Almost certainly the atmosphere is a separate piece of geometry, which I think we can assume.
Couple ways to approach this. Might be a sphere that makes use of fresnel and translucency & transp. depth blending to achieve the effect. Could be tough to tune it so that it falls off properly though.
Another “hackier” approach that might lead to more control would be to have a flat circle ‘billboard’ that is always facing the camera and rotates around the center of the globe. You would have more control over how the fringe scattering looks, but the downside is it would be harder to blend the effect across the face of the planet and there may be some artifacts where the billboard intersects the globe geometry.