James Webb telescope findings support long-proposed process of planet formation
Jayme Blaschke | November 8, 2023
A team of researchers using NASA’s James Webb Space Telescope led by Andrea Banzatti, an assistant professor in the Department of Physics at Texas State University, has made a breakthrough discovery in revealing how planets are formed. By observing water vapor in protoplanetary disks, the researchers confirmed a physical process that provides a highly dynamic picture of planet formation: the drift of icy solids from beyond the orbit of Neptune into the rocky planet zone within a few astronomical units, the region of terrestrial planets in Earth’s solar system.
The findings, “JWST reveals excess cool water near the snowline in compact disks, consistent with pebble drift,” are published in the Astrophysical Journal Letters.
Theories of planet formation 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 process is that as icy pebbles enter into the warmer region within the "snowline"—where ice transitions to vapor—they should release large amounts of cold water vapor. This is exactly what Webb observed.
“Webb finally revealed the connection between water vapor in the inner disk and the drift of icy pebbles from the outer disk,” said principal investigator Banzatti. “This finding opens up exciting prospects for studying rocky planet formation with Webb.”
“In the past, we had this very static picture of planet formation, almost like there were these isolated zones that planets formed out of,” explained team member Colette Salyk of Vassar College in Poughkeepsie, New York. “Now we actually have evidence that these zones can interact with each other. It's also something that is proposed to have happened in our solar system.”
Harnessing the Power of Webb
The researchers used Webb’s MIRI (the Mid-Infrared Instrument) to study four disks—two compact and two extended—around sun-like stars. All four of these stars are estimated to be between 2 and 3 million years old — just newborns in cosmic time.
The two compact disks are expected to experience efficient pebble drift, delivering pebbles to well within a distance equivalent to Neptune’s orbit. In contrast, the extended disks are expected to have their pebbles retained in multiple rings as far our as six times the orbit of Neptune.
The 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. The team chose to use MIRI’s MRS (the Medium-Resolution Spectrometer) because it is sensitive to water vapor 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.
Current research proposes that large planets may cause rings of increased pressure, where pebbles tend to collect. This also could have been a role of Jupiter in this solar system—inhibiting pebbles and water delivery to the small, inner and relatively water-poor rocky planets.
Solving the Riddle
When the data first came in, the results were puzzling to the research team.
“For two months, we were stuck on preliminary results that were telling us that the compact disks had colder water, and the large disks had hotter water overall,” remembered Banzatti. “This made no sense, because we had selected a sample of stars with very similar temperatures.”
Only when Banzatti overlaid the data from the compact disks onto the data from the large disks did the answer clearly emerge: The compact disks have extra cool water just inside the snowline, at about 10 times closer than the orbit of Neptune.
“Now we finally see unambiguously that it is the colder water that has an excess,” said Banzatti. “This is unprecedented and entirely due to Webb’s higher resolving power!”
The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.
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For more information, contact University Communications:Jayme Blaschke, 512-245-2555 Sandy Pantlik, 512-245-2922 |