Why Planets Are Spheres
🌊 Gravity & tides
Question
Why are planets and moons round, while asteroids are lumpy?
Gravity pulls every bit of matter toward the centre. On a small, weak body (an asteroid), the material's own strength holds its shape, so it can be lumpy. On a big body, gravity is so strong that it overcomes the material's strength and pulls everything into the shape where every point on the surface is the same distance from the centre: a sphere. This balance is called hydrostatic equilibrium, and it is the difference between a dwarf planet (rounded by gravity) and a small, lumpy asteroid.
The math
A sphere is the shape of minimum surface area for a given volume - gravity pulls matter to the centre of mass, and the equilibrium surface is the one where every point is equidistant from it. Roughly, bodies over ~600 km across (rocky) or ~400 km (icy) are rounded; smaller ones stay lumpy.
Why it matters
The Earth is round because it is big enough for gravity to win over rock. A flat disc cannot be a gravity equilibrium: the material at the rim would slide toward the centre and the disc would collapse into a sphere. The shape of the planet is itself evidence of gravity doing its job.