NASA’s James Webb Space Telescope (Webb) has made a breakthrough discovery on how planets are made. Webb observed water vapour in protoplanetary disks, which are rotating circumstellar disks of dense gas surrounding young newly formed stars, and eventually develop into planets and asteroids. Observing the water vapour helped Webb confirm a physical process involving the drifting of ice-coated solids from the outer regions of the disk into the inner rocky-planet zone.
Icy pebbles forming in the cold, outer regions of protoplanetary disks have long been believed to be the fundamental seeds of planet formation. According to these theories, the pebbles must drift inward toward the star due to friction in the gaseous disk in order for the planet to be formed. This is because in this way, both solids and water will be delivered to planets.
The theory had predicted that when icy pebbles enter into the warmer region within the snowline (the region where ice transitions to vapour), they should release large amounts of cold water vapour. Since Webb observed cold water vapour, the theory that the drifting of icy pebbles towards the warmer zone in protoplanetary disks produces planets is confirmed.
The study describing the findings was published November 8 in The Astrophysical Journal Letters.
In a NASA statement, the research’s principal investigator Andrea Banzatti of Texas State University said that Webb finally revealed the connection between water vapour in the inner disk and the drift of icy pebbles from the outer disk, and the finding opens up exciting prospects for studying rocky planet formation with Webb.
Colette Salyk of Vassar College in New York, and a team member, explained that in the past, scientists had a very static picture of planet formation, as though there were isolated zones that planets formed out of. Now, scientists have found evidence that these zones can interact with each other, and this is something that is proposed to have happened in the solar system.
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Webb studied protoplanetary disks and examined the presence of water
Webb studied four protoplanetary disks, of which two were compact and two extended. All the four disks were around newborn, Sun-like stars. The extended disk has gaps, while the compact one does not.
The researchers studied the four disks with the help of Webb’s Mid-Infrared Instrument. The stars around which the protoplanetary disks rotate are estimated to be between two and three million years old.
The scientists studied whether compact planet-forming disks have more water in their inner regions than extended planet-forming disks with gaps. 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 rocky inner planets that are beginning to form, then more water will be found in inner regions of the compact planet-forming disks than those in the extended planet-forming disks.
Large planets may result in rings of increased pressure, where pebbles tend to collect whenever there is an increase in pressure. Therefore, the rings serve as pressure traps, and impede the drift of icy pebbles towards the inner circle. Such events occur in the large disks with rings and gaps.
Jupiter is likely to have played a similar role in the solar system. The largest planet in the solar system may have inhibited pebbles and water delivery to the small, inner, and relatively water-poor rocky planets.
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According to NASA, the two compact disks are expected to experience efficient pebble drift, which means they can deliver pebbles to well within a distance equivalent to Neptune’s orbit.
However, the extended disks are believed to have their pebbles retained in multiple rings up to a distance six times the orbit of Neptune.
The reason why the scientists used MIRI’s Medium-Resolution Spectrometer is that it is sensitive to water vapour in disks.
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Compact disks have excess cool water compared to extended disks. Why?
They found that excess cool water is present in the compact disks, compared with the large disks, which means that icy pebbles present on the outer sides of compact disks transport more water and solid mass to the inner sides.
One of the compact disks without rings is called GK Tau disk, and one of the extended disks, which has at least three rings on different orbits, is called CI Tau disk. The scientists analysed warm and cool water in these disks.
Webb’s spectra revealed excess cool water in the compact GK Tau disk, compared with the large CI Tau disk.
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In a NASA statement, Benzatti said that for two months, the researchers were stuck on the preliminary results that revealed that the compact disks had colder water, and the large disks had hotter water overall. According to Banzatti, this made no sense because the researchers had selected a sample of stars with very similar temperatures.
Banzatti overlaid the data from the compact disks onto the data from the large disks and found that the compact disks have extra cool water just inside the snowline.
Therefore, Webb’s MIRI, which is sensitive to water vapour in disks, revealed the difference between pebble drift and water content in a compact disk, and those in an extended disk with rings and gaps.
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Webb found that in one of the compact disks, ice-covered pebbles that drift inward toward the warmer region closer to the star are unimpeded. Their ice turns into vapour as the ice-covered pebbles cross the snowline. This provides a large amount of water to enrich the rocky, inner planets.
Meanwhile, in extended disks with rings and gaps, ice-covered pebbles beginning to move inwards are stopped by the gaps and trapped in the rings. As a result, fewer icy pebbles make it across the snowline to deliver water to the inner region of the disk.
This explains why compact disks have more cooler water than extended disks with rings and gaps.