I wanted to write something constructive yesterday, but was not able to get it right ... second go:
I really like the composition, the foreground and the mojoliths are well done ... but not as much worth as the whole of the composition ... they frame the background which draws my attention. And you are quite right about the background's appearance ... the planets and the star field do not really match. I doubt that using no interpolation for the background, as suggested in another thread, would really improve your art. It may be your aim ... ... ... but not the goal.
What happens to my eyes is, that they are drawn to the moons (moon and planet) and are moving straight downwards out of the image. Having the sharped image as a background might draw my eyes back but ... ... I bet they'll move that much upwards that they leave the image again.
Considering a crisp sharp background, you might want to move the secondary planets just a bit out of the vertical center ...
Using the small moon as a starting point, that the focus is moved by a full diameter of the small moon to the right (maybe left) and half of its diameter upwards. ... just a suggestion to an image that I would like to have rendered myself and would like to improve it ,-)
btw ... good work