Mushroom Colonists' Planet
From the Super Mario Wiki
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From what was seen of the planet in "Star Koopa / Santa Claus Is Coming to Flatbush", it appears to be fairly desolate, consisting mostly of sand and craters. Known native species of the planet include Pokeys and abnormally large Cobrats, although only one of each was ever seen. The only known buildings on the planet appear to be those belonging to the Mushroom Colony.
The Mushroom Colonists' Planet was targeted by Darth Koopa, who had plans to force the Mushroom Colonists into surrendering their entire planet to him. Knowing that the Mushroom Colonists would never give up their planet under his order, Darth Koopa kidnapped Princess Toadstool and warned her that he would destroy the planet with his Birdo Ray unless she ordered the colonists to surrender. When Princess Toadstool refused to cooperate with Darth Koopa, Mouser activated the Birdo Ray, which was set to go off in within thirty minutes.
After escaping from Koop Star in a Garbage Pod, Mario, Luigi, Princess Toadstool and Toad crash landed on the surface of the planet, where they were confronted by a large Cobrat and a Pokey. After a brief fight, Mario managed to take control of the Cobrat, which he and his friends rode to the Mushroom Colony. From there, Obi-Wan Toadi devised a plan for Mario and his friends to take down Koop Star. Because they succeeded, Koop Star, along with the Birdo Ray, was blown up before the planet could be destroyed.