I've attempted many solar systems/galaxies/nebulas, and they are just beasts in Terragen. Super slow. You'll have better, and quicker results in cycles. I have projects I can't even attempt to render. Even on my 24core server they're barely at 25% in a week with all the clouds, scales, and shaders. I just gave up and do smaller stuff. For example you could possibly make a solar system model within the atmosphere of a planet along 0,0,0, at a miniature scale, and it may be doable.
I also don't know if Matt has any plans of integrating with Blender, though that would be cool, I think Terragen is mainly in-house.
PS, I'll try and locate my solar system model. It's true scale, and mainly broken.