Searching for earth analogs around the nearest stars: The disk age-metallicity relation and the age distribution in the solar neighborhood

The chemical composition of Earth's atmosphere has undergone substantial evolution over the course of its history. It is possible, even likely, that terrestrial planets in other planetary systems have undergone similar changes; consequently, the age distribution of nearby stars is an important consideration in designing surveys for Earth analogs. Valenti & Fischer provide age and metallicity estimates for 1039 FGK dwarfs in the solar neighborhood. Using the Hipparcos catalog as a reference to calibrate potential biases, we have extracted volume-limited samples of nearby stars from the Valenti-Fischer data set. Unlike other recent investigations, our analysis shows clear evidence for an age-metallicity relation in the local disk, albeit with substantial dispersion at any epoch. The mean metallicity increases from similar to -0.3 dex at a look-back time of similar to 10 Gyr to similar to + 0.15 dex at the present day. Supplementing theValenti-Fischer measurements with literature metallicity data to give a complete volume-limited sample, the age distribution of nearby FGK dwarfs is broadly consistent with a uniform star-formation rate over the history of the Galactic disk. In striking contrast, most stars known to have ( gas giant) planetary companions are younger than 5 Gyr; however, stars with planetary companions within 0.4 AU have a significantly flatter age distribution, indicating that those systems are stable on timescales of many gigayears. Several of the older, lower metallicity host stars have enhanced [alpha/Fe] ratios, implying membership of the thick disk. If the frequency of terrestrial planets is also correlated with stellar metallicity, then the median age of such planetary system is likely to be similar to 3 Gyr. We discuss the implications of this hypothesis in designing searches for Earth analogs among the nearby stars.