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Five planets are presently believed to orbit the primary star of 55 Cnc, butthere exists a large 5 AU gap in their distribution between the two outermostplanets. This gap has attracted considerable interest because it may containone or more lower--mass planets whose existence is not contradicted bylong-term orbit stability analyses, in fact it is expected according to the"packed planetary systems" hypothesis and an empirical Titius--Bode relationrecently proposed for 55 Cnc. Furthermore, the second largest planet is justthe second farthest and it orbits very close to the star. Its orbit, the mostcircular of all, appears to be nearly but not quite commensurable with theorbit of the third planet, casting doubt that any migration or resonant captureof the inner planets has ever occurred and lending support to the idea of"in--situ" giant planet formation by the process of core accretion. All of theabove ideas will be tested in the coming years in this natural laboratory asmore observations will become available. This opportunity presents itself inconjunction with a physical model that relates the orbits of the observedplanets to the structure of the original protoplanetary disk that harboredtheir formation at the early stages of protostellar collapse. Using only the 5observed planets of 55 Cnc, this model predicts that the surface densityprofile of its protoplanetary disk varied with distance $R$ precisely as$\Sigma (R)\propto R^{-3/2}$, as was also found for the minimum--mass solarnebula. Despite this similarity, the disk of 55 Cnc was smaller, heavier, andless rotationally supported than the solar nebula, so this system represents adifferent mode of multi--planet formation compared to our own solar system.

55 Cancri: A Laboratory for Testing Numerous Conjectures about Planet Formation