Good question! Most planets and stars are near-spherical (they are actually usually spheroidal– squashed spheres) because of the gravitational force that they exert. Planets and stars form when gases and small pieces of solid matter (like asteroids and planetoids) accrete or gather due to the force of gravity, which exists between all objects with mass. Larger objects have larger mass, and therefore stronger gravity. The gravitational field around an object points to its center of mass (COM). The center of mass for planets and stars is in the middle of the body. The gravitational potential vectors point toward the COM center of mass and so matter accelerates toward this point. Matter that builds up around the COM of a planet or star with “want” to go right to the COM, but material that built up previously will be in the way. This is why planets are spheroidal; they are not perfect spheres because their rotation sort of squashes them through centripetal force. Sometimes a planet with experience gravitational differentiation after it is fully built up. For example, most of the iron and nickel (heavy elements) were drawn in to form the Earth’s core when the planet was still young.

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