Please do yourself a favor and try this program. It is literally LIMITLESS. It has a close-to-complete catalog of known celestial phenomena, from comets to exoplanets, procedurally generates planet, whole planet surfaces, asteroid fields, volumetric nebulae and whole galaxies.
You can also export textures for use as game and 3DCG assets, and import your own models. There are a variety of pilotable spaceships included as well. I can’t stress this enough… THIS IS WHAT YOU’VE BEEN WAITING FOR.
It’s still in alpha at the moment, and is pretty buggy, but a few minutes browsing the forums got me up and running pretty reliably. It is extremely system-heavy, so don’t expect any miracles if you’re stuck with a dinosaur of a computer, but this will provide you with a literally endless universe to explore. I was dumbfounded when I stumbled across it, because I’ve been looking for an alternative to Celestia (abandonware?) for quite some time now. This was precisely the resource I needed.
I believe it’s Windows-only at this point, but I wouldn’t be surprised if Linux support was somewhere on the horizon. Try it. You just have no idea how cool this is.
Seriously, try it.