Our knowledge is mostly theoretical of course, but the gas giants are thought to have a core comprised mostly of rock and ice. This is surrounded by a layer liquid metallic hydrogen, and the outer layer is composed of molecular hydrogen.
In cake speak, this translates to a core made of mudcake, surrounded by almond butter cake, surrounded by a tinted vanilla Madeira sponge. There's a crumb coat of vanilla buttercream underneath the fondant.
The Earth Cake has been super-sized.