Astronomers think they know the reason for Uranus’s eccentric axis : ScienceAlert

Astronomers think they know the reason for Uranus's eccentric axis : ScienceAlert
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Uranus marches to the beat of its strange little drum.

Although it shares many similarities with our Solar System’s other ice giant, Neptune, it has a lot of quirks of its own.

And one of these is impossible to miss: its axis of rotation is so skewed that it might as well be lying down. That’s a whopping 98 degrees tilt from the orbital plane.

And to top it all off, it spins clockwise, in the opposite direction to most other planets in the Solar System.

A new study has found a plausible explanation for this strange behavior: a moon migrating away from the planet, causing Uranus to turn on its side. And it wouldn’t even have to be a big moon. Something with half the mass of our own Moon could have done it, although a larger moon would be the most likely contender.

The reasoning has been set out in a paper led by astronomer Melaine Saillenfest of France’s National Center for Scientific Research. This article, not yet peer reviewed, has been accepted into the journal. astronomy and astrophysics and available on preprint resource arXiv.

Scientists have devised models to explain this strange behavior, such as a massive object colliding with Uranus and literally knocked him sidewaysbut the most favored Explanation it’s a bunch of smaller objects.

However, this hypothesis raises problems that are even more difficult to explain: namely, those annoying similarities with Neptune.

The two planets have extremely similar masses, radii, rotation rates, atmospheric dynamics and compositions, and quirky magnetic fields. These similarities suggest that the two planets could have been born together, and they become much more difficult to reconcile when you add planet-tip impacts to the mix.

This has led scientists to look for other explanations, such as a wobble that could have been introduced by a giant ring system or a giant moon early in the history of the Solar System (albeit with a different mechanism).

But then, a few years ago, Saillenfest and his colleagues found something interesting about Jupiter. Thanks to its moons, the gas giant’s tilt could increase from its current slight 3 percent to about 37 percent in a few billion yearsthanks to the outward migration of its moons.

They then looked at Saturn and found that its current tilt of 26.7 degrees could be the result of rapid outward migration of its largest moon, Titan. They found that this could have happened with almost no effect on the planet’s rotation rate.

Obviously, that raised questions about the most tilted planet in the Solar System. So the team ran simulations of a hypothetical Uranus system to determine whether a similar mechanism could explain its peculiarities.

It is not unusual for moons to migrate. Our own Moon is currently moving away from Earth at a rate of about 4 centimeters (1.6 inches) per year. Bodies orbiting around a mutual center of gravity exert a tidal force on each other that causes their rotations to gradually slow down. In turn, this loosens gravity’s grip and the distance between the two bodies widens.

Returning to Uranus, the team ran simulations with a variety of parameters, including the hypothetical moon’s mass. And they found that a moon with a minimum mass of about half that of Earth’s Moon could tilt Uranus 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 with a size comparable to Ganymede was more likely, in the simulations, to produce the tilt and spin that we see in Uranus today. However, the minimum mass, about half that of Earth’s Moon, is about four times the combined mass of the currently known moons of Uranus.

The work also accounts for this. With an inclination of about 80 degrees, Moon it destabilized, triggering a chaotic phase for the spin axis that ended when the moon finally collided with the planet, effectively “fossilizing” Uranus’s axial tilt and spin.

“This new image of Uranus’s tilt looks pretty promising to us.” researchers write.

“To our knowledge, this is the first time that a single mechanism is capable of both tilting Uranus and fossilizing its spin axis in its final state without invoking a giant impact or other external phenomena. Most of our successful runs reach its maximum point in the location of Uranus, which appears as a natural result of the dynamics”, To follow.

“This image also looks attractive as a generic phenomenon: Jupiter today is about to start the tilt phase, Saturn may be halfway there, and Uranus would have completed the final stage, with the destruction of its satellite.”

It is unclear whether Uranus could have hosted a moon large enough and with a high enough migration rate to produce this scenario, and the researchers say it will be a challenge to show it with observations.

However, a better understanding of the current migration rate of Uranus’s moons would go a long way toward resolving these questions. If they are migrating at a high rate, this could mean that they formed from the remains of the old moon after its destruction many eons ago.

To provoke that uranus probe.

The research has been accepted astronomy and astrophysics and is available in arXiv.

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