I like the planet but the asteroids are too evenly spaced and straight on a circle. Also I'd reduce the polycount on them a lot, the level of detail compared to the planet is way to high.
The Background needs some more and better distributed stars, they kind of cluster in groups now.
It looks very nice but I would try to make it a little bit brighter and maybe light it from the side since you will probably get more interesting shadows.
The quality of your coloring borders are too similar to the detail level of the mesh. I'd make the bordes blurred. The way you did it breaks the interesting graphical shadow effect you get from low poly art. Of course just my opinion, other may like exactly that effect.
You're using color mix instead of shader mix. Green shader sockets has to connect to other shader sockets and nothing else.
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