More than one blow to knock Uranus over

Alessandro Morbidelli from the Observatoire de la Cote d’Azur, Nice led an international team of cosmologists in answering a longstanding question:  How did Uranus get its tilt?  Computer simulations indicate that it took at least two thumps from planet-sized bodies.

Uranus is unusual in that its rotational axis is almost perpendicular to the plane of its orbit around the sun.  In other words, it’s spinning on its side.  Other planets have some degree of tilt, the Earth is tilted by 23 degrees (which accounts for our seasons), but none come close to the 98 degree tilt of Uranus.

Until recently, astronomers thought that a single collision with an object larger than the Earth knocked Uranus onto its side.  However, in that scenario, Uranus’s moons should have remained in the horizontal plane (orbiting around Uranus pole to pole).  Instead, they apparently flipped over when Uranus did and now orbit in the plane of Uranus’s equator.  A single collision could not account for rotating the entire Uranus plus moons system.  In contrast, two or more smaller collisions occurring before the moons were fully formed could have resulted in the system we see today.