Planet-forming disks, which consist of swirling gas and dust around young stars, act as the birthplaces of planetary systems, including our own solar system. Recent discoveries have revealed more about the gas movements that continuously shape and modify these disks.
In the visible universe, over 3,000 stars are born every second. Many of these stars are encircled by what astronomers refer to as a protoplanetary disk—a dynamic “pancake” of heated gas and dust that is the starting point for planet formation. Despite this knowledge, the exact mechanisms behind star and planetary system formation remain somewhat of a mystery.
Researchers from the University of Arizona, utilizing NASA’s James Webb Space Telescope, have made strides in uncovering the dynamics influencing protoplanetary disks. Their observations provide a glimpse into the early conditions that characterized our solar system approximately 4.6 billion years ago.
The team successfully tracked what are known as disk winds, which are gas streams emanating from the disk into the void of space. Primarily driven by magnetic forces, these winds can reach speeds of tens of miles per second. The findings of this research, published in Nature Astronomy, significantly enhance our understanding of the formation and evolution of young planetary systems.
Ilaria Pascucci, lead author and professor at the University of Arizona’s Lunar and Planetary Laboratory, emphasizes that a critical process in a protoplanetary disk is the star’s absorption of material from its disk, a phenomenon termed accretion.
“The manner in which a star accumulates mass greatly impacts the evolution of the surrounding disk, including subsequent planet formation,” Pascucci explained. “While the specific mechanisms have been unclear, it seems that magnetic field-driven winds across the disk’s surface are likely vital.”
Younger stars gain mass by absorbing gas from the surrounding disk, but for this to occur, the gas must first rid itself of some of its angular momentum. Otherwise, the gas would continuously orbit the star rather than falling towards it. Astrophysicists describe this process as “losing angular momentum,” yet the precise method of this loss has remained difficult to grasp.
To visualize how angular momentum functions within a protoplanetary disk, consider a figure skater. When she brings her arms close to her body, her spinning speed increases, while extending her arms slows her down. Her total mass is unchanged, meaning her angular momentum stays constant.
For accretion to happen, gas in the disk must discard its angular momentum. However, astrophysicists do not entirely agree on how this process unfolds. Recently, there has been recognition that disk winds play a crucial role by transporting some of the gas—and its angular momentum—away from the disk’s surface, allowing the remaining gas to move inwards and eventually be absorbed by the star.
Recognizing the different processes that influence protoplanetary disks is essential, as highlighted by Tracy Beck, co-author from NASA’s Space Telescope Science Institute.
At the inner edges of the disk, the magnetic forces from the star push materials outwards through a mechanism known as X-wind, while the outer disk is affected by intense starlight, leading to slower, thermal winds.
“To differentiate the magnetic field-driven wind from thermal winds and the X-wind, the high sensitivity and resolution of the James Webb Space Telescope were essential,” Beck noted.
In contrast to the narrowly focused X-wind, the winds investigated in this study originate from a much broader area that spans the inner rocky planets of our solar system—approximately between Earth and Mars. Furthermore, these winds extend significantly higher above the disk compared to thermal winds, reaching heights hundreds of times more than the distance from the Earth to the sun.
“Our observations provide strong evidence that we have captured the first images of winds capable of removing angular momentum, addressing the long-standing question of how stars and planetary systems develop,” Pascucci stated.
The research team focused on four protoplanetary disk systems, all of which appear edge-on from Earth.
“Viewing these systems in this orientation allowed the intervening dust and gas to obscure some of the bright light from the central star, preventing it from overpowering the winds,” explained Naman Bajaj, a graduate student at the Lunar and Planetary Laboratory and contributor to the research.
By tuning the detectors of JWST to target specific molecules in defined transitional states, the researchers were able to observe varying layers of the winds. Their findings revealed a complex, three-dimensional structure consisting of a central jet enveloped within a cone of winds emanating from progressively greater distances in the disk, much like the layers of an onion. A noteworthy discovery was the consistent identification of a distinct central void within the cones formed by molecular winds across all four disks.
Looking ahead, Pascucci’s team aims to expand their observations to include more protoplanetary disks to better ascertain the prevalence of the observed wind structures across the universe and their evolutionary patterns over time.
“We suspect these structures are common, but with just four subjects, it’s challenging to draw definitive conclusions,” Pascucci explained. “We intend to increase our sample size with the James Webb, while also seeking to observe any changes in these winds as stars form and planets evolve.”
