Hey emphasized, welcome to Pixelation
First I'd point out you have a mixture of pixel art (every pixel was placed by you) and non-pixel art (your asteroids have auto anti-aliasing). While there's nothing wrong with this per se (except perhaps stylistically), my first advice to you as a beginner is to avoid this for a very long time. The reason is you want to train your ability to work with tones and control the piece as yours. You will not improve if you rely on the computer to do your work for you.
There are some basic things that this piece can use: First, use a single light source, so that the player can perceive everything uniformly. As well, it makes your piece consistent. Second, reduce your color count. It's very challenging starting out, I know, but bring it down to about 4-5 colors per object to start out. This lets you focus on the more important aspects of pixel art (pretty much any 2D art): the shapes. Finally, use a reference when drawing the asteroids. Always use references.
So, to recap:
1. Control all aspects of your piece. Don't let the computer do it for you.
2. Use a single light source.
3. Use only 4-5 colors per object.
4. Use a reference.
Looking forward to seeing your improvements.