To be honest... I wouldn't have guessed that it should be a planet, you need something like clouds, mist or whatever... it really kinda looks like a marble..
it just needs more texture.. less shine.. and a background. your on the right track... you should ask Charles for some more input.
You did a really great job with this! I would agree about it looking like a marble if it had backlighting, but to me it looks rather planet-ish, like a gaseous planet with a liquid core or something.