Moon and Jupiter 
Our solar system is comfortable and settled. The planets ply their orbits around the Sun without getting in each other's way. An occasional comet or asteroid does slam into Jupiter, which is the solar system's largest planet, or even into Earth, although there's a longer gap between impacts.
But many other planetary systems are young and vigorous, with lots of action.
In the youngest systems of all, planets are still taking shape. Bits of rock and ice clump together to form larger and larger bodies, eventually making planets. Gas adds to the heft of planets in the cold outer regions of the system, creating giants.
Collisions between planet-sized bodies are fairly common. The collisions can pulverize both worlds, or they can pulverize one world while stripping part of the other to make a moon -- the process that probably gave birth to our own moon.
And even if the planets don't ram together, they can still have a major impact on each other. Gravitational interactions can kick some planets out of the system, while causing others to spiral closer to their parent star. In fact, some planets may be nudged into the star itself -- destroying the planet and giving the star a "dusty" atmosphere. More about this process tomorrow.
Look for two objects in our own solar system huddling close together tonight. Jupiter looks like a brilliant star just below the Moon. They rise around midnight and are high in the sky at first light.
Script by Damond Benningfield, Copyright 2011