just watched it, and at 1:34:51, when Carol comes onto the bridge, Kirk turns and says, "My God, Carol, look at it." You can see the viewer as he speaks, and you can see a rain of molten rock falling onto a forming planet that is clearly expanding. Makes sense if it is forming a planet out of the nebula material.
This could also be explained as: matter thrown this way by the explosion and caught Regula's gravity well - the observed "expansion" of the planet could actually be just the Enterprise closing in on Regula - the glow is the same phenomenon we see in the presentation video.
Boy, this is fun