Consider lots of mass in isolated 3D space, close to each other. Consider that only the gravitational force (Newtonian) exists. Also consider that there is no rotational motion.
It is evident that a sphere has the geometry/configuration that allows for the least possible gravitational potential in such a configuration. Hence, most of our planets/stars are spheres.
This post Is it possible to prove that planets should be approximately spherical using the calculus of variations? gives a mathematical proof of this fact.
What would be the most stable shape if gravity was proportional to various interesting $r^\alpha$ instead for various $\alpha$, rather than $r^{-2}$? Can we tweak the linked proof to get that shape?
