James Webb Space Telescope Discovery
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Webb Findings Support Long-Proposed Process of Planet Formation
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.
Credit: NASA, ESA, CSA, L. Hustak (STScI), A. Banzatti (Texas State University)
Water abundance in GK Tau disk (MIRI emission spectrum)
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.
Credit: NASA, ESA, CSA, J. Olmsted (STScI)
Icy pebble drift (infographic)
This artist’s concept compares two types of typical, planet-forming disks around newborn, Sun-like stars. On the left is a compact disk, and on the right is an extended disk with gaps. Scientists using Webb recently studied four protoplanetary disks—two compact and two extended. The researchers designed their observations to test whether compact planet-forming disks have more water in their inner regions than extended planet-forming disks with gaps. This would happen if ice-covered pebbles in the compact disks drift more efficiently into the close-in regions nearer to the star and deliver large amounts of solids and water to the just-forming, rocky, inner planets.
Current research proposes that large planets may cause rings of increased pressure, where pebbles tend to collect. As the pebbles drift, any time they encounter 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 also could have been a role of Jupiter in our solar system — inhibiting pebbles and water delivery to our small, inner, and relatively water-poor rocky planets.
Credit: STScI
Two Protoplanetary Disks (Artist Concept)