Astronomers have discovered new details about gas flows that shape planet-forming disks and shape them over time, offering a glimpse into how our solar system likely formed
Every second, more than 3,000 stars are born in the visible universe. Many of them are surrounded by what astronomers call a protoplanetary disk - a swirling "pancake" of hot gas and dust from which planets form. However, the exact processes that lead to the formation of stars and planetary systems are still not well understood.
A team of astronomers led by researchers from the University of Arizona used NASA's James Webb Space Telescope to gain some of the most detailed insights into the forces shaping protoplanetary disks. The observations offer glimpses of what our solar system looked like 4.6 billion years ago.
Specifically, the team was able to track so-called disc ghosts in unprecedented detail. These winds are streams of gas that blow from the planet-forming disk into space. Driven primarily by magnetic fields, these winds can travel tens of kilometers in one second. The researchers' findings, published in Nature Astronomy, help astronomers better understand how young planetary systems form and evolve.
According to the paper's lead author, Ilaria Pascucci, a professor at the University of Arizona's Lunar and Planetary Laboratory, one of the most important processes at work in a protoplanetary disk is the star eating material from the disk surrounding it, known as accretion.
"The way a star gains mass greatly affects how the circumstellar disk evolves over time, including how planets form later," Pascucci said. "The specific ways in which this happens have not been understood, but we think that winds driven by magnetic fields over most of the disk area may play a very important role."
Young stars grow by pulling gas from the disk spinning around them, but for this to happen, the gas must first lose some of its inertia. Otherwise, the gas would consistently orbit the star and never fall onto it. Astrophysicists call this process "loss of angular momentum", but it is not yet understood exactly how this happens.
To better understand how angular momentum works in a protoplanetary disk, imagine an ice skater: folding her arms at her side would make her spin faster, while her spin would slow her down. Since its mass does not change, its angular momentum remains the same.
For gas accretion to occur, gas along the disk must lose angular momentum, but astrophysicists have difficulty agreeing on exactly how this happens. In recent years, disc winds have become important players that channel some of the gas from the disc's surface—and with it, angular momentum—allowing the remaining gas to move inward and eventually fall onto the star.
Because there are additional processes that operate and shape protoplanetary disks, it is very important to distinguish between the various phenomena, according to the paper's second author, Tracy Beck of NASA's Space Telescope Science Institute.
While material at the inner edge of the disc is pushed outward by the star's magnetic field in what is known as the X-wind, the outer parts of the disc are eroded by intense starlight, leading to thermal winds, which blow at much lower speeds.
"To distinguish between the wind driven by the magnetic field, the thermal wind and the X-wind, we really needed the high sensitivity and resolution of the JWST (James Webb Space Telescope)," Beck said.
Unlike the focused X-wind, the winds observed in the current study originate in a wider region that would include the inner rocky stars of our solar system - roughly between Earth and Mars. These winds also spread farther above the disk than thermal winds, reaching distances hundreds of times the distance between the Earth and the Sun.
"Our observations suggest that we have obtained the first images of the winds that can remove angular momentum and solve the long-standing problem of how stars and planetary systems form," Pascucci said.
For their study, the researchers chose four protoplanetary disk systems, all of which appear sideways when viewed from Earth.
"Their orientation allowed the dust and gas in the disk to act as a mask, blocking some of the light from the bright central star, which would otherwise have obscured the winds," said Naaman Bajaj, a graduate student at the Lunar and Planetary Laboratory who contributed to the study.
By aiming JWST's detectors at certain molecules in certain transition states, the team was able to track different layers of the winds. The observations revealed a complex three-dimensional structure of a central jet, nested within a cone-shaped envelope of winds originating at progressively increasing disc distances, similar to the layered structure of an onion. An important new finding, according to the researchers, was the consistent discovery of a prominent central hole within the cones, created by molecular winds in each of the four discs.
Next, Pascocchi's team hopes to extend these observations to other protoplanetary disks, to gain a better understanding of how common the observed disk wind structures are in the universe and how they evolve over time.
"We believe they could be the common or standard case, but with four objects, it's a little hard to say," Pascucci said. "We want to get a larger sample with James Webb, and then also see if we can detect changes in these winds as stars gather and planets form."
More of the topic in Hayadan: