It looks off because there's no focus. Its kinda odd 'cause you got the nova in the center which should be the focal point but its a lil' small, and then you got the planet in the bottom right, right in the viewers face so it distracts from the nova. I'd suggest making the nova way bigger and making the planet WAY smaller - like pin-point small - just so you can see it against the nova but so it doesn't distract. But otherwise its a good concept and you got some pretty nice brushing ^ ^.
[Edit] also just chop it at the sides to give it some height instead of it being a square, that'll give it alot more depth and give the feeling of height.
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Push it, Push it real good...