A very strong and moving piece of artwork. You've demonstrated nearly all of the aspects that make digital art as beautiful as it can possibly be, and I can really appreciate the effort you put into this, even though its a "practice" piece.
Moving on, I really do not think you could have pulled off the lighting and shadow any better here, everything is perfectly scaled and aligned, this really makes it comforting on the eyes, making for something really enjoyable to look at. The colors go very well too, not too saturated, with a very realistic "metal" look. Nothing is oversharpened or too blurred, everything fits well and the composition/depth is pretty much flawless.
Having said that, there was ONE thing I found that seemed disrtacting, possibly a monitor flaw due to brightness/contrast/gamma settings, making you unable to view it on your screen. The far end of the ring, where it dissapears behind the plannet, seems to have an unecesary bright spot, possibly from the smudge or dodge tool. If you stretched that little piece back behind the planet a little more without throwing off the perspective, this would be absolutely perfect!
I went hard on this one, and even that gave good reviews. This is a remarkable piece of work man, deffinately one I will keep comming back to.