they all look alright. A little amateurish though. If you have been working on learning just photo manuipilations and collages for a year, then you should be a little better than this.
So I'll assume that you have been casually learning your technique, which is fine.
Anyway A reoccurring mistake I see in almost all of these is
in pictures 2, 3 and 4 you have lots of different objects with conflicting light sources. You lack a bounce lighting. Number 4 is missing a bunch of shadows that it needs.
Lighting is one of the first things to break an illusion of a photo manipulation, but it is also one of the hardest things to master.
Lets take picture number 2 for an example. The earth like planet and moon have a light source coming from what would be right over the left shoulder of the viewer. You have another planet to the upper right from those who has a light is still to the left, so its a good start, but it is more farther to the left than the first light source. Then the third planet closer to the bottom of the page has its lighting to the extreme left. its pretty much a I would call it almost profile lighting.
The individual assets in most of these are not too bad, but almost all of them can be learned and produced in a short amount of time from just a couple of tutorials. And your end compositions are very much slapped together leaving out little details that would make them good instead of mediocre.
So keep practicing and study your lighting. Be sure to preplan a lot when making these. It makes the end result better and easier to achieve.