Astronomers Think They Know The Reason For Uranus's Kooky Off-Kilter Axis


The strange little drum Uranus plays is its own.

It possesses a number of peculiarities that are all its own, despite having many similarities to Neptune, the other ice giant in our Solar System.

One of these is particularly noticeable since it appears to be lying down due to its tilted rotating axis. That enormous inclination from the orbital plane is 98 degrees.

Additionally, it rotates in the opposite direction from most of the other planets in the Solar System—clockwise—which is the cherry on top.

A recent study discovered a likely cause for this peculiar behaviour: Uranus is tipped onto its side as a result of the moon moving away from the planet. Furthermore, the moon wouldn't even have to be large. Although a larger moon would be the more plausible candidate, anything with half the mass of our own moon may have pulled it off.

The justification was presented in a study written by astronomer Melaine Saillenfest of the French National Center for Scientific Research. This article has been accepted for publication in the journal Astronomy & Astrophysics and is now accessible on the preprint server arXiv. It has not yet undergone peer review.

Models have been proposed by scientists to explain this peculiar behaviour, such as a huge object slamming into Uranus and literally knocking it sideways, but a collection of smaller particles is currently the most popular theory.

This hypothesis, however, introduces problems that are even harder to explain, particularly those annoying resemblances to Neptune.

The masses, radii, rotation rates, atmospheric dynamics and compositions, and bizarre magnetic fields of the two planets are very similar. These similarities imply that the two planets might have formed simultaneously, and they become much more challenging to explain when planet-tipping impacts are added to the equation.

This has prompted scientists to look for alternative theories, such as a wobble that may have been caused by a massive ring system or a massive moon early in the history of the Solar System (albeit with a different mechanism).

Saillenfest and his associates did, however, discover something intriguing about Jupiter a few years ago. The gas giant's moons' outward migration may cause its tilt to increase from its current low 3 percent to about 37 percent in a few billion years.

Then they turned their attention to Saturn, where they discovered that Titan, its largest moon, may have rapidly migrated outward to cause its current tilt of 26.7 degrees. They discovered that this could have happened essentially without having any impact on the planet's spin rate.

That naturally led to inquiries regarding the planet with the highest inclination in our solar system. Therefore, the team undertook simulations of a potential Uranian system to see if a comparable process might account for its idiosyncrasies.

Moon migration is a common occurrence. The present distance between the Earth and our own Moon is around 4 centimetres (1.6 inches) every year. The tidal force exerted by bodies orbiting a shared centre of gravity slows their rotations over time. Gravity's hold is loosened as a result, increasing the gap between the two bodies.

Returning to Uranus, the researchers ran simulations using a variety of variables, including the fictitious moon's mass. And they discovered that if a moon moved by more than 10 times Uranus' radius at a rate greater than 6 millimetres per year, a moon with a minimum mass of roughly half that of the Earth's moon could tilt Uranus towards 90 degrees.

The simulations indicated that the tilt and spin we observe in Uranus today were more likely to result from a larger moon with a size comparable to Ganymede. The minimal mass, which is about equal to half an Earth moon, is four times greater than the total mass of all currently recognised Uranian moons.

This is also considered in the work. A chaotic phase for the spin axis began when the moon ultimately collided with the planet, essentially "fossilising" Uranus' axial tilt and spin, at a tilt of roughly 80 degrees, the moon became destabilised.

The researchers add, "This new picture for the tilting of Uranus appears pretty promising to us."

This is the only instance that comes to our knowledge of where a single process can both tilt Uranus and fossilise its spin axis in its final condition without relying on a massive collision or other extraterrestrial occurrences. The majority of our successful runs peak there, which seems to be a natural result of the dynamics," they go on to say.

"This picture also appears appealing as a general phenomenon: Today, Jupiter is about to start the tilting phase, Saturn may be halfway through, and Uranus would have finished the ultimate stage, with the annihilation of its satellite."

It's unclear if Uranus could have supported a moon large enough and migrating at a fast enough rate to achieve this situation, and the researchers think it will be difficult to demonstrate with measurements.

However, a deeper comprehension of the moons of Uranus' present rate of migration would go a long way toward answering these queries. They might have developed from the remains of the ancient moon after it was destroyed many ages ago if they are travelling rapidly.

Bring on that probe to Uranus.

Reference: The research has been accepted into Astronomy & Astrophysics and is available on arxiv

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