Icy pebble drift (infographic)

Scientists using the NASA/ESA/CSA James Webb Space Telescope made a breakthrough discovery in revealing how planets are made. By observing water vapour in protoplanetary disks, Webb confirmed a physical process involving the drifting of ice-coated solids from the outer regions of the disk into the rocky-planet zone.

Theories have long proposed that icy pebbles forming in the cold, outer regions of protoplanetary disks — the same area where comets originate in our solar system — should be the fundamental seeds of planet formation. The main requirement of these theories is that pebbles should drift inward toward the star due to friction in the gaseous disk, delivering both solids and water to planets. A fundamental prediction of this theory is that as icy pebbles enter into the warmer region within the "snowline" — where ice transitions to vapour — they should release large amounts of cold water vapour. This is exactly what Webb observed.

Webb observations were designed to determine whether compact disks have a higher water abundance in their inner, rocky planet region, as expected if pebble drift is more efficient and is delivering lots of solid mass and water to inner planets. Researchers chose to use MIRI’s MRS (the Medium-Resolution Spectrometer) because it is sensitive to water vapour in disks. The results confirmed expectations by revealing excess cool water in the compact disks, compared with the large disks. As the pebbles drift, any time they encounter a pressure bump — an increase in pressure — they tend to collect there. These pressure traps don’t necessarily shut down pebble drift, but they do impede it. This is what appears to be happening in the large disks with rings and gaps.

This graphic is an interpretation of data from Webb’s MIRI, the Mid-Infrared Instrument, which is sensitive to water vapour in disks. It shows the difference between pebble drift and water content in a compact disk versus an extended disk with rings and gaps. In the compact disk on the left, as the ice-covered pebbles drift inward toward the warmer region closer to the star, they are unimpeded. As they cross the snow line, their ice turns to vapour and provides a large amount of water to enrich the just-forming, rocky, inner planets. On the right is an extended disk with rings and gaps. As the ice-covered pebbles begin their journey inward, many become stopped by the gaps and trapped in the rings. Fewer icy pebbles are able to make it across the snow line to deliver water to the inner region of the disk.

These results appear in the 8 November 2023 edition of the Astrophysical Journal Letters.

[Image description: This infographic compares the structure of a compact protoplanetary disk on the left to the structure of an extended protoplanetary disk on the right. The disks are angled slightly toward the viewer such that the top as well as the interior are visible. Both disks are circular, composed of a bright glowing yellow orb in the centre, surrounded by concentric red rings. The compact disk looks continuous, with no obvious gaps between rings of red material. The extended disk is significantly wider and thicker than the compact disk.]

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