Frequency of debris disks around solar-type stars:: First results from a Spitzer MIPS survey

We have searched for infrared excesses around a well-defined sample of 69 FGK main-sequence field stars. These stars were selected without regard to their age, metallicity, or any previous detection of IR excess; they have a median age of similar to 4 Gyr. We have detected 70 mu m excesses around seven stars at the 3 sigma confidence level. This extra emission is produced by cool material (< 100 K) located beyond 10 AU, well outside the "habitable zones'' of these systems and consistent with the presence of Kuiper Belt analogs with similar to 100 times more emitting surface area than in our own planetary system. Only one star, HD 69830, shows excess emission at 24 mu m, corresponding to dust with temperatures greater than or similar to 300 K located inside of 1 AU. While debris disks with L-dust /L-* >= 10(-3) are rare around old FGK stars, we find that the disk frequency increases from 2% +/- 2% for L-dust/L-* >= 10(-4) to 12% +/- 5% for L-dust/L-* >= 10(-5). This trend in the disk luminosity distribution is consistent with the estimated dust in our solar system being within an order of magnitude greater or less than the typical level around similar nearby stars. Although there is no correlation of IR excess with metallicity or spectral type, there is a weak correlation with stellar age, with stars younger than a gigayear more likely to have excess emission.