Researching how to work with instanced meshes. I have used VertexBuffers before and it seems that those may be similar... still haven't made the connection yet though.
basically you'll have a VB,and possibly an IB to go with it, then you have an "instance buffer", with a whole bunch of world transform matrices in it. the shader applies the transform to the original mesh (the VB) then draws it. then applies the next transform in the instance buffer to the original mesh and draws another copy. and so on for all your instances.
Was going to billboard nebulae and gas clouds and then particle the systems they can travel to (so that they can have a label attached to them by the nav HUD)
if the "world" (galaxy?) is large, you'll want a "world map" with star systems etc, and an "active targets list" that you page star systems into and out of as they come into and go out of range. then just draw your active targets. planets and such aren't really particle systems, they're more like world objects, like a big ship or starport.
now for the REAL challenge:
how on earth are you going to draw what it looks like when they get shot down and fly through the atmosphere and crash land on an alien planet? if you draw a planet, they're going to want to land on it. you know how player's are, they want to be able to do everything. <g>.
drawing atmospheric reentry is one of the big challenges in space flight sims.
And yes, i'm just kidding. Almost no space sim does this due to the difficulty.