I'd go with a large planet. I played around with this a bit and this is what I came up with:
This is the composition you have now. There are a lot of distinct elements, but nothing to tie them together:
Add a planet however, and you have something which overlaps all the elements, bringing a 'wholeness' to the work over all. Not to mention creating a strong triangular composition:
I also noticed that the ring overlaps one of the stars:
So maybe you can add a lens flare there to fill in the middle a bit, and create a bit more lively composition:
That's just what I'd do. Hope it gives you some ideas!
