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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.

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.

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)


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