JWST Shows Ice-Covered Pebbles Delivering Water to New Planets

The JWST has delivered a breakthrough in planetary science. Its observations show that a long-proposed theory of planet formation is true. Up until now, thick veils of dust in young solar systems have obscured the evidence.

But the JWST saw through it all, and now we know the truth: Ice-covered pebbles from outer solar systems deliver water to still-forming planets closer to their stars.

The debate over Earth’s water has raged for decades. Somehow, Earth ended up with oceans of water, but it’s not clear where it came from.

Earth formed about 4.5 billion years ago in the inner Solar System. It formed inside the rotating disk of gas and dust called the protoplanetary disk. These disks form around young stars throughout the galaxy. The most persistent idea is that the water came from elsewhere in the protoplanetary disk and not from Earth’s vicinity in the inner Solar System. The most well-known hypothesis is that Earth received its water from comets and asteroids after it formed and cooled.

The protoplanetary disks around young stars are still a bit of a mystery. We can see them around other stars in the galaxy, and we can even see the gaps traced in the disk by young, still-forming planets. But seeing inside these disks is tough. One of the reasons the JWST was built was to probe these disks better than any other telescope.

“This finding opens up exciting prospects for studying rocky planet formation with Webb!”

Andrea Banzatti, Texas State University

Protoplanetary disks are a very active area of research because there’s so much we don’t know about how solar systems and planets form. But the origin of Earth’s water is also a hot topic, and the two are linked. The idea that our planet’s water came from objects further out in the Solar System has staying power. The inner Solar System was too hot for water to stick around, the hypothesis goes. But further away, beyond the frost line, frozen water was taken up by comets and asteroids.

These are ALMA‘s high-resolution images of nearby protoplanetary disks, which come from the Disk Substructures at High Angular Resolution Project (DSHARP). The gaps and rings are where planets are forming. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

The early Solar System was a chaotic place, and asteroids, comets, and even icy planetesimals from the outer Solar System continually rained down on the Earth. Over time, water accumulated and formed the oceans, according to this explanation. Now we have an Earth rich in water.

But there’s always been some stubborn problems with this hypothesis. Some isotope ratios indicate that Earth had a sizeable portion of its water very early in its lifetime. Studies of lunar isotope ratios also show that Earth received much of its water prior to the Giant Impact that created the Moon only a few million years after the Solar System formed.

But now, the powerful JWST has entered the chat. Its evidence supports another long-held theory of planet formation. That theory states that Earth’s water arrived as Earth formed. Ice-covered pebbles drifted inward from the outer Solar System, and as they reached the warmer inner Solar System, the water sublimated into vapour. The vapour and the pebbles were both accreted into young planets. This is known as the icy pebble drift theory.

These findings are in a new paper in The Astrophysical Journal Letters titled “JWST Reveals Excess Cool Water near the Snow Line in Compact Disks, Consistent with Pebble Drift.” The lead author is Andrea Banzatti, an assistant professor of physics at Texas State University, San Marcos, Texas.

“Webb finally revealed the connection between water vapour in the inner disk and the drift of icy pebbles from the outer disk,” said Banzatti. “This finding opens up exciting prospects for studying rocky planet formation with Webb!”

Banzatti and his colleagues used the JWST’s MIRI (Mid-Infrared Instrument) to study four young, Sun-like stars only two or three million years old and surrounded by protoplanetary disks. Two of them were compact disks about 10 to 20 AU in size, and two of them were extended disks around 100 to 150 AU. The extended disks also have large radial gaps in them. The pebble drift theory states that compact disks deliver icy pebbles more efficiently into the inner solar system, well within the equivalent of Neptune’s orbit in our Solar System. Conversely, the theory also states that extended disks are less efficient at it.

The JWST’s powerful Medium-Resolution Spectrometer (MRS) works in conjunction with MIRI. The MRS can differentiate cool water from warm water, a critical part of untangling the complicated picture in protoplanetary disks. MRS data shows that there’s more cool water in the compact disks than there is in the extended disks.

“We present new JWST-MIRI spectra of four disks, two compact and two large with multiple radial gaps, selected to test the scenario that water vapour inside the snow line is regulated by pebble drift,” the article states. “Observation of this process opens up multiple exciting prospects to study planet formation chemistry in inner disks with JWST.”

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 extended CI Tau disk, which has at least three rings on different orbits. MRS can 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. Image Credit: NASA, ESA, CSA, Leah Hustak (STScI)

The pebble drift theory says that pebbles behave differently in different-sized disks. Some disks are much larger than others, and the hydrodynamical forces are much stronger in these larger disks. They effectively lock pebbles in place in their outer rings, inhibiting inward drift.

But compact rings don’t have the same strong hydrodynamical forces. Icy pebbles are freer to drift inward, becoming part of the rocky planets that typically form in the inner regions of Solar Systems. Up until now, researchers have had trouble gathering observations to test this theory.

Researchers using ALMA imaged numerous protoplanetary disks around young stars with telltale gaps showing where planets are forming. Based on these images and other data, scientists developed models showing that the presence or lack of gaps in protoplanetary disks regulated pebble drift into the inner solar systems.

However, previous observations lacked the high resolution needed to see inside these disks and test these ideas. Thanks to the James Webb Space Telescope and MIRI, scientists have finally observed the predicted pebble drift behaviour in both large and compact disks.

This graphic is an interpretation of data from Webb’s MIRI and its MRS, 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. Image Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

Here’s how pebble drift works. In compact disks like the one on the left in the above image, ice-covered pebbles drift inward toward the warmer region closer to the star unimpeded. When they cross the snow line, or astrophysical frost line, their ice sublimates to vapour. That delivers a large amount of water to the still-forming, rocky planets nearer the star. The right side of the above image shows an extended disk with rings and gaps likely created by large planets that are still forming. These rings have higher pressure than the gaps. As the ice-covered pebbles drift inward, more of them are stopped by the rings in the protoplanetary disk and trapped there. So fewer icy pebbles make it across the snow line to deliver water to the inner regions.

It took a while for Banzatti and his colleagues to understand the Webb data. It wasn’t initially clear what the observations were telling them. “For two months, we were stuck on these 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.”

The answer became clear when the researchers combined the data from compact and large rings. It showed that the compact disks have extra cool water just inside their snowlines, at about ten times closer than the orbit of Neptune.

This figure from the research article shows how pebble drift is more efficient in compact disks. Compact disks that lack the large rings where massive planets form. These rings have higher pressure and impede icy pebbles from delivering their water to the inner solar system. Image Credit: Banzatti et al. 2023.

“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!”

Now that scientists have observed this in other solar systems it’s very likely and almost certain that this is how Earth got its water. It also shows how the regions of a solar system interact with each other and that the inner regions of a solar system aren’t isolated from the outer regions.

“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.”

This is a huge finding. But scientists are cautious by nature, so the door isn’t closed on other explanations for the origins of Earth’s water. Earth may have gotten its water through different pathways, and icy impactors could still have delivered some of our planet’s water.

Artist’s rendering of a comet headed towards Earth. Comets, asteroids, and icy planetesimals could still have delivered some of Earth’s water. Image Credit: Public Domain.

But the results lead to some other intriguing avenues of inquiry. It starts with observing more disks beyond the four in this study.

“The findings of this work open up a number of exciting prospects,” the authors write in their article. “While this work includes the first four spectra from program GO-1640, which was set up with the specific goal of studying water emission in connection to pebble drift, a large number of disk spectra will be observed with MIRI in Cycle 1 and future cycles.”

That means a larger sample size, a critical part of solidifying these findings. With that in hand, researchers can start to piece together more detail in the pebble drift scenario by asking some pertinent questions. How common is the cool water excess? Does it vary with disk size? How does the size of the disk and the location and size of the dust rings affect everything? How does stellar size and luminosity affect it all? How do all of these factors affect the molecular chemistry in inner solar systems?

Stay tuned. The JWST isn’t finished yet.

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