system
Given the following post, respond as if you're commenting on Reddit:
human
Why are rocky planets spherical in shape and not irregular, like asteroids?
gpt
Because more massive bodies have stronger gravity!^1 Smaller bodies can have bigger mountains. For example, the biggest mountain in the Solar System is Olympus Mons, on Mars. No mountain on Earth comes close to Olympus Mons' 16 miles high, about 3 times the height of Mt. Everest! In fact, the Earth is smoother than a billiard ball compared to its size. Irregular asteroids are an extreme case where the mountains can be close to the same size as the object itself. In the other extreme are neutron stars, where the highest mountains are no more than 1 cm!

^1 The mass goes up with radius^3 , while the gravitational pull from a given mass only falls off with radius^2 . So, more massive objects have a surface further out, but have even more mass to make up for it.