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How Uranus ended up with its eccentric axis

It appears that early in its life Uranus experienced a succession of small punches instead of a single knock-out blow.

Washington: Uranus’ highly tilted axis makes it an oddball in our Solar System. The accepted wisdom is that Uranus was knocked on its side by a single large impact, but now a new research has finally solved the mystery of how Uranus became so tilted.

By using simulations of planetary formation and collisions, it appears that early in its life Uranus experienced a succession of small punches instead of a single knock-out blow.

Uranus`s spin axis lies 98 degrees off of its orbital plane with the Sun. This is far more pronounced than other planets, such as Jupiter (3 degrees), Earth (23 degrees), or Saturn and Neptune (29 degrees). Uranus is, in effect, spinning on its side.

The generally accepted theory is that in the past a body a few times more massive than the Earth collided with Uranus, knocking the planet on its side. There is, however, one significant flaw in this notion: the moons of Uranus should have been left orbiting in their original angles, but they too lie at almost exactly 98 degrees.

This long-standing mystery has been solved by an international team of scientists led by Alessandro Morbidelli from Observatoire de la Cote d’Azur in Nice, France.

Morbidelli and his team used simulations to reproduce various impact scenarios in order to ascertain the most likely cause of Uranus’s tilt. They discovered that if Uranus had been hit when still surrounded by a protoplanetary disk -- the material from which the moons would form -- then the disk would have reformed into a fat doughnut shape around the new, highly-tilted equatorial plane.

Collisions within the disk would have flattened the doughnut, which would then go onto form the moons in the positions we see today.

However, the simulation threw up an unexpected result: in the above scenario, the moons displayed retrograde motion -- that is to say, they orbited in the opposite direction to that which we observe. Morbidelli’s group tweaked their parameters in order to explain this.

The surprising discovery was that if Uranus was not tilted in one go, as is commonly thought, but rather was bumped in at least two smaller collisions, then there is a much higher probability of seeing the moons orbit in the direction we observe.

The study was recently presented at the EPSC-DPS Joint Meeting in Nantes.


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