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Project Progress

In the last year, Haghighipour completed a systematic study of the formation of habitable planets in the habitable zone of binary-planetary systems. The motivation behind this study comes from the fact that among more than 230 extrasolar planets discovered to-date, approximately 25% are within binary star systems. Among these planetary systems, the two binaries of γ Cephei and GJ 86 are unique in the sense that their separations are smaller than 20 AU, and their primary stars host giant planets. Haghighipour’s study focused on understanding whether such binary-planetary system can harbor habitable planets. In collaboration with Sean Raymond from NAI/Colorado, Haghighipour simulated the interactions of several hundred Moon- to Mars-sized objects, in a region between the giant planet and the primary of a binary system, and for different values of the mass and orbital parameters of the binary. Results indicate that, binary-planetary systems can form and harbor Earth-like objects in their habitable zones, however, the efficiency of these processes are strongly affected by the eccentricity of the binary. As shown in figure 1, in binaries with high orbital eccentricities, the interaction of the secondary star with planet-forming material is strong and results in the ejection of water-carrying objects. As a result, in such systems, final terrestrial planets are mostly dry and un-habitable. However, binaries with lower eccentricity and separations between 20 to 40 AU show a more hospitable environment for the formation and long-term stability of Earth-like planets around their primary stars. Figure 2 show this more clearly. As shown here, binary systems in which the giant planet maintains its orbit for a long time (i.e., systems below the dashed line) are more favorable to formation of habitable planets. The results of this study are in press and will appear in the September 2007 issue of the Astrophysical Journal.

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