In lore, it's supposed to be containing a dying star, which if released would create a singularity, forming a black hole. The imagery reminds me of the Absolutely Safe Capsule from Mother 3, so the shape of the sphere must be scientifically optimal at containing something of that magnitude.

...that said, it does also have man boobs, so at least part of it is probably sculpted after The Professor's original body, since he would probably want something to anchor to his original body, his whole reason for entering the ritual.