Although planetary systems seem stable, they change over time.  Wandering asteroids occasionally hit planets and moons, and the influence of gravity, while it may work slowly, can cause moons to collide with each other and orbits to change. How can you set up a system that will work long enough for your life forms to flourish?

     Scientists are working with computer models to study orbits and the relationships between planets and moons.  Gravity can affect orbits even at great distances, and with billions of years to work, even small gravitational influences can have power.

     The force of gravity is strongest between objects that are close to each other. Space out your planets and moons!