Hot off the disk: New detections of complex molecules in warm planet-forming disks

Recent observations of protoplanetary disks—the birthplaces of planets—are challenging our understanding of the chemistry that shapes planetary systems. Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), researchers detected an unexpected abundance of complex organic molecules (COMs) in two disks, HD 100546 and IRS 48. These molecules, including methanol (CH₃OH), methyl formate (CH₃OCHO), dimethyl ether (CH₃OCH₃), and ethylene oxide (c-H₂COCH₂), are crucial because they are considered precursors to life’s building blocks, such as amino acids and DNA.

Interestingly, the researchers found that larger molecules, like methyl formate and dimethyl ether, were more abundant than methanol in these disks—a surprising result. This suggests two possibilities: either the methanol abundance is being underestimated due to observational challenges, or methanol is being broken down by energetic processes into smaller fragments that recombine into more complex species. Understanding these processes is critical to uncovering how the chemical environment in these disks shapes the formation of planets and potentially habitable worlds.

Protoplanetary disks form as gas and dust swirl around young stars, providing the raw material for planet formation. These disks are much smaller than the molecular clouds from which they originate, spanning only a few hundred times the Earth-Sun distance (AU). To study them in detail, astronomers rely on ALMA, one of the most sensitive telescopes in the world. By observing light in the submillimeter range, ALMA can detect both tiny dust particles and the molecular signatures of various chemicals.

Emissions of molecules studied by Booth et al. 2024 toward IRS 48. The bright emissions indicate the presence of that molecule in that region of space. This data was collected by ALMA, a radio telescope.

The new findings, led by Alice Booth and her team at the Harvard & Smithsonian Center for Astrophysics, focus on disks around hot, bright Herbig Ae stars—less commonly studied compared to cooler, dimmer stars like T Tauri stars. The discovery of COMs in these disks marks a significant advancement in understanding planet-forming environments. Notably, only IRS 48 showed dimethyl ether and ethylene oxide, while both disks contained methanol and methyl formate, highlighting the need for further research to explain these differences.

These molecules are thought to form when icy particles in the disks heat up and release trapped chemicals. ALMA’s ability to spatially resolve the emission of these molecules offers a detailed view of where and how they form. Future studies of additional molecules and disks will help clarify the processes that govern the chemical composition of planet-forming environments and how the ingredients for life might arise in the cosmos.

This article made use of the following publications:

Booth, A. S., Leemker, M., van Dishoeck, E. F., et al. 2024, AJ, 167, 164. 

Booth, A. S., Temmink, M., van Dishoeck, E. F., et al.  2024, AJ, 167, 165.