Ummm, the planet and Aeris answered when they had that whole cutscene at the end where Lifestream and the white materia combined to destroy the meteor. I'm pretty sure they had shown Aeris's plain ole self superimposed onto Lifestream, along with the white materia, signifying that she was playing a role.

Cloud's referring to Lifestream when he says he can meet her there... since all people return to it when they die... and Aeris is dead as disco. But, Cloud decided to hang on, since he realized he had himself a lifetime of double D's, provided he survived everything.