Scientists Explain Why Uranus is Tilted

Uranus lies on its side.  It has an axial tilt of over 90 degrees relative to the plane of the solar system.   As a result, each pole gets 42 YEARS of continuous sunlight, followed by 42 years of continuous darkness.  It has always been assumed that during the formation of the solar system, a protoplanet collided with Uranus, knocking it askew.

Now, two astronomers at the Observatoire de Paris in France have proposed a mechanism to explain this unusual orientation:

Boue and Laskar's idea is that Uranus once had a moon of the required size and orbit, which caused the planet to tilt during the planetary migration, but that this moon was ejected during a close encounter towards the end of the migration.

This thesis has been supported by computer modeling, and it offers an additional benefit of explaining why Uranus has rings but not another moon.

The planet was named for the Latinized form of the Greek god of the sky, but the pronunciation of the planet's name has always been the source of some embarassment to English-speaking astronomers because the public does not appreciate that the emphasis should be on the first syllable.

Link and image credit to MIT's Technology Review.