The life cycle of a planet usually begins when the gravity of the star attracts clouds of rocks and dust until it forms into a blob that attracts more matter towards its center.
Since gravity works in all directions equally, this is the reason why planets are round-shaped instead of any other form.
Although the roundness of the planets is not perfect, every planet belongs in its own spherical category, like the Earth is an "oblate spheroid."
Due to the differences in the planet's sizes, bigger planets such as Saturn may gather extra mass around its middle area and get thicker.
The idea of a cube-shaped moon is not entirely impossible. However, for it to happen, it will take a much longer time before we are able to see one.
Asteroids and moons are generally smaller than planets therefore it goes to show that they both have lesser gravity thus they are less round than planets.
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