An international team of astronomers, including scientists from the University of Arizona, has used NASA's James Webb Space Telescope to study the disk of gas and dust around a young, very low-mass star. The results reveal the largest number of carbon-containing molecules seen to date in such a disk. The findings have implications for the potential composition of any planets that might form around this star.

Rocky planets are more likely than gas giants to form around low-mass stars, making them the most common planets around the most common stars in our galaxy. Little is known about the chemistry of such worlds, which may be similar to or very different from Earth. By studying the disks from which such planets form, astronomers hope to better understand the planet formation process and the compositions of the resulting planets.

The scientists' findings, published today in the journal Science, build on a 2009 study led by Ilaria Pascucci, a UArizona professor of lunar and planetary sciences, who is also a co-author of the new study. In their previous work, Pascucci's team used the Spitzer Space Telescope to identify that the gas composition of disks around very-low mass stars differs from that around solar-type stars or stars that have higher mass. Spitzer detected acetylene and hydrogen cyanide, which are simpler molecules that have a smaller number of carbons. 

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"The James Webb spectrum is fantastic. With higher resolution and sensitivity than Spitzer, it enabled detection of many carbon-bearing molecules – even complex ones like benzene," Pascucci said. 

The study's findings demonstrate that the gas composition is very hydrocarbon rich and expands scientists' knowledge of the chemical complexity of disks around very low-mass stars, Pascucci said. These exoplanets could build an early atmosphere that is hydrocarbon rich – very different from the early atmosphere that Earth built.

Planet-forming disks around very low-mass stars are difficult to study because they are smaller and fainter than disks around high-mass stars. A program called the MIRI Mid-INfrared Disk Survey, or MINDS, aims to use Webb's unique capabilities to build a bridge between the chemical inventory of disks and the properties of exoplanets.

"Webb has better sensitivity and spectral resolution than previous infrared space telescopes," said lead study author Aditya Arabhavi, a Ph.D. student at the University of Groningen in the Netherlands. "These observations are not possible from Earth, because the emissions from the disk are blocked by our atmosphere."

In the new study, researchers explored the region around a very low-mass star known as ISO-ChaI-147, a 1 to 2 million-year-old star that weighs just 0.11 times as much as the sun. The spectrum revealed by Webb's Mid-Infrared Instrument, or MIRI, shows the richest hydrocarbon chemistry seen to date in a protoplanetary disk – a total of 13 different carbon-bearing molecules. The team’s findings include the first detection of ethane outside of our solar system, as well as ethylene, propyne and the methyl radical CH3.

"These molecules have already been detected in our solar system, like in comets such as 67P/Churyumov–Gerasimenko and C/2014 Q2 (Lovejoy)," Arabhavi said. "Webb allowed us to understand that these hydrocarbon molecules are not just diverse but also abundant. It is amazing that we can now see the dance of these molecules in the planetary cradles. It is a very different planet-forming environment than we usually think of."

The team indicates that the results have significant implications for the chemistry of the inner disk and the planets that might form there. Since Webb revealed the gas in the disk is so rich in carbon, there is likely little carbon left in the solid materials from which planets would form. As a result, the planets that might form there may ultimately be carbon-poor like Earth.

"This is profoundly different from the composition we see in disks around solar-type stars, where oxygen-bearing molecules like water and carbon dioxide dominate," said team member Inga Kamp, professor at the University of Groningen. "This object establishes that these are a unique class of objects."

"It's incredible that we can detect and quantify the amount of molecules that we know well on Earth, such as benzene, in an object that is more than 600 light-years away," said team member Agnés Perrin, research director at the Centre National de la Recherche Scientifique in France.

Next, the science team intends to expand their study to a larger sample of such disks around very low-mass stars to develop their understanding of how common or exotic such carbon-rich terrestrial planet-forming regions are. 

"The expansion of our study will also allow us to better understand how these molecules can form," said team member and principal investigator of the MINDS program Thomas Henning, director of the Planet and Star Formation department at the Max-Planck-Institute for Astronomy in Germany. "Several features in the Webb data are also still unidentified, so more spectroscopy is required to fully interpret our observations."