|Studies of extrasolar systems offer astronomers increasing opportunity to test their ideas about planet formation. The three innermost planets of star 55 Cancri, for example, have orbits smaller than that of Mercury. These three planets are separated from a much more massive world by a region of apparent stability, which is predicted to harbor another planet. This region encompasses the star´ habitable zone, where surface temperatures would allow a suitable planet to support liquid water. The number above each planet in the bottom panel shows its minimum mass, expressed in Earth masses.