Radiation-pressure mixing of large dust grains in protoplanetary disks

Planet formation: crystals from the dust Protoplanetary disks, the clouds of interstellar gas and dust thought to be the precursors of solar systems, ours included, consist largely of amorphous grains of silicate. Yet the grains found in comets and meteorites (representing the early Solar System), and traced in the spectra of young stars, include large crystalline grains that must have undergone annealing or condensation at temperatures above 1,000 K, despite being surrounded by material that has never experienced such heating. This apparent anomaly has been the subject of much discussion and theorizing. Two papers published in this issue add to the discussion. Ábrahám et al. report mid-infrared features in the outburst spectrum of the young solar-like star EX Lupi that they attribute to crystalline forsterite. These features were not present before EX Lupi's recent outbust, so this may be the first direct observation of the crystal formation process in a celestial object. Annealing by heat from a stellar outburst is a crystal source not previously considered for protoplanetary disks. Dejan Vinković suggests another new mechanism that might produce crystals: infrared light arising from a protoplanetary disk can in theory lift grains bigger than 1 μm out of the inner disk, where they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation-pressure support for a given grain size and solid density.