Hundreds of tiny worlds circling beyond Neptune have been thought of as frozen in time, almost unchanged since they formed at the birth of the solar system five billion years ago. A discovery now has some astronomers entertaining visions of icy volcanoes roiling on some of them, at least at one time. David J. Stevenson, a professor of planetary science at the California Institute of Technology, speculates that the decay of radioactive elements within some of these worlds, known as Kuiper Belt Objects, would produce enough warmth to melt a mixture of water ice and ammonia, an antifreeze. The liquid could then rise up through cracks and erupt onto the surface.