render a planet's atmosphere

can anyone point me to an article or tutorial on rendering an atmosphere around a sphere?

You can do simple atmosphere by rendering a blended blue sphere with a larger radius over you earth.

very simple but not very realistic

Look for the “Earth” demo at

Source (in Delphi) can be found at

It uses some basic raycasting calculations, which are then used as vertex colors for an atmosphere mesh in real-time (an overlay actually). The raycasting and OpenGL rendering code is in the TForm1.GLDirectOpenGL1Render of Unit1.pas.

thank you guys