You should make the emission shader aware of the Alpha information from the image. At present indeed, the antialiased border of the globe is emitting as it was fully opaque.
You can do so by connecting the Alpha socket to the Strength input of the emission shader, and adding a "Math>Convert" node in the middle, set to multiply by 0.8.
(in your setup, the second - half transparent - stripe would have wrongly appeared completely white)
[ As cegaton says, however, for your application the
add shaders can be removed altogether: your globe isn't supposed to diffuse received light. ]