I don't think this would help much, as I don't believe that rendering is the actual bottle neck. When you experience lag when moving towards large structures, this isn't because of physics (which are computation heavy, regardless) or graphics, but simply because I/O (reading it from the disk/transferring it via IP) is expensive.
Also, those planets models are actually static. Proof? Look at an old cookie planet from farther away.
Assuming rendering was so much more expensive, an actual model still wouldn't do much in terms of performane (also, generating them would be expensive, too).