James Webb Space Telescope reveals evidence of winds that could solve mystery of how planet-forming disks are shaped

More than 3,000 stars are born in the visible universe every second, many surrounded by disks of swirling gas and dust.

It is from those disks that planets form. But exactly how that happens is still poorly understood.

An international team of researchers that includes Clemson Professor of Physics and Astronomy Sean Brittain used data from NASA's James Webb Space Telescope to obtain the most detailed insight yet into the forces that sculpt those disks and shape them over time. The team's findings were published in the journal Nature Astronomy.

"The James Webb Space Telescope allows us to interrogate these disks and understand planet formation in ways we couldn't before," Brittain said.

Young stars form from the collapse of gas and dust in molecular clouds. The stars grow by pulling in and eating - or accreting - gas from the disk swirling around them.

"The question we have is, what's the mechanism that causes disks to accrete," Brittain said, adding that relying on the viscosity of the circumstellar gas to drive accretion doesn't work.

Brittain continued, "The density of the gas in the disk is 10 million times less than the air we breathe. Since the viscosity is so low, the gas in the disk, which comprises 99% of the mass, should survive more or less indefinitely. It would take the age of the universe to accrete a disk, but we know that disks are gone in a few million years, so something has to be causing the material that's in orbit around the star to spiral in."

Leading theory

The leading theory is that disk winds add torque to the material in the disk, which would allow the angular momentum to move outward and the mass to move inward. The angular momentum would still be conserved, but the mass would be moved onto the star. To better understand how angular momentum works in a protoplanetary disk, it helps to picture a figure skater on the ice: Tucking her arms alongside her body will make her spin faster, while stretching them out will slow down her rotation. While the mass doesn't change, the distribution of the mass does so the angular momentum is conserved even while the speed at which the skater spins changes.

"How a star accretes mass has a big influence on how the surrounding disk evolves over time, including the way planets form later on," said Ilaria Pascucci, a professor at the University of Arizona's Lunar and Planetary Laboratory and the paper's lead author. "The specific ways in which this happens have not been understood, but we think that winds driven by magnetic fields across most of the disk surface could play a very important role."

Because there are other processes at work that shape protoplanetary disks, it is critical to be able to distinguish between the different phenomena, according to the paper's second author, Tracy Beck at NASA's Space Telescope Science Institute.

While material at the inner edge of the disk is pushed out by the star's magnetic field in what is known as X-wind, the outer parts of the disk are eroded by intense starlight, resulting in so-called thermal winds, which blow at much slower velocities.

"To distinguish between the magnetic field-driven wind, the thermal wind and X-wind, we really needed the high sensitivity and resolution of JWST (the James Webb Space Telescope)," Beck said.

Unlike the narrowly focused X-wind, the winds observed in the present study originate from a broader region that would include the inner, rocky planets of our solar system - roughly between Earth and Mars. These winds also extend farther above the disk than thermal winds, reaching distances hundreds of times the distance between Earth and the sun.

First images

"Our observations strongly suggest that we have obtained the first images of the winds that can remove angular momentum and solve the longstanding problem of how stars and planetary systems form," Pascucci said.

This study included four protoplanetary disk systems, all of which appear edge-on when viewed from Earth.

"Their orientation allowed the dust and gas in the disk to act as a mask, blocking some of the bright central star's light, which otherwise would have overwhelmed the winds," said Naman Bajaj, a graduate student at the Lunar and Planetary Laboratory who contributed to the study.

By tuning JWST's detectors to distinct molecules in certain states of transition, the team was able to trace various layers of the winds. The observations revealed an intricate, three-dimensional structure of a central jet, nested inside a cone-shaped envelope of winds originating at progressively larger disk distances, similar to the layered structure of an onion. An important new finding, according to the researchers, was the consistent detection of a pronounced central hole inside the cones, formed by molecular winds in each of the four disks.

"Now we can see that, yes, the winds are there. This doesn't necessarily prove that disk winds are driving accretion. Maybe there's some other theory we haven't thought of yet. But it's the first really tangible evidence we have supporting this model for how disks accrete. And it also solves a lot of other puzzles about the planet formation process itself," Brittain said.

Next, the team hopes to expand these observations to more protoplanetary disks, to get a better sense of how common the observed disk wind structures are among forming stars and how they evolve over time.

"The James Webb Space Telescope is expanding our knowledge of the universe in incredible ways. Every observation is a new discovery," Brittain said. "Since the launch of satellites capable of observing gamma-ray, X-ray and ultraviolet radiation in the 70's through the scores of astronomical observatories put into orbit since then, and now with the James Webb Space Telescope, it's been an incredible span of one discovery after another. We truly are living in the Golden Age of astronomy."

Adapted from a media release by the University of Arizona.