Heidelberg Joint Astronomical Colloquium

Earth-like planets and the solid cores of gaseous planets like Jupiter are formed in circumstellar disks by the coalescence of solids (grains) that were originally mixed with gas in the parental molecular cloud. This process involves growth over 13 order of magnitude in size and more than 40 orders of magnitude in mass, from the sub-micron grains in the interstellar medium to Earth. The physical and chemical processes that occur at different times in this path are complex and not well understood. In this talk, I will review the more important processes that take place in circumstellar disks, including grain settling, coalescence and fragmentation. I will then discuss the observational evidence we have of grain growth in disks, and the challenge it poses to the planet formation theories.