as long as only two objects are involved, the resulting motions are simple. Kepler’s laws describe the regular, repeating elliptical orbits of planets around the Sun. When more than two objects are involved, however, the resulting motions may be anything but simple and regular. Each planet in the Solar System moves under the gravitational influence of the Sun combined with that of all the other planets. Although these extra influences are small, they are not negligible. Over millions of years they lead to significant differences in the locations of planets in their orbits. In many possible extrasolar planetary systems, such interactions among planets might cause planets to dramatically change their orbits or even be ejected from the system entirely