Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
The host stars of extrasolar planets (ESPs) tend to be metal rich. We have examined other properties of these stars in search of systematic trends that might distinguish exoplanet hosts from the hoi polloi of the Galactic disk; we find no evidence for such trends among the present sample. The a-element abundance ratios show that several ESP hosts are likely to be members of the thick disk population, indicating that planet formation has occurred throughout the full lifetime of the Galactic disk. We briefly consider the radial metallicity gradient and age-metallicity relation of the Galactic disk, and complete a back-of-the-envelope estimate of the likely number of solar-type stars with planetary companions with 6 < R < 10 kpc.
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