I had a marvelous image: a synthetic planet in the Kuiper Belt–deep deep space, far beyond the orbit of Neptune–made solely from water. We let buoyant fusion reactors drift in its depths, fusing deuterium extracted from all that water to provide the light and heat needed to keep the planet liquid. (All that water makes a great neutron absorber). With a large enough volume, it would have a surface gravity close to Earth’s, or at least, strong enough to keep an oxygenated atmosphere from escaping to deep space. Imagine cities floating on the surface, with permanent night above and deeply lit ocean below, with enough cheap energy to fuel a civilization of a billion people, ten times richer than the modern day US, for ten million years….