Hi Rock,

  Planets stay in orbit because of the sun's gravitational force. The sun is much bigger and so has a greater mass than the planets. This causes the sun to have a greater gravitational pull on the planets.Centirpetal force is a force that makes the body follow a curved path. This force is provided by the gravitational force of attraction between the planets and the sun. The planets revolve at great speeds around the sun. They would continue to move in a straight line were it not for the sun. The gravity of the sun causes a change in direction and makes the planets move in elliptical orbits instead of moving in a straight line.

Another way you can think about it is if you tied a tennis ball to the end of a string and swing the tennis ball around over your head. The string is like gravity, and if you let go of the string, well the tennis ball isn’t held by a force spinning around you anymore, it just flies off in a straight line. In the same way, if we have a satellite going around the Earth and for whatever reason the Earth’s gravity switched off, the satellite would just shoot off into space. The same holds for orbits as well.