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Uranus' tilt is due to the tug of a moon, study says


The unusual features of ice giant Uranus have baffled scientists for quite some time. Now, experts believe they have an explanation for one of these mysteries. Why is Uranus so tilted relative to the orbital plane? It is something that brings seasonal changes completely different from those of the other planets. Therefore, the solstices and equinoxes on this planet occur every 21 Earth years, in such a way that one of the planet’s poles receives uninterrupted sunlight for 42 years and the other absolute darkness . The most recent hypotheses were that an impact, or even a succession of collisions, would have caused this tilt.


Uranus, the ‘oddball’ of the solar system

In fact, scientists have long suspected that some kind of violent impact knocked Uranus off its feet. It would have been an object several times more massive than Earth, which would have collided with Uranus long ago. It tilted the young planet hard and left its peculiar system of moons and rings as evidence. Both Uranus and its 27 moons that orbit the planet around its equator flip; almost as if they ‘lie down’.

And, as if that weren’t enough, the planet rotates clockwise , in the opposite direction from most other planets in the solar system.

Now, according to experts from the French National Center for Scientific Research, the impact of any celestial object would not have been necessary: the presence of a moon with half the mass of ours (although larger is the most likely scenario) could have caused the tilt of Uranus. It would be responsible for this overwhelming deviation; it would have pulled the planet to one side , causing it to tilt to its current 98-degree tilt relative to its orbital plane.

Explanations to tilt

The reasoning has been set out in a paper led by astronomer Melaine Saillenfest of France’s National Center for Scientific Research. The study, which has been accepted in the journal Astronomy & Astrophysics and is available on the arXiv preprint service, would also explain why its neighboring planet, Neptune, shares a number of similarities, including masses, rotation rates, dynamics, and compositions. unusual atmospheric conditions and magnetic fields. Where do so many similarities between Neptune and Uranus come from? The wobble produced by a giant moon early in the history of the solar system would be a perfect fit to solve this problem.

The team ran computer simulations of a hypothetical Uranus system to determine whether a mechanism similar to the one once discovered on Jupiter and its moons (which, thanks to them, could increase the gas giant’s tilt from its current slight 3% to about 37% in a few billion years, thanks to the outward migration of its moons), could explain its peculiarities.

The simulations showed that a hypothetical moon with a minimum mass of about half that of Earth’s moon could tilt the planet by 90 degrees if it migrated more than 10 times the radius of Uranus at a rate greater than 6 centimeters per year . However, a larger moon comparable in size to Jupiter’s Ganymede would be more likely to produce the tilt and spin we see in Uranus today.

With this tilt, the moon could have been equally destabilized, triggering a chaotic phase for the spin axis that ended when it finally collided with Uranus, finally settling the planet’s tilt and axial spin.

“This new image of Uranus’ tilt looks quite promising to us,” the authors wrote. “To our knowledge, this is the first time that a single mechanism has been able to tip Uranus and fossilize its spin axis in its final state without invoking a giant impact or other external phenomena. Most of our successful runs peak in the location of Uranus, which appears to be a natural result of the dynamics.

Reference: Tilting Uranus via the migration of an ancient satellite 2022 Astronomy & Astrophysics arXiv:2209.10590 [astro-ph.EP] (or arXiv:2209.10590). v1 )

Ida , S. , Ueta , S. , Sasaki , T. et al. Uranian satellite formation by evolution of a water vapor disk generated by a giant impact. Nat Astron 4, 880–885 (2020).

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