Heaven marches to the beat of its own strange little drum.
Although it shares many similarities with our Solar System’s other ice giant, Neptune, it has a host of quirks that are all its own.
And one of them is impossible to miss: Its rotation axis is so tilted that it might as well be lying down. That’s a massive 98 degree tilt from the orbital plane.
And, what’s more, it rotates clockwise – the opposite direction to most of the 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 be pulled to its side. And it wouldn’t even have to be a big moon. Something half the mass of our own Moon could have done it, although a larger moon would be the most likely candidate.
The rationale has been laid out in a paper led by astronomer Melaine Saillenfest of the National Center for Scientific Research in France. This paper, which has not yet been peer-reviewed, has been accepted into the journal Astronomy & Astrophysics and is available in the arXiv preprint resource.
Scientists have come up with models to explain this strange behavior, such as a massive object colliding with Uranus and literally knocking it on its side, but the most favorable explanation is a bunch of smaller objects.
However, this hypothesis raises issues that are even more difficult to explain: namely, those disturbing similarities to Neptune.
The two planets have remarkably 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 it becomes much more difficult to reconcile them when you throw impacts from the planet’s reversal into the mix.
This 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 Solar System’s history (albeit by 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 inclination could increase from its current slight 3 percent to about 37 percent in a few billion years, thanks to the outward migration of its moons.
They then took a look at Saturn and found that its current tilt of 26.7 degrees could be the result of the rapid outward migration of its largest moon, Titan. This could have happened, they found, with almost no effect on the planet’s rotation rate.
Obviously, this raises 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 uncommon 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 rotating 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 the grip of gravity so that the distance between the two bodies widens.
Turning back to Uranus, the team ran simulations with a range of parameters, including the mass of the hypothetical moon. And they found that a moon with a minimum mass of about half that of Earth’s Moon could tilt Uranus 90 degrees if it migrated more than 10 times the radius of Uranus at a rate of more than 6 centimeters per year.
However, a larger moon comparable in size to Ganymede was more likely, in the simulations, to produce the tilt and rotation we see in the Uranus today. However, the minimum mass – about half that of Earth’s moon – is about four times the combined mass of the current known moons of Uranus.
The play explains that too. At an inclination of about 80 degrees, the moon became destabilized, causing a chaotic phase for the spin axis that ended when the moon finally collided with the planet, effectively “fossilizing” the axial tilt and rotation of Uranus.
“This new picture of the tilt of our Sky looks quite promising,” the researchers write.
“To our knowledge, this is the first time that a single mechanism has been able to tilt Uranus and ossify its spin axis in its final state without invoking a giant impact or other external effects. Most of our successful runs culminate in position of Uranus. appearing as a natural outcome of the dynamic” they continue.
“This picture also seems attractive as a general phenomenon: Jupiter today is about to begin the declination 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 host a moon large enough and with a high enough migration rate to create this scenario, and, the researchers say, it will be difficult to show with observations.
However, a better understanding of the current rate of migration for the moons of Uranus would greatly assist in resolving these questions. If they migrate at a high rate, this could mean that they formed from the debris of the ancient moon after it was destroyed many centuries ago.
Bring that Uranus tracker.
The research was accepted in Astronomy & Astrophysics and is available on arXiv.