I think that the asteroids should be at the side, it's a little bit in the way of the focal point. As for the second planet, the one that looks like the closest one on the left, I think the piece would look better with out it. Try and make the star a little spiker.
The planet on the right looks extraordinary, loving the texture. The composition is a bit overwhelming though, 4 planets (and very large ones at that!) + asteroids. Hard to detect any main focus. Other than that a well made piece indeed