Universe Sandbox is exactly what the name says: a sandbox simulator game of the universe! It uses real physics, and personally I find it to be very enjoyable, with achievements that have very nerdy references. You can just open an existing simulation, like our solar system, or create a whole new galaxy.
I can also make planets with custom textures. I made a post on the US forum where I tried to gather as many textures as possible, check them out here
The game costs 4.99 euro on Steam now, and it might get even cheaper later in this christmas sale. Even at full price it's still
worth the money. Buy it here.