Planets form in disks of gas and dust orbiting young stars. The MIRI Mid-Infrared Disk Survey (MINDS), led by Thomas Henning at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, aims to create a representative disk sample. By studying their chemical and physical properties with MIRI (Mid-Infrared Observatory) on board the James Webb Space Telescope (JWST), the collaboration will link these disks with the properties of planets that may form there. JWST opens a new window on the chemistry of planet-forming disks “These observations are not possible from Earth because the associated gas emissions are absorbed by the atmosphere,” explains lead author Aditya Alabhavi from the University of Groningen in the Netherlands. “So far, we have only been able to detect acetylene emissions (C2H2) from this object. But the higher sensitivity of JWST and the spectral resolution of its instruments allowed us to detect faint emissions from molecules with low abundance. ” The MINDS collaboration found gas at a temperature of about 300 Kelvin (about 30 degrees Celsius) that is very rich in carbon-containing molecules but does not contain oxygen-rich species. “This is fundamentally different from the composition found in disks around Sun-like stars, where oxygen-containing molecules such as water and carbon dioxide dominate,” added team member Inga Kamp of the University of Groningen. A notable example of an oxygen-rich disk is the disk of PDS 70, where the MINDS program recently found a large amount of water vapor. From previous observations, astronomers have concluded that disks around very low-mass stars evolve differently from those around more massive stars like the Sun, which could potentially have implications for the search for rocky planets with properties similar to Earth. Such environments within the disk set the conditions for the formation of new planets, so each of these planets is likely to be rocky but very different from Earth in other aspects.

What does this mean for rocky planets orbiting very low-mass stars? The amount of material and its distribution throughout these disks limits the number and size of planets that the disks can supply with the necessary material. The observations therefore suggest that rocky planets of a size similar to Earth form more efficiently in disks around very low-mass stars, the most common stars in the universe, than around gas giants like Jupiter. As a result, the overwhelming majority of terrestrial planets are hosted by very low-mass stars. “Many of the primary atmospheres of these planets are probably dominated by hydrocarbon compounds, and not by water or oxygen-rich gases such as carbon dioxide,” Henning emphasizes. “In our previous work, we showed that the transport of carbon-rich gas to the regions where terrestrial planets usually form occurs faster and more efficiently in these disks than in those of more massive stars.” It is clear that disks around very low-mass stars contain more carbon than oxygen, but the mechanism of this imbalance is still unclear. The composition of the disk is the result of carbon enrichment or oxygen depletion. If carbon is enriched, the source is probably solid particles in the disk, which evaporate and release that carbon into the gaseous content of the disk. The dust particles, stripped of their original carbon, will eventually form rocky planetary bodies. These planets will be carbon-poor, like Earth. Nevertheless, carbon-based chemicals may dominate at least the main atmosphere produced by the disk gas. Thus, very low-mass stars may not provide the best conditions for searching for Earth-like planets. JWST discovers abundant organic molecules To identify the disk gas, the research team used MIRI’s spectrometer to split the infrared light received by the disk into features in a small range of wavelengths, similar to how sunlight is split into a rainbow. In this way, the team isolated abundant individual features that can be attributed to different molecules. As a result, the observed disk contains the richest hydrocarbon chemistry ever observed in a protoplanetary disk, consisting of 13 carbon-containing molecules up to benzene (C6H6). This includes the first extrasolar discovery of ethane (C2H6), the largest fully saturated hydrocarbon detected outside the solar system. The research team also successfully detected ethylene (C2H4), propyne (C3H4), and the methyl radical CH3 in a protoplanetary disk for the first time. In contrast, the data did not contain evidence of water or carbon monoxide in the disk. Sharpen the view of disks around very low-mass stars The science team next plans to expand the study to a larger sample of such disks around ultra-low-mass stars to better understand how common such exotic, carbon-rich, terrestrial planet-forming regions are. “By expanding the study, we will be able to better understand how these molecules arise,” Henning explained. “Several features in the data have yet to be identified and additional spectroscopy is required to fully interpret our observations.”

source: https://dx.doi.org/10.1126/science.adi8147