The giant planets are still contracting and converting their gravitational potential energy into thermal energy today as they did when they first formed, but they are doing it more slowly. The annual amount of contraction necessary to sustain their internal temperature is only a tiny fraction of their radius. For Jupiter, this is only 1 millimeter (mm) or so—a hundred-billionth of its radius—per year. If this same rate were to continue for the next billion years, Jupiter would shrink by only 1,000 km over that time, a little more than 1 percent of its radius. In addition, in Saturn’s case, and perhaps Jupiter’s as well, under the right conditions, liquid helium separates from a hydrogen-helium mixture and rains downward toward the core. As the droplets of liquid helium sink, they release their gravitational potential energy as thermal energy. Planetary physicists think that most of Saturn’s internal energy and perhaps some of Jupiter’s come from this separation of liquid helium.