Below is a method of achieving a volumetric look without using actual volumetrics. It is a look achieved by a shading trick. It is in fact an old trick that was used in old good Bryce 3D. It is quite efficent though. I am putting below some result images -rendered in Cycles- and a screenshot of the related node arrangement.
Btw, in the images below are used two sphere objects, the one inside the other. It is a way of endowing the result with more realism.