Uranus marches to the beat of its strange little drum.
Although it shares many similarities with the other ice giant in our solar system, Neptune, it has its own set of quirks.
And one of these things is impossible to miss: its axis of rotation is so skewed that it might also be lying down. This is a huge tilt of 98 degrees from the orbital plane.
Above all, it rotates clockwise – the opposite direction to most other planets in the solar system.
A new study has come up with a plausible explanation for this strange behavior: a moon migrates away from the planet, dragging Uranus onto its side. And it wouldn’t even need to be a big moon. Something half the mass of our moon could do that, although a larger moon would be the most likely contender.
The reason is explained in a research paper led by astronomer Mylene Silenvest of the National Center for Scientific Research in France. This paper, which has not been peer-reviewed, has been accepted into the journal Astronomy and astrophysics and made available on the prepress resource arXiv.
Scientists have come up with models to explain this strange behavior, such as a massive object literally colliding with Uranus and slapping it sideways, but the most preferred explanation is a bundle of smaller objects.
However, this hypothesis raises issues that are difficult to explain: the disturbing similarities with Neptune.
The two planets have very similar masses, radii, rotation rates, atmospheric dynamics and compositions, and strange magnetic fields. These similarities suggest that the two planets could have been born together, and reconciling them becomes even more difficult when you throw the effects of planetary flipping into the mix.
This has led scientists to search for other explanations, such as an oscillation that could have been provided by the giant ring system or a supermoon 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 the current 3 percent to about 37 percent in a few billion years, thanks to the outward migration of its moons.
Then they took a look at Saturn and found that its current tilt of 26.7 degrees could be the result of a rapid outward migration from Hao’s largest moon, Titan. They found that this could happen almost without having any effect on the planet’s rotation rate.
This obviously raised questions about the most tilted planets in the solar system. So the team ran simulations of a hypothetical Uranian system to determine if a similar mechanism could explain its peculiarities.
It is not uncommon for moons to migrate. Our moon is currently moving away from Earth at a rate of 4 centimeters (1.6 in) per year. Objects rotating around an alternating center of gravity exert a tidal force on each other that gradually slows their rotation. This, in turn, loosens the grip of gravity so that the space between the two objects expands.
Returning to Uranus, the team ran simulations using a range of parameters, including the mass of the hypothetical moon. They found that a moon with a mass at least half that of Earth’s moon could tilt Uranus about 90 degrees if it migrates more than 10 times the radius of Uranus at a rate above 6 centimeters per year.
However, a larger moon similar in size to Ganymede was more likely, in simulations, to produce the tilt and rotation we see in Uranus today. However, the minimum mass – about half of Earth’s moon – is about four times the combined mass of the current known moons of Uranus.
The work also explains this. At a tilt of about 80 degrees, the Moon became unstable, resulting in a chaotic phase of its axis of rotation that ended when the Moon finally collided with the planet, “petrifying” the axial tilt and rotation of Uranus.
“This new image of Uranus tilting looks very promising to us,” write the researchers.
“To our knowledge, this is the first time that a single mechanism has been able to tilt Uranus and rock its axis of rotation into its final state without causing a giant shock or other external phenomena. The bulk of successful runs culminate in the location of Uranus, which appears as a natural consequence of the dynamics, “They continued.
“This image also looks attractive as a general phenomenon: Jupiter today is about to begin its tilt phase, Saturn may be halfway through, and Uranus has completed its final phase, with its satellite destroyed.”
It’s not clear if Uranus hosted a moon large enough and with a high enough migration rate to produce this scenario, the researchers say, would be difficult to show through observations.
However, a better understanding of the current migration rate of Uranus’ moons will go a significant way toward resolving these questions. If they were migrating at a high rate, it could mean that they were formed from the debris of the ancient moon after it was destroyed many eons ago.
Attend that Uranus probe.
The search has been accepted Astronomy and astrophysics It is available on arXiv.