I don't know what exactly it is about this piece, but I like it a lot.
I will say that, from a technical perspective, there are a couple of flaws, but, for me, they don't actually hold this back. The planet itself seems to be lit by a star that is behind the viewer, but there is a lens flare that seems to have its source just off the canvas to the left. Technically, we should only be able to see a crescent of the planet, the majority should be in shadow from our perspective. The same is true for the smaller moon. Incidentally, if the light source were in fact intended to be behind the viewer then the angle of the shadow on the moon should probably be the same as that of the larger planet.
The planet textures themselves are great, I really like them.
The rings should have a shadow from the planet on them, but are otherwise awesome.
Finally, the background nebula has a great shape to it but I feel like you could have taken a bit more time to develop fine details therein.
I like the flare, and I like the unusual composition, and I like the piece overall.
Thank you very much. Actually all good points of things that I missed. I almost want to go back and fix them.
You can if you like, sure, but in general I prefer to take what I learned from a piece and just apply it to make the next one that much better (saves me a lot of hassle). It's still a great piece.