Mushroom Colonists' Planet
From the Super Mario Wiki
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From what is seen of the planet in "Star Koopa", 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 is targeted by Darth Koopa, who 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 kidnaps Princess Toadstool and warns 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 activates 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 land on the surface of the planet, where they are confronted by a large Cobrat and a Pokey. After a brief fight, Mario manages to take control of the Cobrat, which he and his friends ride to the Mushroom Colony. From there, Obi-Wan Toadi devises a plan for Mario and his friends to take down Koop Star. Because they succeed, Koop Star, along with the Birdo Ray, is blown up before the planet could be destroyed.