Skydomes and spheres are actually alternatives to skyboxes and selected in the 'Tab/Tab' menus same as default. Mostly the planets in these are geometry built into the skydome and a scrolling texture shader is used to animate these, the trick in built in geometry is how to keep the 'shadow' on say, a planet rotating in a direction which appears as static while the planet itself rotates beneath this layer
So they are all exactly same as skyboxes which you never reach the horizon etc and move with the player view, except they are...umm..... spheres or domes without any of the issues you get with a box.
Yo can see the effect a little with the spaceship video where the camera is moved in on the craft, although it appears the ship is moving it is in fact static and it is all a trick of the eye with camera movement. But as you can see the ship gets closer as I move in and further as I move out but the skysphere keeps a constant distance....hope that makes sense
Mostly there a few sphere layers with alpha transparency overlaying and flipped uv's so they rotate in different directions across each other to get more movement in the animations. The speed of animation and distances of, for example, asteroids from each other is completely unrealistic in real terms but for the sake of game 'Art' it is completely acceptable to take liberties in this case
The real bonus here is a fully enclosed scene which would require a minimum of entities to flesh out a level, without hitting fps or memory limits you practically have an entire environment right off the bat, These could also be used as simple skies for terrain levels, to me the really nice thing is being animated as theyy are they create a nice dynamic background which blows skyscroll out of the picture entirely. I had a hand in getting skyscroll into GG way back but would be quite happy to see it dropped in favour of this method, looks a lot better. There is always room for improvement.
"There is a vacancy for 'Village idiot' in the realm.... but I fear you may be over-qualified...." King Lear.