Circumstellar and debris disks are the most visible signposts of other planetary systems, representing indirect evidence of planet formation. We will present some new exciting results from the Spitzer Space Telescope regarding the evolution of debris disks around normal stars.

In the first part of the talk, Murray Silverstone will focus on a survey of 74 young (3-30 Myr) solar-mass stars using Spitzer/IRAC 3-8 micron photometry to search for excess emission above the photosphere. The results show that optically thick dust inside of 1 AU is cleared by 30 Myr from such systems, confirming results of ground-based near- and mid-IR studies. Ongoing analyses of the Spitzer/IRS spectra of the systems with IRAC excesses show evidence of dust grain evolution, particularly the growth of amorphous silicate grains, and suggest increasing crystalline silicate composition in these disks.

In the second part of the talk, Kate Su will present recent studies of infrared excesses at 24/25 um, based on a sample of 266 A-type (~2.5 solar mass) stars observed with IRAS and Spitzer. The behavior of infrared excesses qualitatively agrees with previous studies, suggesting a decline of debris disk activity with stellar age. However, there is a large variety of excess amounts at any given age. These new results show both a large range in initial planetary system structures and also that individual disks may be dominated by debris created in individual events. Detailed modeling of two systems, Vega and HD 69830, shows that short-lived small dust particles dominate the radiometric properties of the disk. These two systems are most likely the sites of major debris-producing incidents that occurred within roughly the past million years.

In the third part of the talk, Serena Kim will present the discovery of Kuiper Belt (KB)-like disks of Sun-like systems based upon Spitzer observations. We find that most of these systems are ~1-3 billion years old. Based on spectral energy distributions and modeling analysis, we suggest that these KB systems are cold (50 - 60 K), and represent materials generated by collision of planetesimals at similar locations (20 - 50 AU away from the central star) as our KB. We speculate on the nature of these systems through comparisons to our own KB, and on the likely perturber responsible for stirring the system and ultimately releasing dust through collision.

Refreshments will be served in the Steward Observatory lobby at 3:30pm.