Unexpected discoveries in protoplanetary disks observed with the James Webb Space Telescope

Newly formed stars are accompanied by massive disks of gas and dust in which planets for in a few million years (or less).

The dust particles they contain, which form the building blocks of future planets, asteroids and comets, range in size from sub-micron, as found in the interstellar medium, to mm-sized pebbles and larger. To build ever larger solid particles, models indicate that it is crucial to strongly concentrate the dust in small volumes, which is likely achieved in disks through the effect of the drag force dust grains experience when traveling through the gas component. One consequence of these forces is to concentrate the larger dust grains into a very thin layer in the midplane of the disks whereas smaller grains, which are strongly coupled to the gas, remain distributed through the full vertical extent of the disks. Testing this prediction is most readily achieved by imaging disks that are seen “on the edge”, as their vertical structure is readily apparent. Considering the characteristic size of protoplanetary, this is possible by combining the exquisite resolution of some of the most powerful observatories: the Hubble Space Telescope (HST), the Atacama Large Millimeter Array (ALMA) and, most recently, the James Webb Space Telescope (JWST).

During the first year of operation of JWST, A team led by scientists at the Institut de planétologie et d’Astrophysique de Grenoble (IPAG-OSUG, CNRS/UGA) has obtained images of 4 large protoplanetary disks, which had already been imaged with HST and ALMA. These images have confirmed that the vertical stratification of dust grains in present in all cases, although not necessarily with the same amount of coupling. In particular, in some systems, grains as large as 10 micron appear lofted all the way up to the surface of the disk, which may indicate that grains are particularly fluffy, as is often seen in cometary dust in our own Solar System. Yet, in other systems, similarly-sized grains are already subject to significant settling. Beyond testing theories of planet formation, these observations also brought to light a number of unexpected features, such as a bright X-shape feature in one system or a large fluffy cocoon surrounding another one. In both cases, these structures extend significantly beyond the outer reaches of the disks, as traced by their dust and gas components, suggesting the presence of winds that could contribute to the dissipation of the disk and that had only been surmised in unresolved images.

Like all new groundbreaking astronomical observatory, the unprecedented resolution and sensitivity of JWST not only achieves, or even surpasses, the original goals it was designed for, but it produces totally unexpected results that will open new areas of research.

This work benefitted from funding from the European Research Council (ERC) under the European Union’s Horizon Europe research and innovation program (project Dust2Planets, PI F. Ménard).

This article was initially published by UGA.