Prospects for Habitable "Earths" in Known Exoplanetary Systems

We have examined whether putative Earth-mass planets could remain confined to the habitable zones (HZs) of the 111 exoplanetary systems confirmed by 2004 August. We find that in about half of these systems there could be confinement for at least the past 1000 Myr, though in some cases only in variously restricted regions of the HZ. The HZ migrates outward during the main-sequence lifetime, and we find that in about two-thirds of the systems an Earth-mass planet could be confined to the HZ for at least 1000 Myr sometime during the main-sequence lifetime. Clearly, these systems should be high on the target list for exploration for terrestrial planets. We have reached our conclusions by detailed investigations of seven systems, which has resulted in an estimate of the distance from the giant planet within which orbital stability is unlikely for an Earth-mass planet. This distance is given by nR_H, where R_H is the Hill radius of the giant planet and n is a multiplier that depends on the giant's orbital eccentricity and on whether the Earth-mass planet is interior or exterior to the giant planet. We have estimated n for each of the seven systems by launching Earth-mass planets in various orbits and following their fate with a hybrid orbital integrator. We have then evaluated the habitability of the other exoplanetary systems using nR_H derived from the giant's orbital eccentricity without carrying out time-consuming orbital integrations. A stellar evolution model has been used to obtain the HZs throughout the main-sequence lifetime.