Let me take you through it step by step:
1. Create a RampShader Material
2. In Common Material Attributes|Color, set two color handles. The one on the left should be black, and its selected position 0 and the second one blue (or the color of the atmosphere you want) and its selected position 1.
3. Set Interpolation to Linear, and Color Input to Light Angle.
In Common Material Attributes|Transparency, set two color handles. The one on the left should be black and its selected position 0, and the second one white, and its selected position 0.250 (play with this value as it determines the amount the atmosphere covers your sphere).
4. In Special Effects, set Hide Source to True and set a value for Glow Intensity to a value you like.
For the sphere that you map this shader to, go to Render Stats and uncheck Double Sided. I don't have Opposite checked.
This sphere should be slightly larger than the sphere you have your planet texture mapped to. Let's say your planet sphere's scale is 50 then your atmosphere scale could be something like 50.1 or thereabouts.
I have a light that is linked to the Atmosphere only, so that you can tweak the glow without affecting the planet.
I suggest you also read up on the default shaderGlow1 node that Maya puts in the Hypershade.
Good luck and hope this helps.
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