A hypothesis for Uranus's unusual tilt angle and opposing spin

A small group of academics from Sorbonne Université, the University of Maryland College Park, the University of Pisa, and the Université Côte d'Azur proposed a theory to explain Uranus' unusual tilt and opposing spin. The group has released an article outlining their study on the preprint service arXiv and is awaiting peer review results before publishing it in the journal Astronomy & Astrophysics. Uranus stands out among the planets in our solar system for two reasons. The first is its unusual tilt angle of 98 degrees off its orbital plane, which is far steeper than that of any other planet. The other difference between it and the other planets in the solar system is its clockwise spin orientation.

Space scientists have proposed hypotheses to explain Uranus's unusual properties during the last few years. Some speculate that the tilt was produced by a collision with another huge planet, while others speculate that it was impacted by a collection of smaller bodies. Due of the lack of evidence of such bodies, either possibility has been difficult to support. The researchers have proposed a fresh idea in this new attempt, claiming that the distinctive tilt angle is caused by the migration of its moons.

Members of the crew noted that Jupiter's tilt was growing due to the movement of its moons a few years ago. According to math estimates, its tilt will shift substantially over the next billion years. When they looked at Saturn, they saw comparable effects, owing mostly to Titan's movement. This motivated the scientists to investigate Uranus and its unusual tilt angle. To determine if moon migration was responsible for the enormous tilt angle, the researchers constructed computer simulations of several lunar migrations, altering the size and speed of the moons. They discovered that a moon half the size of the one orbiting Earth could elevate Uranus' tilt angle to about 90° over millions of years.

However, they discovered that the moons that presently orbit Uranus do not have enough mass to cause such a tilt. However, their simulations revealed that if a huge moon increased Uranus' tilt to 80°, things would become unstable and the moon would collide with the planet, which, the researchers discovered, might explain the amount of tilt as well as the planet's opposing rotation.

Journal Information: Tilting Uranus via the migration of an ancient satellite, arXiv:2209.10590 [astro-ph.EP] arxiv.org/abs/2209.10590