Why are all planets shaped like balls?
Gravity pulls massive space objects into spheres by squeezing them evenly from all directions.
A space object's shape is determined by the tug-of-war between gravity and material strength. When an object reaches a specific mass, its gravity becomes strong enough to crush solid rock. This process, called hydrostatic equilibrium, pulls all material toward the center until the body becomes a sphere. While small asteroids stay lumpy, planets must be round because their gravity is too powerful to allow irregular shapes.
Nerd Mode
The transition of a celestial body into a spherical shape is governed by the principle of hydrostatic equilibrium. This occurs when the internal pressure of a body balances the inward pull of gravity. For rocky objects in our solar system, this transition typically happens when the body reaches a diameter of roughly 600 kilometers or a mass of about 10 to the power of 20 kilograms. Smaller objects like the asteroid Itokawa lack the mass necessary for their gravity to overcome the structural strength of their chemical bonds.The International Astronomical Union (IAU) established this as a key criterion for defining a planet in 2006. To be classified as a planet or dwarf planet, an object must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape. This threshold is often called the 'Potato Radius' because objects below this limit retain irregular, elongated shapes resembling potatoes. Large icy bodies can reach this state at smaller sizes than rocky ones because ice is easier to deform than silicate rock.Even though these bodies are considered spheres, they are rarely perfect. The centrifugal force from a planet's rotation causes it to bulge at the equator, a shape known as an oblate spheroid. For example, Earth's diameter is about 43 kilometers wider at the equator than it is from pole to pole. This phenomenon demonstrates that while gravity acts to create a perfect sphere, other physical forces like rotation continue to influence the final geometry of large celestial bodies.
Verified Fact
FP-0001956 · Feb 16, 2026