When it comes to finding baby, still-forming planets around young stars, the Atacama Large Millimeter/submillimeter Array (ALMA) observatory is astronomers' most adept tool. ALMA has delivered many images of the protoplanetary disks around young stars, with gaps and rings carved in them by young planets. In new research, a team of researchers used ALMA to image 16 disks around young class 0/1 protostars and found that planets may start forming sooner than previously thought.
These findings will be published in Astronomy and Astrophysics and titled "FAUST. XXVIII. High-Resolution ALMA Observations of Class 0/I Disks: Structure, Optical Depths, and Temperatures." The lead author is Dr. Maria Jose Maureira Pinochet, an Astronomy Postdoc at Max Planck Institute for Extraterrestrial Physics. FAUST stands for Fifty AU STudy, an ongoing research program that uses ALMA to study the envelope/disk systems of solar-like Class 0 and I protostars on scales of approximately 50 au.
In the past, astronomers thought that planet formation succeeded star formation. But there's growing evidence that planet formation begins earlier, taking place while the star is a still-forming protostar.
"Growing evidence suggests that the planet formation process begins during the embedded protostellar stages (Class 0/I), making the characterization of protostellar disks key to study both the protostar accretion process and the initial phases of planet formation," the authors of the new research write. The embedded protostellar stage is when young protostars are deeply embedded inside their natural gaseous, dusty envelopes. Protostars are actively accreting new material during this stage, and is when protostars build up most of their mass.
But protostellar disks are difficult observational environments. The thick gas and dust obscures what's going on inside them. Fortunately, ALMA is up to it. The researchers used ALMA to observe 16 very young systems with Class 0/1 protostars.
“These baby disks bridge the gap between the collapsing cloud and the later planet-forming stages,” said Paola Caselli, Director at the Center for Astrochemistry a MPE and one of the main authors of the study. “They provide the missing link for understanding how stars and planets emerge together.”
While surveys of these types of young systems have improved in resolution, there's still a need to see more. A current goal is to recognize when dusk substructures like the ones in Class II disks appear in Class 0/1 disks. In Class II disks, the protoplanetary disk is still thick, but the young star itself no longer heavily embedded.
So far, astronomers have looked at almost 60 Class0/1 disks, but only five of them have clearly defined substructures, and all five of them were in Class 1 disks. "These results suggest either that planet formation begins during the Class I stage or that many younger disks remain too optically thick at ∼ 1 mm, preventing the clear detection of substructures," the researchers explain.
The researchers only identified one definite substructure, and it had been identified by previous researchers. They also found an additional potential substructure. This doesn't mean their work is for nothing. The nature of this pair of substructures suggest that more are hiding just out of sight, beyond ALMA's reach. "These results support the idea that annular substructures can emerge as early as the Class 0 stage but are often hidden by optically thick emission," the authors explain.
Beyond that result, their work also shows that these young disks are about ten times brighter than more evolved disks. It's mostly because they're so thick and so massive, in fact they're far thicker and more massive than thought. The results also shed light on the forces that shape these extremely young disks.
“Our results show that self-gravity and accretion heating play a major role in shaping the earliest disks,” added Hauyu Baobab Liu from the Department of Physics at the National Sun Yat-sen University Taiwan. “They influence both the available mass for planet formation and the chemistry that leads to complex molecules.”
It's just like Nature to conceal its secrets in thick, dusty, regions. And it's just like humans to keep trying to see inside them and find those secrets. But the thick dust is in the way. It makes it hard to determine dust grain sizes, an important indicator of planet formation.
ALMA will continue to play a role in future efforts to see the earliest stages of planet formation in protostellar disks. So will the Very Large Array, another radio interferometers. But upcoming facilities like the Square Kilometre Array and the Next Generation VLA (ngVLA) will also be a part of the effort. Together, they will observe these obscuring disks at longer wavelengths.
"Observations at longer wavelengths are necessary to overcome these issues and thus future observations with SKAO and ngVLA along with more sensitive observations with ALMA to reach wider and fainter populations, will be key for advancing our understanding of early disk and planet formation and evolution," the authors conclude.