All the planets revolve in orbit, but what is their motive, what propels them forward at that speed?

There is nothing that pushes those planets, but something that pulls them, namely the sun. The fact that they do not fall on the sun is because they left with an initial velocity that did not point to the sun. The sun is constantly accelerating the planet, but it never hits it, due to its lateral motion. The force is ‘strong’ because the sun has a lot of mass and therefore gravitational pull, and ‘stable’ because the sun doesn’t change too quickly (and because gravity itself is a fundamental law of nature).

If we send a rocket to the Moon or to Mars, then propulsion is needed to give it an initial speed so that it can get away from our Earth. But on the way she doesn’t need a constant ‘motor’: you keep the movement you got, unless something slows you down, and in empty space this isn’t too bad.