Astronomers think they know how Uranus got flipped onto its side. According to detailed computer simulations, a body about twice the size of Earth slammed into Uranus between 3 to 4 billion years ago. The impact created an oddity in our Solar System: the only planet that rotates on its side.

A study explaining these findings was presented at the American Geophysical Union’s (AGU) Fall Meeting in Washington DC held between December 10th to 14th. It’s led by Jacob Kegerreis, a researcher at Durham University. It builds on previous studies pointing to an impact as the cause of Uranus’ unique orientation. Taken altogether, we’re getting a clearer picture of why Uranus rotates on its side compared to the other planets in our Solar System. The impact also explains why Uranus is unique in other ways.



When the Solar System formed, it all started with a massive cloud of gas and dust. The cloud swirled as it became more dense, and eventually the Sun formed in the center, with the rest of the gas and dust swirling around it. The Sun contains about 99.8% of the mass in the Solar System, and as the remaining 0.2% swirled around it, clumps began to form into the planets we know today. As they formed, the rotation of the gas cloud was imparted to the planets. So, all of the planets rotate the same way, except for Uranus, (and Venus, which was likely struck by an asteroid.)