Why Are All The Planets Round Shaped?

Because the way gravity acts is to pull towards the centre of a planet. On a sphere, all the points on a surface are an equal distance away from the centre. Any objects that rise significantly high will have a bigger gravitational pull acting upon them, which will pull them down. So over time gravity will pull the planet into the most efficent shape, a sphere, where ever point on the surface will have an equal gravitational pull acting upon it.

If there were a big object on a planet, gravity would pull it down, which is why you don't find 50 mile high mountains on planets. Equally, if the planet were shaped like a cube, the corners of the cube would be further away from the centre than the rest of the cube, meaning there would be a bigger gravitational force pulling here, so would eventually they would be pulled down, resulting over time, in the planet having a spherical shape. You see?

Round is the smallest form any mass can achieve. The planets are round because if the gravity on that particular planet.. It pulls all the mass into the smallest form possible.

It's all due to gravity, and inward pulling force, and pressure, and outward pushing force. What happens is all the matter is being pulled into the core or center as a result of gravity, which is created by mass. Remember Newton's 3rd law! Pressure, a pushing force, is created as a result. So we have matter being pulled in, and then we have it being pushed back out by pressure. When these two forces balance, it's known as hydrostatic equilibrium.

Because nothing in this universe is perfect, there are no perfect spherical celestial bodies. The balance is never perfect, and the spheres are generally oblong. For example, the Earth is an oblate spheroid, meaning it is slightly wider at the equator than at the polar regions. Because of this, the equatorial diameter is greater than the polar diameter.

Newton's 3rd Law: For every action or force, there is an equal and opposite reaction or force.