At present, because of observational selection effects, we know of no exoplanetary systems with any planetary masses close to that of the Earth. We have therefore used computer models to see whether such planets could be dynamically stable in the presence of the more massive planets known to be present, and in particular whether 'Earths' could remain confined to the classical habitable zone (HZ) for long enough for life to have emerged.Measured stellar properties have been used to determine for each system the present location of the classical habitable zone (HZ). We have also determined the critical distances from the orbit of each 14/6/06 5:41 PM BST

We present the results of detailed dynamical simulations of the effect of the migration of the four giant planets on both the transport of pre‐formed Neptune Trojans and the capture of new Trojans from a trans‐Neptunian disc. The cloud of pre‐formed Trojans consisted of thousands of massless particles placed on dynamically cold orbits around Neptune's L4 and L5 Lagrange points, while the trans‐Neptunian disc contained tens of thousands of such particles spread on dynamically cold orbits between the initial and final locations of Neptune. Through the comparison of the results with the previous work on the known Neptunian Trojans, we find that scenarios involving the slow migration of Neptune over a large distance (50 Myr to migrate from 18.1 au to its current location, using an exponential‐folding time of τ= 10 Myr) provide the best match to the properties of the known Trojans. Scenarios with faster migration (5 Myr, with τ= 1 Myr), and those in which Neptune migrates from 23.1 au to its current location, fail to adequately reproduce the current‐day Trojan population. Scenarios which avoid disruptive perturbation events between Uranus and Neptune fail to yield any significant excitation of pre‐formed Trojans (transported with efficiencies between 30 and 98 per cent whilst maintaining the dynamically cold nature of these objects –e < 0.1, i < 5°). Conversely, scenarios with periods of strong Uranus–Neptune perturbation lead to the almost complete loss of such pre‐formed objects. In these cases, a small fraction (∼0.15 per cent) of these escaped objects are later recaptured as Trojans prior to the end of migration, with a wide range of eccentricities (<0.35) and inclinations (<40°). In all scenarios (including those with such disruptive interaction between Uranus and Neptune), the capture of objects from the trans‐Neptunian disc (through which Neptune migrates) is achieved with efficiencies between ∼0.1 and ∼1 per cent. The captured Trojans display a wide range of inclinations (<40° for slow migration, and <20° for rapid migration) and eccentricities (<0.35), and we conclude that, given the vast amount of material which undoubtedly formed beyond the orbit of Neptune, such captured objects may be sufficient to explain the entire Neptune Trojan population.

A stellar evolution computer model has been used to determine changes in the luminosity L and effective temperature T e of single stars during their time on the main sequence. The range of stellar masses investigated was from 0.5 to 1.5 times that of the Sun, each with a mass fraction of metals (metallicity, Z) from 0.008 to 0.05. The extent of each star's habitable zone (HZ) has been determined from its values of L and T e . These stars form a reference framework for other main sequence stars. All of the 104 main sequence stars known to have one or more giant planets have been matched to their nearest stellar counterpart in the framework, in terms of mass and metallicity, hence closely approximating their HZ limits. The limits of HZ, for each of these stars, have been compared to its giant planet(s)'s range of strong gravitational influence. This allows a quick assessment as to whether Earth-mass planets could exist in stable orbits within the HZ of such systems, both presently and at any time during the star's main sequence lifetime. A determination can also be made as to the possible existence of life-bearing satellites of giant planets, which orbit within HZs. Results show that about half of the 104 known extrasolar planetary systems could possibly have been housing an Earth-mass planet in HZs during at least the past billion years, and about three-quarters of the 104 could do so for at least a billion years at some time during their main sequence lives. Whether such Earth-mass planets could have formed is an urgent question now being investigated by others, with encouraging results.The detection of Earth-mass extrasolar planets is, at the moment, beyond most current technology. This is shortly due to be redressed, however, with the launch of planet detecting satellites such as the transit-detecting Kepler mission (2008) and the direct imaging missions Darwin (ESA) and TPF (NASA) (2015), plus imaging by extremely large 30-100 m telescopes with high performance adaptive optics (e.g. ESO's OWL). We can, meanwhile, use computer models to predict in which of the currently known exoplanetary systems Earth-mass planets could exist, where 119 extrasolar planets of the order of Jupiter's mass have been found in orbit around 104 stars (catalogued by Jean Schneider at http://www.obspm.fr/encycl/encycl.html). Existence requires that the giant planets would allow terrestrial bodies to remain in stable orbits around their

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Abstract.We have investigated whether terrestrial planets can exist in orbits in known exoplanetary systems such that life could have emerged on those planets. Four contrasting systems have been examined in which giant planets have been detected. Mixed-variable symplectic numerical integration has been used to investigate the orbits of putative terrestrial planets within the habitable zone of each system (the range of distances from the star within which water at the surface of a terrestrial planet would be in the liquid phase). We have shown that Rho CrB and 47 UMa could have terrestrial planets in orbits that remain confined to their habitable zones for biologically significant lengths of time. We have also shown that the Gliese 876 and Ups And systems are very unlikely to have such orbits.