Would it be possible to make planets made out of like um... shadows metal ect?

well im not sure how exstensive the plant creation system will be however i can tell you that like the creature editor... and well.... most everything else..... chaces are you will....

I think Will Wright and the developers are going for as random as possible but who knows what parts of the game are customizable.

From what I understand, the planet you start with and all others at the start of the game/discovery are randomly generated. Meaning we'd have no control over their appearance and features from start.
However, after getting into space and developing your UFO far enough, there was an interview (think it was the NASA one) where Wright stated that a dedicated enough player could literally make any kind of planet he wanted, from an Earth replica to a fantasy world from their own mind.

Also, that if players look hard enough they may even find Sol (our solar system) complete with Earth, Mars, Venus, and all the other wonderful planets we know and love... as to whether the planet will have humans on it... we only got "find out when the game launches."

If I see earth I'm blowing it up. :D