I have been reading some of the post like on backdrops and this one looking for planets. I found some planets when I did a search on this site for free.
Seems its a standalone program for free also.
They seem to have alot of free stuff here. I registered for free and then did a search on the free stuff. Note the keyword I am using is FREE.
I am not affiliated with it or anything like that. I just like it for the free downloads for nearly any kind of 3d program.http://www.renderosity.com
I hope you can find what your looking for I found some cool stuff on the free searchs.