How to contain particles in a circle (or sphere)

I would like a particle system constraint in a circle.

I already tried to constraint their position into a sphere, but because they are blurred, they have to stay way too far from the border or the blur will be visible outside.

It could be by hidding the particles outside the circle, or by drawing the particles in a frame buffer and apply this frame to a sphere... This video is a good example of what I am trying to do, look at the globe on the right: http://youtu.be/iu_XA9lxnu8?t=7m34s

Does anyone has a simple idea? Or just a feasible idea at all? Thank you, Baptiste