Why are planets round - Isostatic adjustment- Thermal Equilibrium- Hydrostatic equilibrium.

Why are planets round

Isostatic adjustment

Thermal Equilibrium

Hydrostatic equilibrium

Stes de Necker

Isostatic adjustment

Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it.

With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere.

The technical name for this process is "isostatic adjustment." 

With much smaller bodies, such as asteroids (50 to 500 miles) the gravitational pull is too weak to overcome the asteroid's mechanical strength. As a result, these bodies do not form spheres. Rather they maintain irregular, fragmentary shapes.

Thermal Equilibrium

Thermal equilibrium is the tendency of thermal energy to flow so that temperature differences are equalized. Equilibrium applies to gravity, too.

Every part of a planet’s surface has a gravitational potential energy. This quantity can be expressed as the product of the object’s mass (m), the acceleration of gravity (g), and the height of the object (h):
PEGravity = m * g * h

So potential energy increases with the mass of the planet (because a more massive planet will have a larger value of g), and with the distance from the planet’s center.

Gravity pulls all the parts of an object toward its center of mass.

Gravity always works to minimize the potential energy of all parts of a planet. High places have a larger gravitational potential energy, and are pulled downward more strongly than low places.

Take a large tray and cover it with sand or gravel heaped into "mountains." If you shake the tray, the sand will quickly settle to a flat surface where the gravitational potential energy is the same everywhere. This is an example of equilibrium.

Large bodies in the solar system have enough mass so that their strong gravity forces their surfaces to have the same potential energy everywhere. The result is the most symmetric shape possible: a sphere.

The Earth is as round and smooth as a billiard ball.

Small asteroids, however, do not have enough gravity to overcome the strength of the rock they are made of. So they have irregular shapes.

Asteroid Ida with its tiny moonlet Dactyl

Hydrostatic equilibrium

Planets are round also because their mass is large enough to trigger hydrostatic equilibrium.  Hydrostatic equilibrium is where an objects own gravity pulls high places down and pushes low places up until all pressures are equalized and the object is round.  

It is very much like a bubble.  The pressure on the inside and outside of a round bubble are the same. 

If a part of the bubble is sticking out, there is more pressure on the outside of the part sticking out than on the inside of the part sticking out.  That pressure pushes the bubble until it is not sticking out any more.  It doesn't keep pushing on that part, because then there would be more pressure on the inside of the bubble than the outside, and the part being pushed in would be pushed back out again.