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Information in this article may not be canonical to the storyline of the games or any other Sonic continuity.
Planet Eggs are large gemstones of varying shape and color. Each Planet Egg embodies the life force of its home planet.
Removal of a Planet Egg
Once a Planet Egg is removed from its spot, the planet begins to die, which can take many different forms. On Sonic's homeworld, the plant life began to die out in the surrounding area once the Planet Egg had been extracted from Angel Island. On another occasion, the Planet Egg's harvesting turned a desert planet into a gigantic ocean. These effects were reversed after Sonic and co. recovered the Planet Egg, though they had to fight off two powerful Metarex in order to save the planet.
Another world became covered in ice when the Planet Egg was gone, a fact that greatly saddened Cosmo until the Egg was returned. Later, the gang of heroes found a world that had become overgrown by plants. Using the Planet Egg of this world, Cosmo was able to communicate with the vegetation, but it went out of control as a result.
Historical records on Cosmo's homeworld revealed that Dark Oak (then known as Lucas), the leader of the Metarex, had first discovered a way to use the Planet Eggs to stabilize the unnatural transformation process he and other male Seedrians were capable of. He used its power to make himself and the other male Seedrians immortal, turning them into the Metarex. Because of this, the female Seedrians led by Earthia left their homeworld behind, thinking that the males would be destroyed. Instead, they survived and built metal bodies to support their frail structures.
Dark Oak planned to use the Planet Eggs of every planet to grow stronger. For years, the Metarex stole the Planet Eggs of every planet they could reach, as part of their plan to rule the galaxy.
Some time later, the Blue Typhoon and Doctor Eggman arrived at a Metarex fortress. After making their way through the defensive gauntlet, they discovered a Planet Egg that had been enlarged by energy from two Chaos Emeralds. Unfortunately, a battle with the Metarex resulted in the Egg being jettisoned and lost.
When all the combatants in the conflict reached Galactic Coordinates 000, the true plan of the Metarex was revealed: to receive enough energy to conduct a special event that happens once every a thousand years. Using the Planet Eggs as a source of water and the Chaos Emeralds as a source of light, the three leaders of the Metarex, Pale Bay Leaf, Black Narcissus and Dark Oak transformed into the Final Nova planned to channel the energy from a breach in space time to destroy all non-plant life in the galaxy. Fortunately, their plans were thwarted. After the Metarex were destroyed, all the Planet Eggs scattered back to their home planets in a flash of many lights.
Different Planet Eggs
No two Planet Eggs are alike, but only a few are known. The Planet Egg of Sonic's home is a pink square jewel, which is located in the forest under Angel Island. The Planet Egg of the jungle planet is a green sphere with little pillars. Its location might seem to be the middle of the jungle.
- The Planet Egg from Sonic's homeworld resembles the shape of the original Chaos Emeralds as depicted in Sonic the Hedgehog, especially the pink colored one (discarded color in later games).