All of the planets are round because of gravity. When our Solar System was forming, gravity gathered billions of pieces of gas and dust into clumps which grew larger and larger to become the planets. The force of the collision of these pieces caused the newly forming planets to become hot and molten. The force of gravity, pulled this molten material inwards towards the planet's center into the shape of a sphere. Later, when the planets cooled, they stayed spherical. Planets are not perfectly spherical because they also spin. The spinning force acts against gravity and causes many planets to bulge out more around their equators.
						

							
OUR SOLAR SYSTEM BEGAN AS A CLOUD OF SWIRLING OF DUST AND GAS. DUE TO THE EXTERNAL FORCE LIKE EXPLOSION OF STARS SOMEWHERE. THE  CLOUD BURST  , DUE TO THE BURST DUST AND GAS BEGAN TO  COLLECT INTO DENSER REGION  THESE DENSER REGION PULLED MORE MATTER , CONSERVATION OF MOMENTUM CAUSED THEM TO ROTATE AND THE PRESURE CAUSED THEM TO  HEAT UP  HENCE MOST OF THE MATERIAL ENDED AS A BALL 
						

							
Force of gravity is the main cause.
Moreover, we know that among geometric figures of same  volume, the surface area of a sphere is the least.Thus,the planets are spherical in order to lower their surface area.