The solubility of rare gases in a simulated model of fused silica is evaluated by the test particle method. It is shown that the order of magnitude of the rare gas solubility in liquid silica is mainly governed by the entropy of cavity formation and only marginally by the solvation energy of the solute in the melt. Hence, the hierarchy of solubilities is dictated by the size of the noble gases : The smaller the atom the higher the solubility. Moreover, the solubility exhibits only a moderate temperature dependence at fixed density. In silica glass the same solubility hierarchy is found (in agreement with the experimental data) although the energetic contribution to the absorption process may become significant, particularly when the temperature decreases. These results in silica are in strong contrast with those obtained in liquid water where the energetic contribution to the rare gas solubility is large enough to overcompensate the entropic loss of cavity formation, the net result being that light noble gases (e.g., He) are less soluble than heavier ones (e.g., Xe). These contrasting behaviors are explained by pointing out that the liquid phase occurs in very different temperature ranges for these two systems (SiO2 and H2O) while the rare gas-solvent interaction energy is essentially of the same order of magnitude. Finally, the structure of silica around helium and neon atoms is found to be interstitial (cristobalitelike) while in the case of heavier rare gases, the structure of cavities is more reminiscent of the polyhedral arrangement encountered with clathrasils.