Magrathea is a great planet, although it is mostly asleep. Located in the Horsehead Nebula, in orbit around the twin suns Soulianis and Rahm, this planet is the home of an important economic industry: luxury planet-building. During the days of the Galactic Empire, this planet designed and built planets for the wealthiest people in the universe.
Magrathea should be extremely important to humans. Why, you ask? Because Magrathea designed and built Earth. In fact, Earth is actually a computer set on determining the answer to the life, the universe, and everything.
This is the planet that created us:
Isn't she lovely?
For more information on Magrathea, read Hitchhiker's Guide to the Galaxy, or watch the movie. Or the TV series. It's all over the place. Douglas Adams is the man.
Picture from The Hitchhikers Wiki.