A reader recently asked why all the planets in our solar system share a similar shape, even though they vary greatly in size, composition, and other characteristics. This question prompts a deeper look into the physics of celestial bodies and the forces that shape them over time. Ouest-France has provided an answer to this intriguing inquiry. Planets in our solar system appear round because of a natural process called hydrostatic equilibrium. This occurs when a planet's gravity pulls its mass inward, overcoming the outward pressure of its internal materials. As a result, the planet forms a shape that is roughly spherical. However, not all celestial bodies are perfectly round. Smaller objects, such as asteroids and comets, often have irregular shapes because their gravitational pull is too weak to overcome the rigidity of their solid structures. The Earth and other large planets are nearly spherical, but they are not perfectly round. Due to rotation, planets tend to bulge slightly at the equator and flatten at the poles. This phenomenon is known as an oblate spheroid. For example, Earth's equatorial diameter is about 43 kilometers larger than its polar diameter. The same principle applies to other planets, although the degree of flattening varies depending on their rotation speed and mass. In summary, while planets may not be perfectly round, their overall shape is primarily determined by gravity and rotation. This explains why, despite their differences, all the planets in our solar system have a similar, rounded appearance. Understanding this helps clarify the appearance of these distant worlds and the forces that shape them over billions of years.