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Smear a little on the thread of a screw-in ceiling spotlight bulb to make it much easier to replace when the time comes.
If you want a huge planet, try a children's toy space hopper instead of a balloon. They are around 50cm diameter. It might be a good idea to place a piece of string threaded through a piece of stiff card on to the surface of the hopper before applying papier mache so as to have something to hang it up by.