J.R.R. Tolkien created Middle-earth as the setting for The Hobbit and The Lord of the Rings.
Middle-earth is a fictional region of Tolkien’s larger mythology, populated by peoples such as Hobbits, Elves, Dwarves, Men, and Orcs. The stories of Bilbo Baggins and Frodo Baggins take place there during different historical periods.
The name does not mean a planet separate from Earth. Tolkien used it as a modern English rendering of an old Germanic concept for the inhabited world, related to terms such as Old English middangeard.
Another common mistake is treating Middle-earth as the title of a single book. It is instead the setting and cultural-historical landscape in which many of Tolkien’s connected tales unfold.