The Earth is the planet on which Springfield is located. It's also called the world and, less frequently, Gaia. It is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets and the only astronomical object known to accommodate life.
Treehouse of Horror
Kang and Kodos frequently look down on Earth, waiting to attack it when the humans are on their weakest point.
Appearances
The Earth appears in all episodes, because Springfield does and Springfield's located on Earth.