I have been curious about how this kind of seamless transition from space to planet can be implemented. It would be nice if someone could explain this as I can't really wrap my head around the solution.
Over time, move the camera from far away from the planet to close to the planet surface. This is analogous to the way that you can seamlessly transition from one side of a Quake level to the other side, merely by moving the camera from one side to the other.
There really is no fundamental difference between the two.
This guy did exactly what your looking to do, his site goes into tons of details.