Protoplanetary disk art

NASA’s James Webb Space Telescope has provided evidence supporting the theory that icy pebbles drift inward from the colder parts of protoplanetary disks to form planets, a process now confirmed by the observation of vapor transitions of water.

Drifting pebbles supply water to interior regions of planet-forming disks

How are planets born? Scientists have long proposed that ice-covered pebbles are the seeds of planet formation. These icy solids are thought to drift toward the newborn star from the cold, outer reaches of the disk surrounding it. The theory predicts that when these pebbles entered the hottest region, closer to the star, they would release significant amounts of cold water vapor, bringing both water and solids to the nascent planets.

Now the James Webb Space Telescope witnessed this process in action, revealing the connection between water vapor in the inner disk and drifting icy pebbles from the outer disk. This discovery opens up exciting new perspectives in the study of the formation of rocky planets.

Two protoplanetary disks

This artist’s concept compares two typical types of planet-forming disks around newborn Sun-like stars. On the left is a compact disk and on the right is an extended disk with spaces. 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 contain more water in their internal regions than expanded planet-forming disks with gaps. This would happen if the ice-covered pebbles in the compact discs drifted more efficiently toward regions near the star and delivered large quantities of solids and water to the forming rocky inner planets.
Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

Findings from NASA’s Webb Space Telescope support long-proposed planet formation process

Scientists use NASAThe James Webb Space Telescope has just made a revolutionary discovery by revealing how planets are made. By observing water vapor in protoplanetary disks, Webb confirmed a physical process involving the drift of ice-covered solids from the outer regions of the disk into the area of ​​rocky planets.

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 the pebbles drift toward the star due to friction in the gaseous disk, delivering both solids and water to the planets.

Confirmation of theoretical predictions

A fundamental prediction of this theory is that when icy rocks enter the warmer region of the “snow line” – where ice turns to vapor – they should release large quantities 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 Andrea Banzatti of Texas State University, San Marcos. , in Texas. “This discovery opens up exciting perspectives for the study of the formation of rocky planets with Webb! »

Water abundance (Webb MIRI emission spectrum)

This graph compares spectral data for hot and cold water in the GK Tau disk, which is a compact disk with no rings, and the extended CI Tau disk, which has at least three rings in different orbits. The science team used the unprecedented resolving power of MIRI’s MRS (Medium Resolution Spectrometer) to separate the spectra into individual lines that probe water at different temperatures. These spectra, visible in the top graph, clearly reveal an excess of cold water in the compact disk GK Tau, compared to the large disk CI Tau.
The bottom graph shows the excess cold water data in the GK Tau compact disk at least the cold water data in the extended CI Tau disk. The actual data, in purple, is overlaid on a model spectrum of cold water. Note how well they line up.
Credits: NASA, ESA, CSA, Leah Hustak (STScI), Andrea Banzatti (Texas State University)

“In the past, we had this very static picture of planet formation, almost as if there were isolated areas from which planets formed,” said team member Colette Salyk of Vassar College in Poughkeepsie. , New York. “We now have evidence that these areas can interact with each other. This is also something that would have happened in our solar system.

Harnessing the Power of Webb

The researchers used Webb’s MIRI (Mid-Infrared Instrument) to study four disks – two compact and two extended – around Sun-like stars. These four stars are estimated to be between 2 and 3 million years old, which would only be newborns in cosmic time.

Both CDs should experience effective pebble drift, delivering pebbles a distance equivalent to Neptunethe orbit. In contrast, extended disks should have their pebbles held in several rings up to six times the orbit of Neptune.

Pebble drift infographic

This graph is an interpretation of data from Webb’s MIRI, the mid-infrared instrument, which is sensitive to water vapor present in the disks. It shows the difference between pebble drift and water content of a compact disc versus an expanded disc with rings and spaces. In the compact disc on the left, as the ice-covered pebbles drift toward the hottest, closest region to the star, they are unimpeded. When they cross the snow line, their ice turns to vapor and provides a large quantity of water to enrich the forming rocky interior planets. To the right is an extended disk with rings and spaces. As the ice-covered pebbles begin their journey inward, many of them find themselves stopped by the gaps and trapped in the rings. Fewer icy pebbles are able to cross the snow line to carry water to the inner region of the disc.
Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

Webb’s observations were designed to determine whether compact discs have a higher abundance of water in their rocky interior region, as expected if pebble drift is more efficient and provides plenty of solid mass and water to the inner planets . The team chose to use MIRI’s MRS (Medium Resolution Spectrometer) because it is sensitive to water vapor present in the disks.

The results confirmed expectations by revealing an excess of cold water in compact discs, compared to large discs.

As the pebbles drift, whenever they encounter a pressure bump – an increase in pressure – they tend to accumulate there. These pressure traps do not necessarily stop the drift of stones, but they do hinder it. This is what seems to happen in large disks with rings and spaces.

Current research suggests that large planets could cause rings of increased pressure, in which pebbles tend to accumulate. It could also have been a role of Jupiter in our solar system – inhibiting the delivery of rocks and water to our small, rocky, relatively water-poor interior planets.

Solving mysteries with Webb data

When the data first came in, the results were confusing to the research team. “For two months, we were stuck on these preliminary results that told us that compact discs had colder water, and that large discs had warmer water overall,” Banzatti recalls. “This made no sense, because we had selected a sample of stars with very similar temperatures.”

It was only when Banzatti superimposed the compact disc data on the large disc data that the answer became clear: Compact discs contain very cold water just inside the snow line, about ten times closer than the orbit of Neptune.

“Now we finally see unambiguously that it is the coldest water that has an excess,” Banzatti said. “This is unprecedented and entirely due to Webb’s higher resolving power!”

The team’s results appear in the November 8 edition of Astrophysical journal letters.

Reference: “JWST Reveals Excess Cold Water Near the Snow Line in CDs, Consistent with Cobble Drift” by Andrea Banzatti, Klaus M. Pontoppidan, John S. Carr, Evan Jellison, Ilaria Pascucci, Joan R. Najita, Carlos E. Muñoz-Romero, Karin I. Öberg, Anusha Kalyaan, Paola Pinilla, Sebastiaan Krijt, Feng Long, Michiel Lambrechts, Giovanni Rosotti, Gregory J. Herczeg, Colette Salyk, Ke Zhang, Edwin A. Bergin, Nicholas P. Ballering, Michael R. Meyer and Simon Bruderer, November 8, 2023, Letters from the astrophysical journal.
DOI: 10.3847/2041-8213/acf5ec

The James Webb Space Telescope is the world’s first space science observatory. Webb solves the mysteries of our solar system, looks beyond distant worlds around other stars, and probes 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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