The Tilting of Uranus

When I stumbled across the article with the title, “A New Approach to Tilting Uranus,” I knew immediately I had to include the topic in the newsletter. Uranus is one of those puns that everyone continues to love, despite “growing up.” I know some people who publicly claim to disdain this line of joking, but I also know secretly they love it. It was great timing, too, as the topic intersects with the seasonal clockwork, which we’ll delve into in the next issue on Friday.

We have axial tilt to thanks for the seasons. Earth’s axial tilt is 23.4 degrees. What exactly does that mean? Axial tilt is the angle between a planet’s rotational axis at its north pole and a line perpendicular to the orbital plane of the planet. Check out this handy image for a visual representation.

Essentially, our planet tilts toward and away from the sun as it travels around our star. When one hemisphere tilts closer to the sun that hemisphere experiences spring and summer. When one hemisphere tilts away it experiences autumn and winter.

Most planets in our solar system have an obliquity, another word for axial tilt, similar to ours, in the ballpark of 0 to 30 degrees. But you may have learned growing up that Uranus is special. In other news, the planet of Uranus is also special (sorry, couldn’t help myself). Schoolhouse Rock, anyone? Uranus has an axial tilt of 98 degrees. That measurement means Uranus spins on its side in relation to its orbit around the sun.

Scientists have long believed the odd tilt of Uranus was due to impacts with very large space objects. But the authors of the article I noted above have offered a new theory, after identifying weaknesses with the previous model:

If Uranus were impacted many times, causing a strange tilt, but Neptune was not, their rotation rates would probably be drastically different. They only differ by 6%
Impacts would disrupt the moons of Uranus; instead, their masses fall within the “appropriate” range of a planet its size.
Computer models find it difficult to imagine a scenario where a single impact was large enough to tilt Uranus.
Impacts would have heated Uranus so much that its interior ice would have ejected into space, which would have caused the moons to be icy; but the satellites have little ice.

So what is their new idea?

When the ice giants of our solar system formed, they had what is called a “circumplanetary disk.” These disks are made up of space matter that orbits the planet and is either assimilated into the orb or forms a moon. These disks last for only about the first million years of the planet’s life cycle.

The new study proposes one of these disks might have been responsible for the tilt of Uranus. Computer models showed that certain scenarios related to the circumplanetary disks can cause tilting. One projection produced as much as 70 degrees of tilt on Uranus. You might recall we said Uranus is tilted 98 degrees. The authors believe the disk got Uranus most of the way there, but then an impact did the rest of the work.

So the old theory wasn’t all wrong.

It’s a fascinating article and, to top it all off, the article comes from the American Astronomical Society. In partnership with Nova, the website on which I found the article is aasnova.org. That’s very close to perfect. I apologize for being 8 years old. Article link below!