Astronomers have a new theory explaining why Uranus spins sideways

One of the most bizarre phenomena in our solar system is the strange way Uranus spins sideways. It’s a puzzle because all the other planets rotate upright. What could possibly have happened to make Uranus so different, especially from its neighbor Neptune, which formed around the same time under similar circumstances?

Conventional thought holds that shortly after the solar system was formed, Uranus was knocked over by a series of collisions with some of the many planetesimals that swept through the region at that time.

The problem with this theory is that Neptune survived the same conditions unscathed. This suggests that another process was responsible for Uranus’ bizarre behavior. But what could it be?

reverse orbit

We now have a potential answer thanks to the work of Melaine Saillenfest at the Paris Observatory in France, and her colleagues, who believe that Uranus could have tilted sideways in another way. They say the tilt can be explained if Uranus once had a large, ancient satellite whose orbit gravitationally interacted with the planet’s own rotation in a way that slowly flipped it sideways.

First, a little background. Astronomers have long understood that the gravitational relationship between planets and their satellites can be complex and long-lasting. Indeed, small satellites can have a significant impact on their larger hosts by their repeated gravitational nudges during their orbit.

When the nudges occur at a frequency that resonates with a property of the host planet, the effects can be greatly magnified, especially as the satellite slowly moves away from its host.

Astronomers know that the Moon slowly migrates away from Earth at a rate of about 1.5 inches (3.8 centimeters) per year. But recent observations have revealed that the satellites around Jupiter and Saturn are also migrating.

This suggested to Saillenfest and his colleagues that something similar might have happened with Uranus. Under these conditions, gravitational nudges from a large satellite could have resonated with the precession of Uranus’ spin axis, gradually tilting the planet sideways.

The team simulated the process with Uranus to determine the conditions under which this could have happened.

It turns out that a satellite barely one thousandth the mass of Uranus could have tilted the planet away by a distance of about 10 times the radius of Uranus. “To achieve the inclination in less than the age of the solar system, the average drift rate of the satellite must be comparable to the current orbital expansion of the Moon,” say Saillenfest and co.

The team’s simulations show that once the planet tilts beyond 80 degrees, its behavior and the satellite’s orbit become chaotic and unpredictable to the point that the satellite can collide with Uranus.

However, when this happens, Uranus’ behavior stabilizes and its rotation locks at this very unusual inclined angle. Tellingly, Uranus today has no large moons, unlike Neptune, which has Triton, Saturn, which has Titan, and Jupiter, which has Ganymede and others.

Fossilized Spin

This is an interesting theory with a lot of merits. “To our knowledge, this is the first time that a single mechanism has been able to both tilt Uranus and fossilize its spin axis to its final state without invoking a giant impact or other external phenomena,” state Saillenfest and co.

However, this is by no means a slam dunk. “The conditions required for the tilt seem broadly realistic, but it remains to be determined whether Uranus could have hosted a large primordial satellite subject to a large tidal migration”, specify the astronomers.

One thing that could help shed more light on this scenario is a better understanding of the migration of Uranus’s satellites today as well as their other properties. For Saturn and Jupiter, much of this detail had to wait for various orbiting spacecraft to visit, such as Galileo, Juno, and Cassini.

Only one spacecraft made the lonely trip to Uranus. Voyager 2 swept through the passage in January 1986 as it exited the solar system. And although various space agencies are planning to send an orbiter, no mission has been approved.

In the meantime, astronomers will have to make do with ever more detailed observations from Earth and the JWST. These data should be useful, say Saillenfest and co: “Our results can serve as a solid starting point for future experiments.”


Ref: Tilting of Uranus via the migration of an old satellite: arxiv.org/abs/2209.10590

Sharon D. Cole