Uranus is individual. Most of the planets in our solar system have their poles, more or less oriented in the same direction. And most of them rotate counterclockwise when viewed from above.
But Uranus? Its poles are oriented 98 degrees from the orbital plane of the solar system, and it rotates clockwise.
The main hypothesis for this oddity is that something large collided with Uranus long ago, toppling it. While this scenario is not impossible, there are several significant gaps in this model.
Astronomers at the University of Maryland have come up with a new scenario that neatly tackles these problems. Uranus could have been tilted sideways by a giant ring system.
Wait a second, you will no doubt think, Uranus does not have a giant ring system. And it is right. This is not the case right now – its rings are weak and thin compared to Saturn's ring system.
But the latest data from the Cassini probe suggests that the rings may be temporary and short-lived – so it is possible that Uranus once had a much larger ring system, 4.5 billion years ago.
According to astronomers Ziv Rogoshinski and Douglas Hamilton of the University of Maryland, if Uranus had a ring system large enough to make it wobble on its axis like a top – a phenomenon called precession – and if that precession coincided with the orbital precession of a planet where the ellipse is slowly shifting around the sun.
You can see these two concepts animated below.
Spin precession (left) and orbital precession (right). (Robert Simmon / NASA; WillowW / Wikimedia Commons).
This alignment of motion is called resonance, and it has occurred several times in the solar system — usually between the orbits of two or more bodies. For example, Pluto and Neptune have an orbital resonance of 2: 3, which means that for every two Pluto orbits around the Sun, Neptune rotates three times.
The resonance between a planet's precession and its orbital precession is known as spin-orbital resonance, and it can generate a large axial tilt. It is believed that this type of resonance could lead to an axial tilt of Saturn, greater than that of Jupiter.
Secular spin-orbit resonance has been previously investigated in connection with the tilt of Uranus, but with resonance caused by the hypothetical Planet Nine. This was ultimately ruled out as highly unlikely.
But in the opinion of Rogozinsky and Hamilton, a large disc might have worked better. They simulated Uranus and Neptune with large disks to see how they interact with the planets. And they found that the large disk of material accumulating on the planet, which we know is part of the formation of giant planets, was the best fit.
But even though he showed the best result among all models, he still could not fully tilt Uranus. For a million years, it only tilted 70 degrees. Which means that the theory of a collision with another cosmic body is still valid.
The study was published in the Astrophysical Journal.
Sources: Photo: SCIEPRO / Science Photo Library / Getty Images
