Simple 1 is to render a quad with high enough quality bi-linear texture (512 perhaps enough, maybe 1024) with black and white smooth sphere behind the planet, using additive blending and no depth-test. You don't need alpha channel for the texture.

Or you could render it on top of planet but then you need to fade out big portion of sphere center in your image editor. This technique would allow texture light up a little of planet surface too that may look more realistic.

Size and position of the quad shouldn't be a problem.

Edit: Oh, you need to invert Modelview matrix, null its position values (x,y,z are on 4th column) and then use it with glMultMatrixf before rendering quad.