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The chemical composition of Earth's atmosphere has undergone substantialevolution over the course of its history. It is possible, even likely, thatterrestrial planets in other planetary systems have undergone similar changes;consequently, the age distribution of nearby stars is an importantconsideration in designing surveys for Earth-analogues. Valenti & Fischer(2005) provide age and metallicity estimates for 1039 FGK dwarfs in the SolarNeighbourhood. Using the Hipparcos catalogue as a reference to calibratepotential biases, we have extracted volume-limited samples of nearby stars fromthe Valenti-Fischer dataset. Unlike other recent investigations, our analysisshows clear evidence for an age-metallicity relation in the local disk, albeitwith substantial dispersion at any epoch. The mean metallicity increases from-0.3 dex at a lookback time of ~10 Gyrs to +0.15 dex at the present day.Supplementing the Valenti-Fischer measurements with literature data to give acomplete volume-limited sample, the age distribution of nearby FGK dwarfs isbroadly consistent with a uniform star-formation rate over the history of theGalactic disk. In striking contrast, most stars known to have planetarycompanions are younger than 5 Gyrs; however, stars with planetary companionswithin 0.4 AU have a significantly flatter age distribution, indicating thatthose systems are stable on timescales of many Gyrs. Several of the older,lower metallicity host stars have enhanced [alpha/Fe] ratios, implyingmembership of the thick disk. If the frequency of terrestrial planets is alsocorrelated with stellar metallicity, then the median age of such planetarysystem is likely to be ~3 Gyrs. We discuss the implications of this hypothesisin designing searches for Earth analogues among the nearby stars.

Searching for Earth analogues around the nearest stars: the disk age-metallicity relation and the age distribution in the Solar Neighbourhood