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Solar system

Why Uranus spins sideways

Crashing into two large objects has tilted the rotation axis of the planet by 98°

Images NASA

One of the great unsolved problems of the astronomy of the solar system may have been cracked by an international team of five researchers, including a Brazilian. Using computer simulations, the team led by Italian Alessandro Morbidelli, from the Côte d’Azur Observatory in Nice, France, has picked up indications that the anomalous inclination of the rotation axis of Uranus is not only due to one great collision with a body the size of Earth as was thought, but to two, with objects of a significant size. The planet spins around an axis whose inclination is 97.7o relative to the plane of its orbit around the Sun. The two crashes happened at different times in the process of the birth of Uranus. “They would explain why Uranus spins lying down,”says Rodney Gomes from the National Observatory in Rio de Janeiro, one of the authors of the study. Presented in October at the European Congress of Planetary Science in Nantes, France, the new hypothesis may change the view we have of the first phase of the formation of the solar system.

The planets began to form 4.5 billion years ago from a disc of gas and dust spinning around the Sun. During its first few millions of years the material in the disc gradually adhered together, forming increasingly larger bodies with proportions similar to asteroids and comets, the so-called planetisimals. As a result of colliding with each other the planetisimals continued to grow until they formed planetary embryos, bodies with dimensions similar to those of current planets. Some of these embryos rapidly captured the gas of the disc, which dissipated in the first few millions of years, forming the huge gaseous and ice planets. The embryos that remained inside the solar system continued to collide with each other until they formed the rocky planets. This scenario implies that all planets were born orbiting in the plane of this primordial disc, whose axis or rotation around itself was perpendicular to this plane. Subsequent encounters between planets, planetisimals and the remaining planetary embryos, however, caused their axis to deviate from this norm. The axis of rotation of the Earth, for example, tilts almost 23o. Uranus, on the other hand, is an extreme case, with an inclination of almost 98o. Because of this its north and south poles are located on the sides of the planetary sphere rather than at the top and bottom.

Since the 1960s, scientists have believed that this pronounced obliqueness was due to a violent shock between Uranus and a large planetary embryo. However, there was always a problem with this explanation: the dozens of moons and rings of Uranus also spin around the planet’s extremely inclined rotation axis. During a sudden collision, say the critics of the hypothesis, there would be no time for these rings and satellites to have accompanied the inclination of Uranus. They should have remained at a less angled orbital plane.

To explain this discrepancy, astrophysicists Gwenaël Boué and Jacques Laskar from the Paris Observatory proposed an alternative theory in 2009. According to them, in the past Uranus had an enormous moon, the size of the Earth. The presence of the massive satellite meant that the precession movement of the rotation axis of the planet, which is similar to the oscillation produced by a spinning top, gradually grew bigger in such a way that, because of a series of interactions, it led the planet to slowly “lie down.”This inclination was such a gradual process that the rings and other satellites accompanied the equator of the planet.

Chaos in the solar system
The problem seemed resolved until Morbidelli and Gomes decided to examine the theory in detail. Last year they came across a contradiction. According to their calculations, the same gravitational influence of the hypothetical satellite, which little by little had pushed over Uranus, would attract other satellites and rings in a way that would prevent them from accompanying the planet in its inclination. The theory of the French, therefore, did not work.

The scientists decided to go back to the idea of a primordial collision, but with modifications. They carried out simulations of the gravitational interactions that would have occurred if a body the size of Earth had collided with Uranus in its infancy, when its moons and rings had still not been formed from the disc of gas and dust. The impact would have knocked Uranus onto its side and detritus from the collision would have formed a second disc around its equator. The gravitational influence of the inner disc would have meant that the material from the first disc had spread out in the shape of a “donut,”technically called a torus, around the equator of Uranus. Over time, the inner disc would have been absorbed by the planet and the torus flattened into the shape of another disc from which the moons and rings had originated.

This scenario explains the inclined axis of Uranus, except for one detail: the moons formed in the simulations spun in the opposite direction to the rotation of Uranus, which is anti-clockwise. For the result of the computer model to agree with the reality of the solar system, the researchers discovered that Uranus must have suffered another collision with another planetary embryo. This shock must have occurred before the one that twisted the planet’s axis, like the disc that gave rise to its moons and rings. “If two collisions of this order happened there must have been many planetary embryos of the size of Earth close to Uranus at that time,”says Gomes.

“It is an interesting idea and one that is entirely probable,”comments the Brazilian astrophysicist Wladimir Lyra, from the American Museum of Natural History in New York. “Research shows that the solar system was a chaotic place at the beginning. There was a lot of interaction between the proto-planets. The eight planets we see today are just the ‘winners’ of a struggle they won at the cost of some scars.”