Webb Shows Planets Really Do Start with Pebbles

Planet-forming disks — 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.
*NASA / ESA / CSA / Joseph Olmsted (STScI)*

Astronomers have used the James Webb Space Telescope to spot a long-sought piece in the puzzle of rocky planet formation. They’ve discovered evidence that icy pebbles drift toward newborn stars, delivering both water vapor and solid material to the inner parts of new solar systems.

Water is thought to play a huge role in making a terrestrial planet like Earth habitable, but where does it come from? Planets form from the detritus of star formation, condensing out of swirling protoplanetary disks of gas and dust. Except, any water originally in the inner part of the disk would soon be evaporated by the ferocity of the fledgling star.

Now, a team led by Andrea Banzatti (Texas State University) has used Webb to explore four such disks around stars in the constellation of Taurus, each of which is just a few million years old. Two of the disks are extremely compact (just 10 to 20 astronomical units, or a.u., across), while two are considerably bigger (100 to 150 a.u.). There is greater friction within more compact disks, which causes distant, icy pebbles to lose their orbital energy and drift inward. The larger disks, on the other hand, have rings and gaps, which can impede such pebble drift.

Once a drifting pebble crosses a region known as the snow line, the temperature rises to the point where the outer veneer of ice sublimates into water vapor. This process leaves behind the bare, rocky material from which terrestrial planets then form. By using the Medium-Resolution Spectrometer on Webb’s Mid-Infrared Instrument, Banzatti’s team found an excess of cool water around the snow line in the two compact disks, compared with the larger ones. The result appears in the Astrophysical Journal Letters.

Two spectra comparing water abundances in different disk systems — This graphic compares the spectral data for warm and cool water in the GK Tau disk, which is a compact disk without rings, and the more extended CI Tau disk, which has at least three rings on different orbits. The two spectra, both shown in the top graph, clearly reveal excess cool water in the compact GK Tau disk, compared with the larger CI Tau disk. The bottom graph shows the *difference* in excess cool water, by showing the compact GK Tau disk spectrum minus the spectrum of the extended CI Tau disk. The actual data, in purple, closely match a model spectrum of cool water, shown in blue.
*NASA / ESA / CSA / Leah Hustak (STScI)*

“Webb finally revealed the connection between water vapor in the inner disk and the drift of icy pebbles from the outer disk,” Banzatti says.

“In the past, we had this very static picture of planet formation, almost like there were these isolated zones that planets formed out of,” says team member Colette Salyk (Vassar College). “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.”

“[Webb] is making a massive difference to this kind of study,” says Thomas Haworth (Queen Mary University of London), who was not involved in the research. “With facilities like ALMA we can study the structure and composition of the colder, outer disk, but JWST is now enabling us to study the composition of the inner disk.”

Pebble drift diagram — This graphic is an interpretation of data from Webb’s Mid-Infrared Instrument, which is sensitive to water vapor in disks. It shows the difference between pebble drift and water content in a compact disk *(left)* versus an extended disk with rings and gaps *(right)*. In the compact disk, ice-covered pebbles drift inward unimpeded toward the warmer region closer to the star. As they cross the snow line, their ice turns to vapor and provides a large amount of water to enrich the rocky planets forming there. 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.
*NASA / ESA / CSA / Joseph Olmsted (STScI)*

It’s still early days, though. The four disks studied so far were chosen so as to be at opposite ends of the size scale. “It could potentially take only one more observation of another disk of some mid-size to get a result that would not fit the beautiful correlations seen for the first four disks,” says Jane Greaves (Cardiff University, UK), who was also not involved in the research.

We won’t have to wait too long for those data, as this quartet of disks is just the beginning of the planned observations with Webb. Perhaps then we’ll have a better idea of the link between icy pebbles and habitable worlds, and in turn, we’ll know the story of how we came to be here a little better.

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Webb Shows Planets Really Do Start with Pebbles

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