Using the normal "glowing" command on planetary size spheres is not a good idea. It's very, very difficult to control, almost impossible. This only works effectively in small spheres (a few meters radius). This may not be a problem to you if scale is not relevant (for example, if you're making a scene entirely in deep space, with no other references). But if the scale is relevant (for example, if the planet is seen from earth and you have montains, clouds, etc, to serve as reference), this will not work well.

I found it much better to simulate glow by duplicating the planet, enlarging it a bit (with the same center) and applying a volumetric material to it. This second sphere will be your atmosphere. You can use similar technique to make a cloud layer, for example.

There was a discussion about this in this forum, some months ago. Let me try to find it.