Large planets become round because gravity pulls matter toward the centre.

What we found

Small asteroids can keep irregular shapes because their gravity is weak. In larger worlds, high points press downward and material redistributes toward gravitational equilibrium. A rapidly rotating planet becomes slightly flattened at the poles. Mountains and craters remain, but they are tiny compared with the planet’s total size. The Ridbusters then compare the wording with the original sources, look for the conditions of each observation and check whether another explanation could fit. That turns a surprising headline into a trail a young investigator can follow: claim, test, result and remaining uncertainty. When an object becomes massive enough, gravity overcomes the strength of rock or ice over geological time and produces a shape close to a sphere.