No, you don’t particularly need to use Cycles to do this …
The “glow” behind the sun-cube could simply be a fuzzy blob on a plane positioned behind the cube. (Why not?) The so-called “billboard” technique. A spotlight positioned at the cube, in-line with the camera (yes, you can put a lamp directly in front of the camera and it won’t be “seen”), can be used to generate any “glow” on the sun-cube itself. (No light need fall on the billboard plane, [I]unless that’s how you’d like to create the “fuzzy glow” on that plane.)
(Even if you “can plainly see” the billboard-plane when setting up the comp, you will only use the channel of information that represents reflected light from the plane when you wire-up the “noodles.” Or, you can just spray-paint the thing, and be done.)
The orbiting planet-cubes can be differentiated from the sun-cube by the simple expedient of Z-depth: how far away they are from the camera.
If you’d like to make the orbiting cubes appear to reflect light from the sun-cube, simply put a light source in their render-layer, where the “glowing” cube is. Since the result will be visually consistent with the notion that “this cube is glowing … this cube is the source of light,” that’s how the audience will interpret the scene.
For maximum post-production flexibility, you might separate the “light that shines from the sun onto the planets” from the basic illumination of those planets, creating a layer of data which consists only of that added (yellow?) light. Having separated it, you can now adjust its relative strength, tint it various colors, and so on. All without re-rendering anything.