The following is about the best I can get. Note the use of the parameter 
Scale which enables you to scale the atmosphere without changing its 
characteristics.

#version 3.7;

global_settings{
     assumed_gamma 1.0
}

camera {
     location <0.0 , 0.0 ,-3.0>
     right x*image_width/image_height
     look_at <0.0 , 0.0 , 0.0>
}
light_source {
     <1000, 1000, 1000>
     color rgb 1
}

//planet
sphere {
     <0, 0, 0>, 1
     pigment { color srgb <1,0,0> }
     scale 0.85
}

//atmosphere
#local Scale = 1.1;
sphere {
     <0, 0, 0>, 1
     pigment { color srgbt 1 }
     interior {
         media {
             absorption srgb <0.1, 0.3, 0.5>*10
             scattering {1, srgb <0.3, 0.2, 0.1>*10/Scale /*eccentricity 
0.2*/ }
             density {
               spherical
               density_map {
                 [0.01 rgb 0]
                 [0.30 rgb 1]
                 [0.85 rgb 1]
               }
            }
         }
     }
     hollow
     scale Scale
}


-- 
Thomas

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