they are listed on the Orion's Arm Celestia page here.
http://www.orionsarm.com/reviews/Celestia.html
Now I come to looka them some of the planets do need a bit more work; gradually over time I'll produce hi-res textures and better meshes for each of the worlds concerned- but they will be available as modifications to the main package. Learning as I go along; I now know how to make better specular maps and bump maps, as well as night textures for the inhabited worlds (in OA that is most of them).
For instance here is a world I have just made using Rassilon's Terraform generator;
after a lot of tweaking, it looks quite earth-like.