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October 07, 2011
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Series of thumps may have thrown Uranus off-kilter
Oct. 7, 2011
Courtesy of Europlanet Research Infrastructure
and World
Science staff
Astronomers say they may have figured out why the planet Uranus seems to be lying on its side with respect to the Sun. A series of bumps from other solar system objects early in its life could explain the odd alignment, they propose. Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted.
(Credit: Lawrence Sromovsky, (U. Wisconsin-Madison), Keck Observatory)
Most planets spin along roughly the same plane they inhabit with respect to the Sun. That is, if we could picture the Sun as a big spinning top on a table, most planets would be small tops around it on the same surface, spinning normally. But Uranus would have to be envisioned as a member of this group that is for some reason spinning on its side. To be more precise, its spin axis is tilted by 98 degrees compared to its orbit around the Sun. Jupiter’s axis, by contrast, is tilted by only 3 degrees; Earth’s by 23; Saturn’s and Neptune’s, 29.
The conventional explanation is that Uranus was at some point knocked on its side by an impact from an object a few times heavier than Earth, which weighs a fifteenth as much as Uranus. The problem with this impact scenario is
that the moons of Uranus should have been left orbiting in their original angles, said Alessandro Morbidelli of the Cote d’Azur Observatory in Nice, France, who led the new research. Instead, they too lie at 98 degrees.
Morbidelli and colleagues used simulations to reproduce various impact scenarios. They found that if Uranus had been hit when still surrounded by a protoplanetary disk – primitive material from which its moons would form – then the whole disk would tilt with it. It would also temporarily reform into a fat doughnut shape, but this would flatten out gradually as a result of collisions within it, and the moons would form about where we see them today.
There remained one problem: in the simulations, the moons ended up orbiting Uranus backward compared to how they are really moving. A tweak fixed this, Morbidelli’s group found. It turned out that if Uranus was not tilted in one go, but rather was bumped in at least two smaller collisions, then the moons are much more likely to orbit in their correct direction.
The finding is at odds with current theories of how planets form, which may now need adjusting, Morbidelli said. “The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk,” he explained. “They should have suffered no giant collisions.” Morbidelli presented the research Oct. 6 at the American Astronomical Society’s Division for Planetary Sciences and the European Planetary Science Congress in Nantes, France.
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Astronomers say they may have figured out why the planet Uranus seems to be lying on its side with respect to the Sun. A series of bumps from other solar system objects early in its life could explain the odd alignment, they propose. Most planets spin along roughly the same plane they inhabit with respect to the Sun. That is, if we could picture the Sun as a big spinning top on a table, most planets would be small tops around it on the same surface, spinning normally. But Uranus would have to be envisioned as a member of this group that is for some reason spinning on its side. To be more precise, its spin axis is tilted by 98 degrees compared to its orbit around the Sun. Jupiter’s axis, by contrast, is tilted by only 3 degrees; Earth’s by 23; Saturn’s and Neptune’s, 29. The conventional explanation is that Uranus was at some point knocked on its side by an impact from an object a few times heavier than Earth, which weighs a fifteenth as much as Uranus. The problem with this impact scenario is the moons of Uranus should have been left orbiting in their original angles, said Alessandro Morbidelli of the Cote d’Azur Observatory in Nice, France, who led the new research. Instead, they too lie at 98 degrees. Morbidelli and colleagues used simulations to reproduce various impact scenarios. They found that if Uranus had been hit when still surrounded by a protoplanetary disk – primitive material from which its moons would form – then the whole disk would tilt with it. It would also temporarily reform into a fat doughnut shape, but this would flatten out gradually as a result of collisions within it, and the moons would form about where we see them today. There remained one problem: in the simulations, the moons ended up orbiting Uranus backward compared to how they are really moving. A tweak fixed this, Morbidelli’s group found. It turned out that if Uranus was not tilted in one go, but rather was bumped in at least two smaller collisions, then the moons are much more likely to orbit in their correct direction. The finding is at odds with current theories of how planets form, which may now need adjusting, Morbidelli said. “The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk,” he explained. “They should have suffered no giant collisions.” Morbidelli presented the research Oct. 6 at the American Astronomical Society’s Division for Planetary Sciences and the European Planetary Science Congress in Nantes, France. n