Dyson Sphere; Planet

If you poke around you will find some criticism of the idea of Dyson Sphere, such as rising levels of heat and other radiation trapped within the sphere and the lack of enough material in a solar system to build one and so on.<br /><br />What if you scaled it down to just be a planet that was encased in a dyson sphere? It would reduce the complexity of the structure by several orders of magnitude. <br /><br />Of course this would beg the question of why would you enclose a world with a dyson sphere? Things like the sarcoffagus enclosing Chernoble come to mind. Some vast danger that the only answer by an advance race was to shut it up. <br /><br />Solar radiation is intercepted so is the world deathly cold or does the natural furnace at its core produce enough heat that the dyson shell acts as a green house? Naturally there would be defenses that would try to prevent the PCs from entering or openning the sphere. There might be drifting hulks of the ships that were the interdiction fleet while the dyson sphere was being built.<br /><br />The danger posed by whats within would have to be on a grand scale to warrent this effort.