system
Given the following post, respond as if you're commenting on Reddit:
human
Why are planets always round? (Not a flat earther at all here btw) just wondering why a planets are not a more random shape?
gpt
The definition of a planet has a few criteria and one of them is that it has to be round. So if an object isn't a sphere, we wouldn't call it a planet.

But that's a bit of a lame explanation. The reason why the things we call planets are round (and the reason that we added "being round" as a requirement for something to be called a planet) is that a sphere is the most energy efficient state for an object to be in when its own gravity is sufficiently strong.

So if a celestial body becomes large enough, its gravity will automatically force it more and more towards a spherical shape. Imagine an object that is otherwise spherical, but contains a large mountain-like structure sticking out from the spherical surface. A rock will roll down from this mountain without requiring additional energy input. This rock rolling down the mountain brings the object closer to a spherical shape. But rolling a rock from the surface up the mountain, making the object less spherical, requires some external force.

So any objects in space that are not spherical are simply too small for their own gravity to pull them together into a ball.