Your lighting does not match your star's (lens flare) location and your planets.
You have your star peaking over the top-left of the yellow planet, but the lighting for that planet hints that the star is off to the right of the picture. Same applies to the white planet.
The rest of the picture looks rather crowded, perhaps vary the size of the planets, and introduce more open space. Then you could perhaps add star fields into the background, using the noise filter, and setting it to lighten blending mode.
You've done well on the yellow planet's texture, and the rings are always tricky to create, so I commend you for that. Would be better if the rings weren't as blurred. Perhaps even introduce some different shades of colour to the planet.
Overall its good for an "experimentation", but has more potential.