A crucial piece of evidence supporting a long-held hypothesis about planet formation has been observed by the James Webb Space Telescope (JWST), meaning astronomers are confident they have correctly understood part of the cosmic process.

JWST data processed by an international team of researchers supports the theory of “icy pebble drift,” believed to be essential for gathering dust and rocks that eventually form into planets like ours.

Simply put, icy pebble drift works like this: when tiny pieces of material covered in ice collide with each other in the outer reaches of a young protoplanetary disk they lose momentum, allowing them to fall toward the star into a warmer zone where their frozen coating sublimates.

It is from this ring of fine debris and water vapor that rocky planets form, effectively serving as a delivery service for building materials across a newborn solar system.

As interesting as the idea is, studies of distant starlight that could indicate the position of this water vapor have until now been too fuzzy to say for sure whether this drift of icy pebbles is really taking place.

Water vapor table
Water vapor in compact or expanded discs. (Banzatti et al., Astrophysical journal letters2023)

Here, the team examined higher-resolution images taken with JWST’s mid-range infrared camera to examine two varieties of protoplanetary disks; compact and extensive. As their name suggests, expanded versions are larger and made up of distinct rings separated by pressure and gravity, while compact protoplanetary disks are more tightly packed together.

What the new research shows is that icy material can indeed move through protoplanetary disks, although this happens more easily on compact disks.

“In the past, we had this very static picture of planet formation, almost as if there were isolated areas from which planets formed,” says planetary scientist Colette Salyk of Vassar College.

“Now we have evidence that these areas can interact with each other. This is also something that would have happened in our solar system.”

By comparing data from compact and expanded disks, the team could see more water vapor collected at the compact disk’s “snow line,” where icy rocks would be expected to lose much more vapor.

This supports the idea that building materials can move toward the interior of the disc, a phenomenon more efficient in compact discs where there is no need to cross large gaps. As the sublimating stream of pebbles continues to fall from the afterlife, they provide both solids and water to create the seeds of a new planet, according to the theory.

It’s a fascinating discovery, made possible by the high-resolution and ultra-sensitive instruments on board the JWST. Astronomers can now continue to study planet formation, knowing that this particular process is actually happening.

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

“This discovery opens up exciting perspectives for studying the formation of rocky planets with Webb!”

The research was published in Astrophysical journal letters.

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