We all knew that planets are round since we starting learning about them back in elementary school. It’s a basic well-known fact of today, and because it’s such a basic fact, it is rarely questioned. Planets are round because they are. Period. But inquisitive little second graders, or people of any age with a curious nature, start to wonder why. It’s not just because they are; there has to be a reason for planets being round, and there is. Well, sort of.
In all actuality, planets are not round. They have a spherical shape, but it is not a true sphere. From a distance, they appear to be round, and so they have taken on this characteristic, although faulty.
So why are planets “round”? Because of gravity. Yes, the same force that made the apple fall on Issac Newton’s head is the same one that makes planets round. Gravity is a force that pulls two objects together, and in the case of planets, gravity pulls everything inward, toward the center. To make it a little clearer, we will go back to the time when our solar system was created, from a scientific standpoint.
According to the Big Bang Theory, after a large explosion about five billion years ago, gravity pulled together pieces of gas and clumps of dust. These clumps began to grow in size as more pieces collided into them and stuck together. With the collision, the clumps were very hot and contained molten material, which eventually became our planets. As they grew in size, they gained a higher gravitational pull, (because larger objects have a larger gravitational pull) and they each started to have their own gravitational pull. Gravity then pulled the materials inward toward its center at even rate on all sides. Because of this, the planets became a spherical shape.
But if gravity pulls everything towards the center evenly, then planets should be perfectly round, right? Well, if gravity was the only force on planets, then they probably would be perfectly round; however, other forces are also at play. One particular action that changes the shape of a planet is its rotation speed, or how quickly it spins on its axis. When a planet spins, it creates a centrifugal force. This force then creates a bulge at the equator of the planet. So the faster a planet rotates, the greater the bulge, and the less round the planet actually is.
Other factors that affect how round a planet is are the planet’s mass and size. The greater the mass and size, the larger the gravitational pull. Since planets are larger and have a higher mass than other objects flying through outer space, such as asteroids, they are rounder.
But this still doesn’t explain why gravity, which pulls everything equally toward the center, allows hills, mountains, and even buildings, to defy it and protrude higher from the Earth. The truth is that these objects are not defying gravity. In fact gravity limits how high these objects can be. If something, either natural or manmade, was too high, then it would crush because of its gravitational pull. These objects are still being pulled toward the center of Earth. The only difference is that there is something blocking their way, such as a cement foundation for buildings.
Everything is pulled toward the center of Earth unless something is blocking its way, even us. That is why when we are not standing solid ground, we sink. Our gravitational pull is strong enough to pull our bodies through things like mud, but not cement.