The known solar system has more than doubled in size since 1992. For the first time in almost two centuries an entirely new population of planetary objects has been found. This 'Kuiper Belt' of minor planets beyond Neptune has revolutionised our understanding of how the solar system was formed and has finally explained the origin of the enigmatic outer planet Pluto. This is the story of how theoretical physicists decided that there must be a population of unknown bodies beyond Neptune and how a small band of astronomers set out to find them. Written by a scientist active in the field, in close collaboration with others involved in the work. A modern story of scientific discovery, showing how theory and practice interact. Clearly explains the interest and importance of these newly found solar system members.