The Little Planet (also given the secondary name of the Miracle Planet in the Japanese instruction manual) is a mysterious moon of Sonic's home planet, which functions as the setting for Sonic the Hedgehog CD. It only appears for one month of every year, in the skies above Never Lake.
The planet hosts the Time Stones, gems that are coveted by Dr. Robotnik due to their mystical ability to control time. Robotnik chains the Little Planet to a giant rock (engraved with his own face, no less) and quickly begins work to transform the entire planet into his new base, using time travel to take over the past and thus control the future.
Seeing this, Sonic sets off to foil his nemesis's latest scheme, and eventually sets the Little Planet free by destroying Robotnik's base and the rock which the planet was chained to. Released, the Little Planet rises up for a short distance, then completely vanishes.
The US instruction manual adds that the Little Planet is normally covered with lush vegetation including tall trees and flowers, but at the start of Sonic CD has been metallised by Dr. Robotnik, and as such is covered only in grim, grey steel.
The Little Planet is featured again years later in Sonic the Hedgehog 4: Episode II. In Episode Metal, the Little Planet comes close enough to Sonic's world for Dr. Eggman to revive Metal Sonic and have him travel across space and back to his base in Mad Gear Zone for repairs.
Over the course of Episode II, the Little Planet can be seen in the night sky, and as Sonic and Tails progress, Dr. Eggman slowly builds a brand new Death Egg known as the Death Egg mk.II around it. By the time Sky Fortress Zone is cleared, construction, Sonic and Tails finally reach it and destroy Dr. Eggman's final machine. Though they destroy the Death Egg mk.II, it is still intact, and Little Planet is left inside. As the Sonic 4 saga ended prematurely, one can either assume the Death Egg mk.II is eventually dismantled, or the Little Planet is stuck inside indefinitely.
The Little Planet is once again featured in Sonic Mania, and is visited twice by the heroes during the course of the game. The first instance the heroes go to Little Planet is when Dr. Eggman uses the Phantom Ruby at the end of Press Garden Zone to displace the heroes, sending them to Stardust Speedway Zone in the Past.
The second instance that the Little Planet is visited is near the end of the game, as it is visible from Lava Reef Zone on Angel Island. The heroes then travel to the Little Planet for the last two zones in the game, Metallic Madness Zone and Titanic Monarch Zone. In the normal ending, the heroes escape the Little Planet and watch Titanic Monarch Zone explode, while in the true ending, only Tails and Knuckles escape as the Little Planet is again set free from Eggman's control and transported away by the Phantom Ruby.