Debris disks provide exciting opportunities to study planet formation through the composition and architecture of planetary building blocks. The Spitzer Space Telescope Infrared Spectrograph (IRS)’s comprehensive survey of more than 500 debris disks enables us to study in detail the composition of dust grains in both the terrestrial temperature zone and the exo-Kuiper belts. These mid-infrared spectra provide direct evidence for planetesimal formation processes (such as differentiation), opening doors to new questions on small planet formation: Do planetesimal compositions in exoplanetary systems display a spatial pattern similar to our solar system? Are extrasolar planets assembled from differentiated planetesimals, similar to the solar system asteroids, or from undifferentiated materials? I begin with an archetypal debris disk, beta Pictoris, to showcase the capability to constrain planetesimal properties as a function of their blackbody temperature, a proxy for stellocentric distance. Using the Spitzer IRS mid-IR spectrum, I discover trends in beta Pic also strikingly present in our solar system: the surfaces of large planetesimals are more Fe-rich and processed closer to the star and become more Fe-poor and primordial farther from the star. Expanding on this work, I investigate the commonality and timescale of the crust-mantle formation of extrasolar, small Earth-like planets. I analyze the most comprehensive debris disk catalog based on Spitzer and Gaia DR3 data. I identify a group of debris disks exhibiting spectroscopic evidence for hematite, an indicator of water present in differentiated terrestrial planets and solar system asteroids. I will discuss the implications of my findings for the formation of small planets and observational prospects with JWST.