If you have ever looked at pictures of our planets, or lined them up side by side to inspect them, then you might notice something interesting–all the planets are round. Naturally, you might ask, “why are planets round?” The answer to this question rests in how planets are formed and how gravity works.
When rocks, gas, and dust start to run into each other in space, some pieces may stick together or clump together. As all these bits and pieces of rock, dust, and gas gather, they begin to create their own gravity, which helps hold it all together. These forming planets are very hot, and they are usually molten.
Gravity then begins to work on the hot and molten materials. Gravity pulls equally from all sides. Since the gravity is at the very center of the material, the material is pulled towards the center of the mass equally. Since the material is pulled towards the center equally, the material begins to take a spherical shape.
As the bulky and heavy materials begin to crash towards the center, the molten materials and pressure begin to push outwards. The combination of weight pushing in and pressure pushing out creates balance. The balance is created and maintained through a spherical shape.
After the material cools, it remains in its spherical shape. This is why our planets are round. However, they are not perfect in shape. Since planets spin, there can be bulges in their round shapes. The faster the planet spins, the bigger the bulges. Meaning faster spinning planets might be a little less spherical than other planets, and they could be a little flattened. Additionally, objects can impact planets, creating craters and bumps on their surfaces.
For comparison, you can look at asteroids. Smaller asteroids are generally not round. In fact, they are jagged, fragmented, and irregular in shape. This is because small asteroids have very weak gravitational pulls, meaning they cannot pull all the material surrounding them towards their center equally. Therefore, unlike planets, these small asteroids are not spherical and round.
Planets are round because gravity causes all the material to gather equally around a gravitational center, but not all planets are perfectly round. They have imperfections in their shapes and on their surfaces, but they generally keep their round, spherical shape.