The earth is not round in the perfect sense of the word. It's an oblate spheroid, meaning it's slightly flattened at the poles and bulging at the equator due to its rotation. As for other planets, they are not flat. This is a common misconception due to the way they are often represented in images or diagrams, but they're also oblate spheroids, to varying degrees depending on their mass and rotation speed. The shape of these celestial bodies is determined mainly by gravity. In the early stages of a planet's formation, it's a chaotic mass of particles. Over time, those particles pull towards each other due to gravity, and the most efficient way for them to pack together is in a sphere (or near-sphere). This is why larger objects in space, like planets and stars, are generally round. If you're interested in astrophysics or the physics of shapes, I'd advise looking up the concept of 'hydrostatic equilibrium' for a more detailed understanding. This isn't directly related to programming, but it's always good to learn new things!