Brim*Star

BrimStar is hollow, red, and open to space through an enormous star-shaped crater, and that crater is the point of it. Light from the MoonStar travels down through the opening to reach the Transformation Chamber, and the chamber turns a caged criminal back into an armoured warlord. One wiki adds that the planet carries rocket engines in its crust, fired from a control room usually manned by Yes-Man, to rotate the world until the crater lines up with the Moon*Star.

Inside the crater sits the fortress, with the throne, Melodia’s pipe organ room and the Deep Space Ejector. The Planet Eater is here too, set in the crust rather than flying, so it belongs to this entry and not to the vehicles.

The spelling is unsettled in our own primary source. The episode data carries Brim*Star in episodes 28 and 62 and BrimStar in 37 and 52, and we have not normalised it.

Sources: the aired episode synopses, thundercats.fandom, thundercats-ho.fandom.