Your intuition/deduction is correct, it's a clipping plane issue. You can adjust the clipping plane range by using the [ ] bracket keys. [ will move the clipping plane closer to the camera and ] will move it further, for portraying very close or very distant things in the realtime preview, respectively (e.g. when you are very far from the planet you may see black specks in the preview for a planet and you could then *increase* the clipping plane distance). Note that this only affects the realtime 3D preview.
- Oshyan