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...
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...
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 Sat*urn 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.