsided lumps of rock inside the planet. This could help explain Uranus’ tilted and off-center magnetic field.

The results were published in the July 2, 2018 issue of the Astrophysical Journal.


J.A. Kegerreis et al. 2018. Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion. ApJ 861, 52; doi: 10.3847/1538-4357/aac725