Science: Tau Ceti, at 12 light-years away, is the closest single star system with a temperature and luminosity similar to those of the Sun. From an analysis of more than 6000 observations, Mikko Tuomi of the University of Hertfordshire in the UK and his colleagues have determined that there may be as many as five planets orbiting Tau Ceti. Slight changes in the star’s motion through space indicate that all five have masses between 2 and 7 times that of Earth, and all are located closer to their star than Mars is to our Sun. The fourth planet from Tau Ceti orbits at a distance similar to that of Venus. Because Tau Ceti emits only 45% as much light as the Sun, this puts the fourth planet in the habitable zone—the region of space where water can exist in liquid form. Because of the small size of the variations in Tau Ceti’s motion, confirmation that the variations are caused by orbiting planets—rather than disturbances on the star itself—will require an improvement in our ability to identify low-mass planets.