Cyan-green combination really stands out here but the main role is of course played by the planets and there are a few things I'd like to pinpoint about.
I like how the texture of the big gas giant looks plastical but it also flattens that planet by great deal. Those parts further from the edge should have been erased to avoid this ugly effect. It is mostly visible on the left side of the planet.
Moons shade should be more round - now it looks like the moon is somehow deformed. Its texture is also a bit more stretched then it should be - loosing its detail.
That slice of land on the bottom of the picture looks good but I would be satisfied even without it.
Starfield is like a coin - it has two sides. Bright and.. ..blurred . That bright side with all the flares and star clusters looks nice. Nothing really difficult to achieve but it is effective. That blurry part with all the unfocused stars and gasses pulls this piece a bit under.
In overal - it's generally a standard piece (well - a bit higher standard) with some nice highlights but when we look closer, some unfortunate details emerge and diminish the size of impression.