How come planets are round?

Answer

Planets – stars for that matter – are indeed round because of their own gravity. Try making a mountain of water in the sea, it always comes back down. For a sphere is the surface that at any point is perpendicular to the direction (downwards) of gravity. If it weren’t a perfect sphere, then there is also a component of the force along the surface (like a rock sliding down a mountain), and the water then moves down. Until the equilibrium is finally found, in a spherical configuration.

A stone is generally not round, because the gravitational pull it exerts on itself is much smaller than the – essentially electromagnetic – forces that hold it together. Self-gravity starts to dominate from a size of the order of 500-1000 km, and such objects in our solar system are indeed quite spherical.

Gravity does not need a medium to propagate, and therefore no ‘friction’. The strength of the force between objects is determined only by their masses and mutual distance. ‘Friction’, in the sense of electromagnetic interaction between the particles, does play a role in the formation of stars and planets, to radiate the released energy and thus cool the center. The role of density is simply that the thinner the center, the longer it all takes: the universe is not completely empty, and as large thin clouds contract they naturally become denser. Ultimately, an equilibrium is found in which the inward gravitation is compensated by an outward pressure force. And that equilibrium is spherical, for the reason stated above.