How are planets formed in the solar system?

Out of an accretion disc. A disc of materials left over from the formation of a star (in this case our sun) orbiting. Small particles bump into each other and stick. Slowly over time it isn't the rigidness of the particles in that environment which causes them to stick, but at a certain size it will begin to bend space and time to where the larger chunks have significant gravity so other particles are drawn towards them. As the bigger pieces collide over time they also begin to hold together until they have what is known as hydrostatic equilibrium. This is where the object has enough gravity to hold a roughly spherical shape. Thus a planet is born. Gas planets are slightly different only in the respect that solid portions are not as visible; they fall towards the center.