The formation of stars and planets has long been attributed to the Nebula Theory, where a massive cloud of gas and dust—known as a nebula—collapses under its own gravity. This collapse gives birth to a new star, and the remaining material forms a disk around it, known as a protoplanetary disk. Over millions of years, this disk evolves, giving rise to planets, moons, and other celestial bodies. Our very own Solar System formed in this manner about 4.5 billion years ago.
However, the precise mechanics of planet formation are still a subject of debate among scientists. Traditionally, the “bottom-up” model has been widely accepted. This theory suggests that planet formation occurs gradually as dust particles coalesce over millions of years. In contrast, the “top-down” model proposes that gravitational instability within the circumstellar disk can cause the material to fragment and form planets much more rapidly.
A recent study conducted by an international team of astronomers has provided compelling evidence supporting the “top-down” model. Using the Atacama Large Millimeter/submillimeter Array (ALMA), they observed a protoplanetary disk located over 500 light-years away from Earth. The study, led by Jessica Speedie, a Ph.D. candidate at the University of Victoria, and her team, was recently published in the journal Nature.
Groundbreaking Observations with ALMA
ALMA, the world’s largest radio telescope, is situated in the Atacama Desert in the Chilean Andes. It is uniquely equipped to observe parts of the Universe that are otherwise invisible, including cold dust clouds, protoplanetary disks, and the earliest galaxies. By focusing on the protoplanetary disk around AB Aurigae, a young star system approximately 530 light-years from Earth, the team aimed to unravel the mysteries of planet formation.
AB Aurigae, a pre-main sequence A-type star, is about 2.5 times the size of our Sun and 2.4 times as massive. Beginning in 2017, ALMA has been observing this star’s protoplanetary disk to gain insights into how planets form around young stars. These observations have revealed the presence of several developing protoplanets within the disk, including a gas giant that is nine times the mass of Jupiter.
These findings challenge the “bottom-up” model, as the protoplanets observed did not have enough time to form through the gradual accumulation of material. To further investigate, Speedie and her Ph.D. advisor, Ruobing Dong, focused on studying the movement of gas within the star’s spiral arms. ALMA’s unparalleled sensitivity and high velocity resolution enabled the team to measure the gas’s motion with remarkable precision.
The Signature ‘Wiggle’ of Gravitational Instability
Dr. Cassandra Hall, an Assistant Professor of Computational Astrophysics at the University of Georgia and a co-author of the study, had previously led a research project simulating the behavior of gravitationally unstable disks. In 2020, Hall and her colleagues predicted that such disks would exhibit distinctive “wiggles” in their velocity field. These wiggles are caused by variations in the density and gravity within the disk, leading to changes in the velocity of gas around the spiral arms.
Using ALMA, Speedie and her team mapped the velocity of carbon monoxide isotopes within the disk’s spiral arms, searching for these predicted wiggles. Their observations produced a three-dimensional data cube, mapping gas velocity and position within the protoplanetary disk. By analyzing slices of this data, they conclusively identified the velocity wiggle, confirming the presence of gravitational instability.
This discovery marks the first direct observational evidence that the “top-down” model of planet formation is accurate. It also suggests that planetary systems may form much faster than previously thought, potentially altering our understanding of astrogeology and exoplanet research.
Future Implications and Continued Research
The implications of this discovery are far-reaching. As Speedie noted, the success of their research was made possible by the combination of Hall’s predictive work and ALMA’s high-quality data. “This is a classic science story of, ‘we predicted it, and then we found it’. The hallmark of gravitational instability,” she said. “The ALMA data provides a clear diagnosis of gravitational instability in action.”
Looking ahead, Speedie and her colleagues plan to continue using ALMA to deepen their understanding of how planetary systems form around young stars. As part of the NSF/NRAO ALMA ambassador program, Speedie is also training with other early-career astronomers to share ALMA’s resources and capabilities with the wider astronomical community.
This research not only strengthens the case for the “top-down” model but also opens new avenues for exploring the complex processes that shape the formation of planets and planetary systems in our Universe.
