The gravitational pull of the sun keeps the planets within the solar system. The counteracting effect of the planet's own velocity keeps the planet from collapsing into the …sun and so merely orbiting it. If you mean what binds a planet together, that's the planet's own gravity. In addition, the solar system contains millions of rocky asteroids and billions of icy comets. All of these objects are held together in a group by the Sun's gravity. The planets, asteroids and comets in the solar system are loose objects left over from the formation of the Sun.
