"Mytilene," he told them, is a pleasure world. Or, rather, I should say worlds. You can fly from any planet to any other in the atmosphere.
"The whole ball, which is much larger than Jupiter, is held together by electro-magnetic forces created and maintained by the machines and the robot-brains on the centre planet - which spins of itself, but does not move within the gaseous envelope..."
First published in book form in 1960, 'The Fatal Fire' was previously serialised in 'New Worlds'. The story is a routine SF thriller by an accomplished writer, with highly inventive plot and background, though somewhat spoilt by its rushed ending suggestive of wrapping the tale up to meet a story-length criteria perhaps.