Uranus has an axial tilt of 98 degrees, giving it the appearance of a bulls-eye as it revolves around the sun.  Its moons revolve comfortably around the planet’s equator.  This unusual arrangement, unique in the solar system, has challenged planetary scientists since its discovery.  A new model accounts for it through a series of gentle bumps from impacts as the planet was forming from dust and gas, but how would one ever test such an idea?

Simplistic models of planet formation from an evolving dust disk around a star should end up with all the planets revolving in near-circular ellipses in the equatorial plane, and rotating with axes perpendicular to the plane.  That contradicts a great deal of observational evidence; the planets have varying degrees of orbital eccentricity, orbital inclination, and axial tilt.  Some of these can be explained by subsequent gravitational interactions.  In the last decade or so, based on observations of “hot Jupiters” around other stars, radial migration has been invoked to get gas giants to form in the distant recesses of the dust disk, then move them closer to the star just in time (8/21/2009).  Uranus, in particular, remains a challenge – both getting it to form, and getting it to tip over.

Previous theories used a single giant impact to explain the tilt.  Uranus, though, has lower orbital eccentricity than the other gas giants save Neptune, and lower orbital inclination than the other gas giants.  It would seem strange such a collision would not have greater effect on the planet’s orbit or moons, which have relatively circular orbits in the orbital plane of Uranus.  Science Daily noted that problem for the single-impact theory, saying, “the moons of Uranus should have been left orbiting in their original angles, but they too lie at almost exactly 98 degrees.”  What to do?

This long-standing mystery has been solved by an international team of scientists led by Alessandro Morbidelli (Observatoire de la Cote d’Azur in Nice, France), who is presenting his group’s research at the EPSC-DPS Joint Meeting in Nantes, France….

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