They are, in Ogre the height is actually the y coordinate. The camera is at +15 the nucleus model is at 0 and the background is at -50.
I even tried setting the model to be at +10 and it still looks like to be behind the background:
I think this has to do with me having to remove the transparency etc. settings from the materials, because Ogre said they where unknown (probably removed in Ogre 2.1).