Astronomers have detected 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, a benzene molecule (C₆h₆) in a planet-forming disk around a young star. Besides benzene, they saw many other smaller carbon compounds and a few oxygen-rich molecules. Observations indicate that, like Earth, the rocky planets that form in this disk contain relatively little carbon. The scientists published their findings in the journal natural astronomy.

The researchers studied the young star J160532 (one-tenth the mass of our Sun) about 500 light-years from the planet Scorpius. about this small young starsMany rocky planets, similar to Earth, are composed of discs made up of gas and dust. Until now, it has been difficult to study molecules in the warm interior of these disks where the majority of planets form due to the limited sensitivity and spectral resolution of previous observatories.

In their research, the scientists used data from the MIRI spectrometer aboard the James Webb Space Telescope. MIRI can see through clouds of dust and is particularly suitable for measuring the hot gas in the inner disks. 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,” says Eoin van Dishoek (University of Leiden), who has been involved in building Webb and the MIRI instrument 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

Besides the first-ever observation of benzene in a planet-forming disk, the researchers also saw the hydrocarbon diacetylene (C₄h₂) for the first time and an unusually large amount of acetylene gas (C₂h₂), a highly reactive hydrocarbon. Remarkably, there is very little water and Carbon Dioxide in this disk. These oxygen-rich compounds are often found in other dust discs. Identification of these molecules requires close collaboration with chemists who measure spectra (chemical fingerprints) in the laboratory.

The researchers believe that benzene and (bi)acetylene are released into the disk after the destruction of carbon-rich dust grains by the active young star. The remaining dust grains contain silicates with relatively little carbon. At a later stage, the low-carbon granules clump together into larger clumps. This eventually becomes rocky planets like the earth. This scenario may explain why our Earth is so poor in carbon.

Fifty tablets to go

Meanwhile, researchers are mining data from more than 30 more dusty disks around young stars, and data is expected on 20 more disks this year. In doing so, they are expected to discover other particles and gain more knowledge about the formation of planets around stars from the smallest to those two to three times the mass of our Sun.

“This work is only the first glimpse into the physical and chemical conditions under which Earth-like planets like Earth were formed,” says study lead author Benoît Tabone (a CNRS researcher now at the University of Paris-Saclay in France and formerly affiliated with the University of Leiden).

Co-author Aditya Arabhavi, Ph.D. Adds a student at the University of Groningen, “More particles 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.”