Therin of Andor wrote:
Yup, the nebula is blasted into concentric layers that shrink back and we see the planet forming from the core outwards from scratch.
And this is essentially how Vonda McIntyre described it in her ST II novelization.
Until the protomatter subplot was retconned into ST III, the main reason the new planet was so unstable was because, instead of the Genesis torpedo being detonated over the surface of an existing planet, and terraforming it, Khan exploded it inside the Reliant
- and the device's program drew in material from the ship, its crew and the nebula gases instead... to create an entire planet. McIntyre also explains how the Genesis Cave and the Genesis Planet both appear to have natural light sources/suns.
That's how I always interpreted it. The Regula planetoid was not visible from the Mutara Nebula even before Reliant exploded, obviously full impulse power for so many minutes away from the planetoid was far enough to leave it behind. We see from the Enterprise looking back the way they came, just Reliant behind her.
So it could still be there in the system, just at some distance from Genesis, being too small and dark to see at all.
As others have pointed out the star is still there in the system close enough and remains there after the planet goes, so it does seem to be the same natural star.