SpaceEngine is a surprisingly good piece of software I also came across it while looking for something else!
Amazing amount of information is displayed on screen as you shoot through the universe or land on planets and explore. The Planets and space sky textures are fantastic and there are extra downloads for specific planets in much greater detail but extra detail means, very large files eg. 700mb to around 5Gb, per planet, may be downloaded, fantastic!
One can edit : Planets, Systems and Textures and make Cylindrical or Cubical Skyboxes and export as, png, tga, dds, tif, jpg. Ready sliced for six sided cubes and easily loaded into other game engines. Unity is one I tried for a quick test, worked seamlessly (make sure when in Unity every one of the six textures is set to 'clamp' and not the Unity default method 'Repeat') or you'll most likely get seams in your skybox.
Try it it's free and fun...