How to measure the unseen building blocks of planets.

Planets form at the midplane of the protoplanetary disks surrounding newborn stars. Understanding the compositions of forming planets  is one of the key goals connecting protoplanetary disk research with exoplanet characterization and astrobiology. The compositions of forming planets are difficult to assess, as the solid "dust" in the protoplanetary disk midplane is optically thick. However, it has now become possible to probe these solids indirectly through accurate accounting of the dust coming into, and out of, the disk.  In this talk, I will first demonstrate a new method for measuring the dust that is "left behind" in protoplanetary disks (McClure 2019; McClure, Dominik, and Kama 2020). This technique allows us to identify both the global composition of solids in the disk as well as specific locations in the disk where these solids are retained. These are the likely birthplaces of future planets. For the second half of the talk, I will describe a cutting edge Early Release Science (ERS) program, Ice Age (P.I. McClure, >50 team members world wide) with the upcoming James Webb Space Telescope (JWST) to measure the composition of dust entering disks. This ERS program will provide the star and planet formation and astrochemistry communities with a non-proprietary dataset of JWST spectra, laboratory data, and chemical models within the 5 months of JWST's science operations. By combining these direct studies of the icy and rocky dust input into protoplanetary disks with the direct measurement of material moving out of the disk onto the central star, we can map out where solid material is being retained in a given disk, which is a first, critical step to forming planets.