I love your blue planet, its very nicely done your cityscape planet is good too I did one once but cant remember which tutorial I used, think it was on deviantart or something. Yours looks alot better than mine did anyway.
Mind if I post a few creations from Photoshop of mine? Here goes.
My favourite colour is blue so I also did a blue planet while ago, I did have these two in 1024 x 768 resolution but I had to reboot my computer the other day and only have these 800 x 600 ones, I think I got the others on my psp hehe so I'll grab em off it later
Basicaly I wanted to have a nice nebula instead of just plain black starscape, and a ring system which took very little time. The over all time for the blue planet was about 2 hours.
The Star system one was my first attempt and I like it but not as much as the blue planet, I think one of the most important things about making a space scene is getting the sun right (if any) in relation to your shadows and also having a nice atmosphere on your planetary bodies too, which you have managed quite will there Creative Inc.