Uranus is one of the prominent planets of the solar system. Although it is very similar to Neptune, another ice giant in the system, it is different from it in many ways.
One such difference is very striking: its axis of rotation is tilted so much that it rotates almost lying on its side. The inclination to the orbital plane reaches 98 degrees.
In addition, Uranus rotates in a counter-clockwise direction to most of the planets in the solar system.
A new study found a possible explanation for this strange behavior: one moon away from Uranus, the planet moved and tilted on its side. It really doesn’t need a big moon. An object half the mass of our Moon can do this, but a more mature moon is likely to be the culprit.
The study was conducted by Melanie Selenfest, an astronomer at the French National Center for Scientific Research. The paper will be published in the journal Astronomy & Astrophysics, and until then is available on the peer-reviewed server arXiv.
To explain this strange behavior, scientists have developed models such as a massive object colliding with Uranus and knocking it on its side, but a more favorable explanation is a cluster of smaller objects.
However, this hypothesis raises some problems that are even more difficult to explain: namely, the striking resemblance to Neptune.
The two planets have extremely similar masses, radii, rotation rates, atmospheric dynamics and composition, and magnetic fields. These similarities indicate that these two planets may have been born together, and their reconciliation becomes much more difficult when deviating planetary collisions are involved.
All of this has led scientists to look for other explanations, such as oscillations that could have been caused by a giant ring system or a giant moon early in the history of the solar system.
But then, a few years ago, Selenfest and his colleagues discovered something interesting about Jupiter. Thanks to its moons, within a few billion years, the gas giant’s current 3-percent inclination could increase to 37 percent, which should be caused by the outward migration of its moons.
They then looked at Saturn and determined that its current 26.7-degree tilt may be due to the rapid outward migration of its largest moon, Titan. They also found that this could happen with almost no effect on the planet’s rotation rate.
Naturally, this raised questions about the most tilted planet in the solar system. Therefore, the team ran simulations of a hypothetical Uranian system to determine whether such a mechanism could explain its peculiarity.
Moons are no strangers to migration. Earth’s moon is currently moving away from our planet by about four centimeters per year. Bodies moving around the common center of gravity exert a tidal force on each other, which causes a gradual slowing down of their rotation. In turn, this weakens gravity so much that the distance between the two bodies widens.
The team then turned to Uranus and ran simulations of various parameters, including the hypothetical moon’s mass. They found that a moon with at least half the mass of Earth’s moon would have to tilt Uranus by 90 degrees if it were to move away from the planet ten times the radius of Uranus, at a rate of more than six centimeters per year.
However, according to the simulations, an adult moon about the size of Jupiter’s Ganymede is more likely to be responsible for the current tilt and rotation rate of Uranus. It is worth noting that the minimum mass, about half that of Earth’s moon, is almost four times the total mass of the currently known moons of Uranus.
This was also taken into account in the research. At about an 80-degree tilt, the Moon became unstable, leading to a chaotic phase of its rotation axis that ended when the Moon finally collided with the planet and “fossilized” Uranus’s axial tilt and rotation.
“This new picture of the inclination of Uranus seems quite promising to us. “As far as we know, this is the first time we have found that a single mechanism can cause both the tilt of Uranus and the fossilization of its spin axis in its final state, without any giant collapse or external phenomenon,” the researchers write.
According to them, such an image also looks attractive as a general phenomenon. Today, Jupiter is at the beginning of its declination phase, Saturn maybe halfway through it, and Uranus should have already completed its final phase by destroying its own satellite.
It is not known whether Uranus had a moon large enough for this and whether this moon had the migration rate necessary to produce such a scenario; According to the researchers, it will be quite difficult to determine this through observations.
Everything will probably be clarified by the spacecraft that NASA will send to Uranus in the future.
Prepared by ScienceAlert.