Colossal Collision Kicked Uranus On Its Side

Uranus and its rings are pictured here in a false color photo taken with the Hubble Space Telescope. Unlike the other planets with their more moderate tilts, Uranus rotates on its side. NASA/ESA

Blessed is the 23.5° tilt of Earth’s axis. Without it there would be no seasons. How fun would that be? All the solar system’s planets spin on tilted axes, but none so much as Uranus, which nods 98°, a smidge past sideways.  Scientists have always wondered how Uranus got tilted so much that it spins on its side; new research on the planet’s early formation points to a major collision that literally knocked the planet over. Four billion years ago, scientists believe a young proto-planet of rock and ice collided with Uranus, causing its extreme tilt. Instead of rotating like a top spinning nearly upright, as Earth does, the planet “rolls” on its side as it circles the sun.


A simulation of the most likely Uranus-impact scenario that caused today’s tilted orbit, according to new, detailed simulations. Light gray represents ice materials from Uranus, while dark gray represents rock materials from Uranus. Purple represents ice materials from the impactor, while brown represents rock from the impactor. Light blue represents Uranus’ atmosphere. Jacob Kegerreis / Durham University

The research team, led by Durham University, UK, in collaboration with scientists at NASA’s Ames Research Center in Silicon Valley, used advanced computing techniques to create the most detailed simulation to date of the suspected impact. They worked through more than 50 impact scenarios using a supercomputer and determined that the object with at least twice the mass of Earth struck the young Uranus with a glancing blow. While the planet kept its atmosphere in the simulations, the blow was powerful enough to push the planet on its side and remix and reshape its interior.

What a topsy-turvy planet. Uranus’s rotation axis is the thinner black line that passes through the center of the planet. The magnetic field, represented by the arrowed arcs, is tilted 59° to the rotation axis and offset from the planet’s center by a third of its radius.

The impact might have left molten ice and lopsided lumps of rock within the planet, which might explain its tilted and off-center magnetic field, too. Rock and ice thrown into orbit would have then clumped together to form the rings and moons around Uranus. Like Earth, Uranus is enveloped in a magnetic field with a magnetic north and south, but the similarity ends there. Earth magnetic field lines up with the planet’s axis. Uranus’s field is tilted 59° to its axis and offset 5,200 miles (8,370 km) from the planet’s center.

The tilts of the other planets, especially those with larger tilts like Earth (23.5°), Mars (25°), Saturn (27°), Neptune (28°) and Venus (177° — it spins backwards!) were likely caused by impacts, too. The early solar system was a rough and tumble place where anything could happen.

3 Responses

    1. astrobob

      Hi Carol,
      Yes, you’re right, but their simulation went beyond that to attempt to show why the planet has an unusual interior and magnetic field. That’s how I read it anyway. And I suppose any additional evidence of impact through supercomputer simulations helps to firm up the idea.

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