The cosmos offers us many mysteries that astronomers are still struggling to solve, but there is one that is in our own solar system and remains unexplained – why do Venus and Uranus rotate in different directions from the other planets in the solar system?
Venus rotates from east to west, while Uranus is so inclined that it practically rotates on its side. Every other planet, including Earth, rotates from west to east, and scientists have not yet determined why.
The planets must rotate in one direction because the solar system was formed by the collapse of a hot cloud of gas, and so far it has been accepted that the spin direction of most planets, such as Earth, has inherited the direction of the ancient cloud.
But Venus and Uranus are exceptions and have retrograde motion, which is opposite of the direction of rotation of the Sun? Why is this so?
One theory is that both planets rotated in the same direction as the others but were hit by massive objects, perhaps even other planets that changed their rotation in the other direction.
In 2011, simulations showed that it was more likely that a series of smaller collisions, rather than a large impact, tilted Uranus’ spin angle to 98 degrees. The alternative hypothesis was that Uranus had a very large satellite that caused the tilt of its orbit, and at one point was hit by another planet.
Venus’ hypotheses are that it began to rotate counterclockwise, then slowed down at rest, and then began to rotate clockwise.
This may explain the very low rotation speed of the planet today – on Venus the days are longer than the year and the Sun rises from the West.