The Earth is a planet far away from the Mushroom World and is the setting of Mario is Missing and Mario's Time Machine. In both of these games, Bowser steals important artifacts from Earth, causing chaos on the planet. As such, Mario or Luigi must retrieve these items to restore balance to the Earth.
The planet is orbited by one moon (which Mario has visited).
Origin Theories
Mario and Luigi are believed to have grown up in Brooklyn, a neighborhood of New York, a city on Earth. Sometime after Mario & Luigi: Partners in Time, Mario and Luigi were sent to Earth, possibly to avoid harm from Kamek or Bowser. On Earth, the brothers grew up, becoming plumbers and eventually stumbling into a Warp Pipe in the game Mario Bros. This warp pipe took Mario and Luigi back to the Mushroom Kingdom, where they have remained ever since.