The reason is what’s known as ‘conservation of angular momentum’. Think of a figure skater doing a spin and pulling in their arms. The Solar System formed from the collapse of a huge cloud of gas and dust. As it collapsed, any net rotation would cause most of the cloud to settle into a flattened, rotating disc. Collisions between dust and friction within the gas would drag material into the same motions as the majority of the gas and dust. Collisions between dust grains would build up to larger bodies – planetesimals – which would ultimately collide to form the planets. The general revolution of the disc would transfer to the rotating planets and to the Sun itself.
Most of the angular momentum of the Solar System is in the outer planets, both in rotation andAmong the planets, only Venus and Uranus fail to share this common rotation. Venus rotates backwards – or it’s tipped upside down – while Uranus is tipped so its axis is close to the plane of its orbit. We believe these are due to violent collisions when the Solar System was still very young.
