I remember seeing a tutorial about using the 'lit rays' preset to make rings for Saturn. I seem to remember that you need to make sure only the 'B' channel is selected in the diffusion setting of this preset or else the bottom of the object becomes invisible. I think this is correct. I know that only one of the channels is needed and I'm pretty sure it was 'B'. If not, try each one individually. Sorry, I'm not at my home computer or I could tell you definitely.