Why Does Everything in the Solar System Rotate? And Why Do Most Rotational Movements Share the Same Direction?
This cannot be a coincidence. Looking down at Earth from above, you will see it rotating counterclockwise. The same goes for the Sun, Mars, and other planets.
Approximately 4.54 billion years ago, our Solar System formed within a hydrogen cloud, which is different from the Orion Nebula or the Eagle Nebula with its magnificent cosmic structures.
Subsequently, it required a few impacts, such as shock waves from a nearby supernova, which created a region of gas that collapsed inward due to gravitational forces. Once it contracted, the cloud began to rotate.
Artist’s illustration of a protoplanetary disk around a newly formed star. (Credit: ESO/L. Calçada).
But Why Do They Rotate?
It is due to the conservation of angular momentum.
Consider a single molecule within the hydrogen cloud. Each particle carries its own momentum and drifts through space. When these molecules combine with another molecule due to gravity, they need to balance the momentum of each particle. Achieving perfect balance of zero is theoretically possible, but it is quite difficult.
This means that some of them will slip past one another. Similar to figure skaters pulling each other to spin faster, the contraction of the protoplanetary nebula, along with the momentum of its balanced particles, causes it to spin faster.
This is when the conservation of angular momentum begins to take effect.
As the Solar System rotates faster, it flattens into a disk with a bulging area in the center. We see similar structures all over the universe: the shapes of galaxies, around spinning black holes…
The Sun formed from the bulge in the center of this disk, while the planets formed outside of it. They inherited their rotational motion from the overall movement of the Solar System itself.
Over the course of several hundred million years, all matter in the Solar System coalesced into planets, asteroids, moons, and comets… Subsequently, strong radiation and stellar winds from the young Sun blew away the remaining material.
With no other unbalanced forces acting on them, the inertia of the Sun and the planets has kept them rotating for billions of years.
And they will continue to rotate until they collide with other objects, billions or even trillions of years into the future.
Earth rotates to conserve its angular momentum.
Finally, Why Does Earth Rotate?
Earth rotates because it was born in the accretion disk of the contracting hydrogen cloud due to mutual gravitational forces and needs to conserve its angular momentum. It continues to rotate due to inertia.
The reason everything rotates in the same direction is that they formed together in the same nebula billions of years ago.
Earth could stop rotating, change its rotation direction, or its axis… only if some unbalanced force were to act on it.
