Trying to find baby planets swaddled in dust

When it comes to finding young, still-forming planets around young stars, the Atacama Large Millimeter/Submillimeter Array (ALMA) an observatory is the most skillful instrument of astronomers. ALMA has provided many images of protoplanetary disks around young stars, with gaps and rings carved into them by young planets. In a new study, a team of researchers used ALMA to image 16 disks around young class 0/1 protostars and found that planets may begin to form earlier than previously thought.

These findings will be published in Astronomy and astrophysics in the article “FAUST. XXVIII. High-resolution ALMA observations of class 0/I disks: structure, optical depth and temperature.” Research currently accessible on arXiv preprint server.

Lead author is Dr. Maria José Maureira Pinochet, a postdoc 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 shell/disk systems of class 0 and class I solar-like protostars at scales of approximately 50 AU.

Astronomers used to think that planet formation succeeded star formation. But there is growing evidence that planet formation begins earlier, when the star is still a protostar.

“Increasing evidence suggests that the process of planet formation begins in embedded protostellar stages (class 0/I), making the characterization of protostellar disks key to studying both the process of protostar accretion and the initial phases of planet formation,” write the authors of the new study. The nested protostellar stage is when young protostars are deeply embedded in their natural gaseous dust envelopes. During this stage, protostars are actively accumulating new material, and this is when protostars gain most of their mass.

But protostellar disks are difficult environments to observe. Thick gas and dust hide what is happening inside them. Luckily, ALMA is ready for this. 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 stages of planet formation,” said Paola Caselli, director of the MPE Center for Astrochemistry and one of the main authors of the study. “They provide the missing link to understanding how stars and planets go out together.”

Although studies of these types of young systems have improved in resolution, there is still a need to see more. The current goal is to recognize when disk substructures similar to those found 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 deeply embedded in it.

So far, astronomers have studied nearly 60 class 0/1 disks, but only five of them have a clear substructure, and all five of them are class 1 disks. “These results suggest that either planet formation begins at the class I stage, or that many younger disks remain too optically thick, around 1 mm, to prevent clear detection of substructures,” the researchers explain.

The researchers identified only one specific substructure, and it was identified by previous researchers. They also discovered an additional potential substructure. This does not mean that their work is in vain. The nature of this pair of substructures suggests that many more are lurking just out of sight, beyond ALMA's reach. “These results support the idea that ring substructures can appear as early as the class 0 stage, but are often obscured by optically dense emission,” the authors explain.

Beyond this result, their work also shows that these young disks are about 10 times brighter than more mature disks. This is mainly because they are so thick and massive. In fact, they are much thicker and more massive than expected. The results also shed light on the forces that shape these extremely young discs.

“Our results indicate that self-gravity and accretion heating play an important role in the formation of the earliest disks,” added Hayou Baobab Liu from the Department of Physics at National Sun Yat-sen University of Taiwan. “They influence both the available mass for planet formation and the chemistry that leads to the formation of complex molecules.”

It's like how nature hides its secrets in dense, dusty lands. And it's like people keep trying to look inside themselves and find these secrets. But thick dust gets in the way. This makes it difficult to determine the size of dust particles, an important indicator of planet formation.

ALMA will continue to play a role in future attempts to see the earliest stages of planet formation in protostellar disks. The same will happen with the Very Large Array, another radio interferometer. But upcoming objects such as Square kilometers array and Next Generation VLA (ngVLA) will also be part of this effort. Together they will observe these darkening disks at longer wavelengths.

“Observations at longer wavelengths are needed to address these issues, and therefore future observations with SKAO and ngVLA, as well as more sensitive observations with ALMA to cover broader and fainter populations, will be key to advancing our understanding of the early formation and evolution of disks and planets,” the authors conclude.