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Young star system reveals gravitational instabilities of planet formation process

For the first time, scientists directly measured gas velocity in the protoplanetary disk surrounding a young star. Photo by ALMA (ESO/NAOJ/NRAO)/T. Paneque-Carreño (Universidad de Chile), B. Saxton (NRAO)
For the first time, scientists directly measured gas velocity in the protoplanetary disk surrounding a young star. Photo by ALMA (ESO/NAOJ/NRAO)/T. Paneque-Carreño (Universidad de Chile), B. Saxton (NRAO)

June 17 (UPI) -- A chaotic, young star system, located 400 light-years from Earth, has offered astronomers new insights into the planet formation process.

Observations of the stellar system Elias 2-27 -- made using the Atacama Large Millimeter/submillimeter Array, or ALMA -- confirmed the presence of significant gravitational instabilities, a phenomenon scientists have long suspected played an important role in planet formation.

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Scientists have long known that planets emerge from the large disks of gas and dust frequently found circling young starts. However, the details of this emergence -- the formation process -- have been difficult to decipher.

Now, for the first time, scientists have used gas velocity data to directly measure the mass of a protoplanetary disk.

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The breakthrough, described in a pair of papers published Thursday in the Astrophysical Journal, suggests gravitational instabilities, in addition to other kinds of structural chaos, help facilitate planet formation.

"How exactly planets form is one of the main questions in our field," lead researcher Teresa Paneque-Carreño, a recent graduate of the Universidad de Chile, said in a press release. "However, there are some key mechanisms that we believe can accelerate the process of planet formation."

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"We found direct evidence for gravitational instabilities in Elias 2-27, which is very exciting because this is the first time that we can show kinematic and multi-wavelength proof of a system being gravitationally unstable," said Paneque-Carreño, now a doctoral student at the University of Leiden and the European Southern Observatory. "Elias 2-27 is the first system that checks all of the boxes."

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In addition to being gravitationally unstable, with a large portion of the system's mass contained within the protoplanetary disk, ALMA observations revealed Elias 2-27 to host a pair of spiral arms -- a features never before seen around a single star.

Followup surveys using the full spectrum of ALMA's observational bands confirmed the presence of these multidimensional arms carved by density waves. The fresh observations also showed an influx of fresh cosmic material was likely responsible for the disk's vertical asymmetry and velocity perturbations.

"The Elias 2-27 star system is highly asymmetric in the gas structure," said Paneque-Carreño. "This was completely unexpected, and it is the first time we've observed such vertical asymmetry in a protoplanetary disk."

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While the latest findings have confirmed a number of theories about gravitational instabilities and spiral arms, there are other aspects of Elias 2-27 that scientists still don't have an explanation for, such as a band of missing material within the inner half of the protoplanetary disk.

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The authors of the new study are hopeful that future ALMA observations will help them solve the remaining mysteries of protoplanetary disk dynamics and uncover the secrets of the planet formation process.

"Previous measurements of protoplanetary disk mass were indirect and based only on dust or rare isotopologues," said Benedetta Veronesi, a graduate student at the University of Milan and postdoctoral researcher at École normale supérieure de Lyon. "With this new study, we are now sensitive to the entire mass of the disk."

"This finding lays the foundation for the development of a method to measure disk mass that will allow us to break down one of the biggest and most pressing barriers in the field of planet formation," said Veronesi, lead author of one of the two papers.

"Knowing the amount of mass present in planet-forming disks allows us to determine the amount of material available for the formation of planetary systems, and to better understand the process by which they form."

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