For a long time it has been suggested that icy ‘pebbles’ in the cold outer regions of protoplanetary disks form the basis for the formation of planets. And now that theory has finally been confirmed.

How do planets see the light of day? It is a pressing question that astronomers have been pondering for some time. Fortunately, scientists have now developed a leading theory to explain this issue. Although until recently it remained just a theory. Now the James Webb space telescope has seen this process in action, finally allowing us to know for sure how planets are born.

The theory
For some time now, theories have suggested that icy ‘pebbles’ in the cold outer regions of protoplanetary disks (the same region where comets form in our Solar System) are the essential building blocks for planet formation. According to the thinking, these icy particles drift from the cold, outer regions of the disk toward the newborn star. The theory states that as these pebbles enter the warmer region closer to the star, they would release significant amounts of cold water vapor, delivering both water and solids to developing planets.

James Webb
Most importantly, theory predicts that icy pebbles should release significant amounts of cold water vapor upon entering the warmer region (where ice turns to vapor). And that is exactly what the Webb telescope has now observed. By observing water vapor in protoplanetary disks, Webb confirmed that icy particles drift from the outer parts of protoplanetary disks into the rocky planetary zone. A breakthrough. “Webb has now finally shown how water vapor in the inner disk is related to the drift of icy pebbles from the outer disk,” says study leader Andrea Banzatti. “This opens up exciting possibilities for studying the formation of rocky planets using Webb.”

Study
The researchers made this discovery after studying four disks – two compact and two extended – around Sun-like stars using Webb’s MIRI (the Mid-Infrared Instrument). These four stars are all estimated to be between two and three million years old, which is considered newborn in cosmic terms.

Artist’s impression of two planet-forming disks around newborn, Sun-like stars. On the left we see a compact disk and on the right an extended disk with openings. Image: NASA, ESA, CSA, Joseph Olmsted (STScI)

The purpose of the Webb observations was to see if the compact disks have more water in their inner, rocky part. This would happen if the pebbles were delivered in a more efficient way, with a lot of mass and water. The team used MIRI’s MRS (Medium-Resolution Spectrometer) because this instrument is good at detecting water vapor.

Confirmed
The results confirmed what was expected. The researchers indeed found a surplus of cold water in the compact disks. “Now it is finally clear that there is a surplus of colder water,” says Banzatti. “This is extraordinary and entirely due to Webb’s improved accuracy.”

Thanks to the study, published in the journal Astrophysical Journal Letters, we now finally know for sure how planets form. “In the past we had a very static view of planet formation, almost as if there were isolated zones from which planets were formed,” says researcher Colette Salyk. “Now we finally have proof that these zones interact with each other. And something similar has probably happened in our own solar system.”

More about James Webb
The James Webb telescope is the successor to the famous Hubble telescope. The telescope is the world’s most important observatory, which promises to solve many mysteries in our solar system, provide an even closer look at distant worlds around other stars and unravel much about the origins of our universe and our place in it. In short, the telescope is expected to answer many questions and radically change our view of the universe and its creation. And it looks like the telescope is right on track to deliver on that promise.




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