system
Given the following post, respond as if you're commenting on Reddit:
human
Why are planets almost perfectly spherical?

Why are there no odd shaped planets, or planets shaped like cubes etc?
gpt
Planets are held together by gravity and once an object is heavy enough its own gravity is strong enough to pull it into a a spherical shape.

Our moon is large enough to be spherical but the moons of mars retain a shape that has been formed some other way.