The first law is that everything moves in a straight line if it is not being influenced by the exertion of force by another object. We know that planets orbit the sun, and the sun is exerting the force of gravity on the planets that orbit it. For the second law, F=MA, we know that every object will continue to go in motion is there is no force acting upon it. Once again, the sun is this force, and if there was no sun, the Earth and other planets would just fling off into space until pulled into orbit by another celestial object. This last law basically dictates our definition of gravity, "every action has an equal or opposite reaction." The force that the Sun exerts on the planets is equally exerted on the sun, and that's entirely what creates the orbit today. Hope this helps!
