When it comes to finding small, still-forming planets around young stars, the Atacama Large Millimeter/submillimeter Array (ALMA) observatory is astronomers’ most adept tool. ALMA has provided many images of the protoplanetary disks around young stars, in which gaps and rings have been carved 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 planet formation may begin earlier than previously thought.
These results are published in Astronomy and Astrophysics under the title “FAUST. XXVIII. High-Resolution ALMA Observations of Class 0/I Disks: Structure, Optical Depths, and Temperatures.” The main author is Dr. Maria Jose Maureira Pinochet, postdoctoral researcher in astronomy at the Max Planck Institute for Extraterrestrial Physics. FAUST stands for Fifty AU STudy, an ongoing research program that uses ALMA to study the shell/disk systems of Sun-like class 0 and I protostars at scales of about 50 AU.
In the past, astronomers thought that planet formation followed star formation. However, there is increasing evidence that planet formation begins earlier and occurs while the star is still a forming protostar.
“Increasing evidence suggests that the planet formation process begins during the embedded protostellar stages (Class 0/I), making the characterization of protostellar disks key to studying both the protostar accretion process and the initial stages of planet formation,” the authors of the new research write. In the embedded protostellar stage, young protostars are deeply embedded in their natural gaseous, dusty envelopes. Protostars actively accumulate new material during this phase and it is during this phase that protostars build up most of their mass.
But protostellar disks are difficult observation environments. The thick gas and dust obscure what’s going on inside them. Fortunately, ALMA is up to it. Using ALMA, the researchers observed 16 very young systems with class 0/1 protostars.
“These baby disks bridge the gap between the collapsing cloud and the later stages of planet formation,” said Paola Caselli, director of the Center for Astrochemistry at MPE and one of the study’s lead authors. “They represent the missing link to understanding how stars and planets form together.”
*This figure shows 14 of the Class 0/1 hard drives examined. The top two rows are class 0 and 1 disks where the nearest protostellar neighbor is larger than 100 AU. The bottom two rows show the same thing, but for systems with a protostellar neighbor below 100 AU. “In contrast to the first group, disk-like circumbinary structures are observed for all sources in the second group,” the researchers write. Image source: Maureira et al. 2025. Astronomy and Astrophysics*
Although the resolution of studies of these types of young systems has improved, there is still a need for further knowledge. A current goal is to detect when twilight substructures such as those 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 is no longer strongly embedded.
So far, astronomers have examined nearly 60 Class 0/1 disks, but only five of them have clearly defined substructures, and all five were in Class 1 disks. “These results suggest that either planet formation begins at the Class I stage or that many younger disks remain optically too thick at around 1 mm, preventing clear detection of substructures,” the researchers explain.
The researchers identified only one unique substructure that had already been identified by previous researchers. They also found an additional possible substructure. But that doesn’t mean their work is free. The nature of this pair of substructures suggests that there are more of them hiding beyond ALMA’s reach. “These results support the idea that ring-shaped substructures can form as early as class 0, but are often obscured by optically thick emission,” the authors explain.
In addition, their work also shows that these young disks are about ten times brighter than more developed disks. This is mainly because they are so thick and massive, in fact they are much thicker and more massive than we thought. The results also provide information about the forces that shape these extremely young disks.
“Our results show that self-gravity and accretionary heating play an important role in shaping the earliest disks,” added Hauyu Baobab Liu from the Department of Physics, National Sun Yat-sen University of Taiwan. “They influence both the mass available for planet formation and the chemistry that leads to complex molecules.”
It’s like nature hiding its secrets in dense, dusty regions. And it’s just like people who keep trying to look inside themselves and find these secrets. But the thick dust is in the way. It makes it difficult to determine dust grain size, an important indicator of planet formation.
*This ALMA image from other research shows the protoplanetary disk around the young star HL Tauri. It is only about 100,000 years old and the disk has distinct rings and gaps that astronomers believe are caused by the formation of planets in the disk. But astronomers need to see more details to understand the process of planet formation. Image credit: By ALMA, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=36643860*
ALMA will continue to play a role in future efforts to observe the earliest stages of planet formation in protostellar disks. This also applies to the Very Large Array, another radio interferometer. But upcoming facilities such as the Square Kilometer Array and the Next Generation VLA (ngVLA) will also be part of the effort. Together they will observe these dark disks at longer wavelengths.
“Observations at longer wavelengths are necessary to overcome these problems and therefore future observations with SKAO and ngVLA, together with more sensitive observations with ALMA to reach broader and fainter populations, will be crucial to improve our understanding of the early formation and evolution of disks and planets,” the authors conclude.
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