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Researchers from the University of Michigan Department of Astronomy reported discoveries of ultraviolet-driven chemical markers that can indicate late-stage planet formation in a paper published on Dec. 8, 2022. The paper states the chemical composition of protoplanetary disks — a rotating disk of dust and gas — is essential for helping human understanding of future compositions of planets and their capability for life.

Jenny Calahan, first author of the paper and Ph.D. candidate in the U-M Department of Astronomy, told The Michigan Daily that when she was looking at images from the Atacama Large Millimeter Array telescope (ALMA), she noticed a specific configuration of the chemical compound methyl cyanide (CH3CN) in a gaseous state when it usually should have been frozen solid. Calahan said she first tried to explain the unexpected state of methyl cyanide, but soon realized that her previous assumptions about energy entering planetary disks were mistaken.

Because planetary disks are very cold, planet formation requires more energy for reactions to occur. Scientists previously believed cosmic rays, an energy source which dominates interstellar space, facilitated this formation. Calahan said the study’s findings provide new evidence in the presence of chemical markers that UV rays play a more important role than other energy types in the later stages of planet formation.

Earth was formed from tiny grains of particulate matter existing in interstellar space. These grains of matter make up other planetary disks and, as they grow, develop into the core of planets. Edwin Bergin, Astronomy professor and chair of the Department of Astronomy, as well as principal investigator of the study, explained to The Daily how these tiny grains that make up the disks highlight the presence of the UV rays.

“At some point, the chemistry (between early and late planetary disks) becomes distinct and different,” Bergin said. “The difference is whether or not UV photons are important. What Jenny found is that, as the grains grow, the UV photons are allowed to penetrate deeper and they basically take over the chemistry.”

According to Calahan, deep UV radiation penetration allows for the methyl cyanide to detach from the grains of particulate matter, causing the methyl cyanide to exist in the gas phase rather than the expected frozen phase. Calahan said the presence of methyl cyanide in the disk has further implications for the future of a planet’s atmosphere formation as well.

“We’re saying that the fact that you have bright methyl cyanide in a disk definitely means that there is a high carbon-to-oxygen ratio in the gas,” Calahan said. “In the disks that we’re looking at, they kind of correspond with later stages of planet formation … but (are) now creating its atmosphere from the gaseous part of the disk from what we think is carbon-rich.”

A carbon-rich atmosphere is essential for life, making the study’s findings a piece in the wider puzzle of what factors contributed to the formation of life on Earth and the possibility of life beyond.

Jackson Proffer, a LSA junior studying astronomy and member of the Student Astronomical Society, told The Daily he’s been interested in astronomy and astrochemistry since he was in middle school. He said he believes understanding planet formation can also lead to clues in better understanding how the universe and Earth itself were formed.

“I think it’s interesting to understand the complete path of (the) history of events that needed to have taken place for our solar system to evolve the way it has,” Proffer said.

In the future, Calahan said she hopes to take what she’s learned from the study to look at other young planetary disks. She said she also hopes other devices, such as the James Webb Telescope, will offer an opportunity to look more closely at the inner disk of the early planet rather than the outer disk, which is all that is visible using the ALMA. 

“I want to see if this kind of chemistry is also happening in the inner disk, or are there even crazier things going on?” Calahan said.

Daily Staff Reporter Isabella Kassa can be reached at ikassa@umich.edu.

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