Prospects for ‘Earths’ in the Habitable Zones of Known Exoplanetary Systems Available to Purchase

We have shown that Earth‐mass planets could survive in variously restricted regions of the habitable zones (HZs) of most of a sample of nine of the 104 main‐sequence exoplanetary systems confirmed by mid‐November 2003. In a preliminary extrapolation of our results to the other systems, we estimate that roughly a half of these systems could have had an Earth‐mass planet confined to the HZ for at least the most recent 1000 Ma. The HZ migrates outwards during the main‐sequence lifetime, and so this proportion varies with stellar age — about two thirds of the systems could have such a planet confined to the HZ for at least 1000 Ma at sometime during the main‐sequence lifetime. Clearly, these systems should be targeted for exploration for terrestrial planets. We have reached this conclusion by launching putative Earth‐mass planets in various orbits and following their fate with mixed‐variable symplectic and hybrid integrators. Whether the Earth‐mass planets could form in the HZs of the exoplanetary systems is an urgent question that needs further study.