When you look at photos from around our solar system, you’ll notice that there are a lot of circles. The sun is roughly spherical, planets are roughly spherical. Even some of the larger moons are pretty neat little balls of rock. What makes them all so round, and why are other bodies, like asteroids, not?
In Short: Gravity
Planets and stars all form in more or less the same fashion when you break it down to the basics. A bunch of objects crashed together to create a ball with enough mass to have a gravitational field of its own. The smallest-known object to hold its spherical shape is one of Saturn’s moons, Mimas, with a radius of about 123 miles. More matter got pulled in, increasing the object’s gravitational pull, spinning all the while. The center of an object’s gravity lies at the core of its mass.
According to Newton’s law of universal gravitation, every particle attracts every other particle in the universe. The force of attraction is directly proportional to the mass of the body and inversely proportional to the square of the distance between them. That’s a lot to take in at once, so let’s break it down a little.
Near And Far
The first part of Newton’s law talks about mass. The more massive an object, the more gravitational force it exerts on its surroundings. The second half of the law explains that the force of an object’s gravity decreases incrementally as you move away from the source. When you map it out, the effect of gravity forms a sort of ball, with the strongest pull toward the center.
Let’s think back to the formation of a star or a planet. The center is formed by a bunch of matter colliding to form a ball, which in turn collects more mass through collision. Eventually, enough mass is concentrated in one place to form an object with a substantial gravitational field. That gravity builds a roughly spherical shell around the center of mass, creating your round planets, stars, and moons.
Look at a photo of Saturn, and you’ll notice that it isn’t a perfect sphere but rather an oblate spheroid. That’s a fancy way of saying it bulges at its equator, and it’s not alone. Earth also has a central bulge, as do many other planets, and that’s because the centrifugal force created by the rotation of the planet pushes its center outward. It’s the same force that keeps the water at the bottom of a bucket when you swing it around in a circle. Now we know why planets are round, but why are asteroids anything but round?
Once again, it all comes down to mass and gravity. Specifically, it’s a complicated relationship of mass, gravity, and composition. In short, some materials are more moldable than others, meaning that it takes less gravity and therefore less mass to give it a rounded shape. The tipping point where gravity becomes strong enough to overcome the strength of the material it’s pulling on is called the potato radius. Seriously, look it up.