I have two suggestions.

1.)  Look to see if you  have zoomed in on the planet.  If you have, your view in the corner of the window in the assembly room will say something other than 100%.  If this has happened, then that might be why the starfield is not what you're looking for.

To fix this, zoom back to 100% and move the camera in the scene to get what you wan rather than rendering with a zoom.

2.)  I would suspect that you have 1 of two possible issues with your lighting.

First, you still have ambient light turned on.  If so, turn that off.  You can find it in the scene settings.  If this is still elusive, just ask here in the thread and we will help out.

or

Second, you have turned on some kind of environment in the scene settings and you are rendering with some level of GI turned on.  If so, you can turn off the GI rendering in the scene and see if that fixes the issue.

I have never used any preset environment before so I don't know what the settings you started with look like (nor do I know if you started with a preset).  My starfield was actually generated using the Digital Carvers Guild plug in starbright (probably my most favorite).

-Kix