Although the stability of planetary motion helped Newton to establish the laws of classical mechanics, new research on the positions of the outer planets suggest they are governed by chaos. Norman Murray from the University of Toronto and Matthew Holman from the Harvard-Smithsonian Center for Astrophysics have developed a new analytical technique which reveals two chaotic resonance effects that could move asteroids and comets into the inner solar system. The resonances are caused by subtle gravitational interactions between Jupiter, Saturn, Uranus and Neptune (Science 283 1877).