Astronomers see benzene in a planet-forming disk around a star for the first time

An international team of astronomers, including several Dutch researchers, has observed for the first time benzene gas (C6H6) in a planet-forming disk around a young star. In addition to benzene, they saw several other smaller carbon compounds and a few oxygen-rich molecules. Observations indicate that planets that form in this disk, such as Earth, contain a relatively small amount of carbon. The scientists will publish their findings in the journal Thursday evening natural astronomy.

The researchers studied the luminous young star J160532 (one-tenth the mass of the Sun) about 500 light-years away in the direction of the constellation Scorpius. Many rocky, Earth-like planets form around such young, young stars. Until now, it has been difficult to study the molecules in the warm interior of the dust disks around young, young stars, because previous generations of telescopes lacked sensitivity and spectral resolution.

In their research, the scientists used data from the James Webb Space Telescope’s MIRI spectrometer. MIRI can see directly through clouds of dust and measures mainly warm gas. The main optics of the MIRI spectrometer were designed and built by the Netherlands Research School for Astronomy (NOVA).

“This is exactly the kind of science the MIRI spectrometer was designed for,” he says. Ewen Van Dishoek (University of Leiden), who has been involved in building Webb and the MIRI device since the beginning. “The spectra contain a wealth of data that tells us something about the chemical and physical composition of the planet-forming disks.”

Too much carbon gas, too little oxygen
In addition to the first-ever observation of benzene in a planet-forming disk, the researchers also saw the hydrocarbon diacetylene (C4H2) for the first time, and the disk also contains a very large amount of the gas acetylene (C2H2). This is a highly reactive hydrocarbon. It was surprising that there was so little water and carbon dioxide in the disc. These compounds are often found in other planet-forming disks around sun-like disks. Identification of these gases requires close collaboration with chemists who measure spectra (chemical fingerprints) in the laboratory.

The researchers believe that benzene and (binary) acetylene gases entered the disk due to the destruction of carbon-rich dust grains near the active young star. This releases carbon compounds from the dust. The remaining granules contain silicates with relatively little carbon. At a later stage, the low-carbon granules clump together into larger clumps. These eventually become rocky planets like Earth. This scenario may also explain why the percentage of carbon on Earth is so low.

Fifty tablets to go
Meanwhile, the researchers are processing data from more than 30 other disks of dust around young stars, with 20 more this year. They are expected to discover other particles and gain more knowledge about the formation of rocky planets in disks around young and old stars, which are two to three times the mass of the Sun.

Study lead author Benoit Tabone (now a researcher at the National Center for Scientific Research at Université Paris-Saclay in France and previously associated with Leiden University) concludes: “This work is only a first glimpse into the physical and chemical conditions under which Earth-like planets formed our Earth.” Author participant Aditya AraphaviAdds the doctoral student at the University of Groningen: “Many more molecules will be discovered, either in the J160532 disk or in other disks. Webb is a ‘playground’ not only for astronomers, but also for experts in molecular physics.”

MINDS (MIRI mid-Infrared Disk Survey)
The search was performed within JWST Guaranteed Time Monitoring (GTO) MINDS software. Co-leads the programme Enga Camp (behind). Astronomers from Groningen, Leiden and Nijmegen are part of the team.

Scientific article
Rich hydrocarbon chemistry and a high C to O ratio in the inner disk around a very low mass star. Written by: b. Taboni et al. in: Natural Astronomy, May 11, 2023. [origineel | preprint]