Water abundance in GK Tau disk (MIRI emission spectrum)

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 compares the spectral data for warm and cool water in the GK Tau disk, which is a compact disk without rings, and extended CI Tau disk, which has at least three rings on different orbits. The science team employed the unprecedented resolving power of MIRI’s MRS (the Medium-Resolution Spectrometer) to separate the spectra into individual lines that probe water at different temperatures. These spectra, seen in the top graph, clearly reveal excess cool water in the compact GK Tau disk, compared with the large CI Tau disk. 

The bottom graph shows the excess cool water data in the compact GK Tau disk minus the cool water data in the extended CI Tau disk. The actual data, in purple, are overlaid on a model spectrum of cool water. Note how closely they align.

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

[Image description: The top graph compares the spectral data for warm and cool water in the compact GK Tau disk with the extended CI Tau disk. The bottom graph shows the excess cool water data in the compact GK Tau disk minus the cool water data in the extended CI Tau disk. View description for details.]