Astronomers discover the largest molecule yet in a planet-forming disc

Researchers have detected the organic molecule dimethyl ether (CH₃OCH₃) in a planet-forming disc for the first time, according to a new study published in the journal Astronomy & Astrophysics.

Dimethyl ether is commonly seen in star-forming clouds but has never been found in planet-forming (proto-planetary) discs – a rotating disc of gas, dust, and ice that develops around a newly forming star from which planets can arise. With nine atoms, this is the largest complex organic molecule (COM) identified in such a disc to date.

COMs are a building block of the larger organic molecules such as amino acids and sugars that are important for the emergence of life. Studying them in planet-forming discs allows us to gain a better understanding of how they’re incorporated into planets, including our own.

“From these results, we can learn more about the origin of life on our planet and therefore get a better idea of the potential for life in other planetary systems,” says lead author Nashanty Brunken, a master’s student at Leiden Observatory in Leiden University, the Netherlands. “It is very exciting to see how these findings fit into the bigger picture.”

The dimethyl ether was found in the disc around the young star IRS 48 (also known as Oph-IRS 48), using the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile – the largest radio telescope in the world. Located 444 light-years away in the constellation Ophiuchus, the star was studied because its disc contains an asymmetric, cashew-nut-shaped region known as a “dust trap”.

Located in the southern part of the disc, this is a region where large numbers of millimetre-sized particles of dust clump together and grow into kilometre-sized objects like comets, asteroids and potentially even planets. It is also a reservoir for dust grains covered in ice that’s rich with COMs.

The COMs are trapped in ice because they form within the extremely cold environments of star-forming clouds (before the stars themselves are born), where atoms and simple molecules like carbon monoxide stick to dust particles. There, they form an icy layer in which they undergo chemical reactions to produce more complex molecules.

“It is really exciting to finally detect these larger molecules in discs,” says co-author Dr Alice Booth, a researcher at Leiden University. “For a while we thought it might not be possible to observe them.”

This is because the trapped molecules only become detectable when heating from IRS 48 causes the ice to turn from a solid to a gas, freeing them.

“What makes this even more exciting is that we now know these larger complex molecules are available to feed forming planets in the disc,” explains Booth. “This was not known before as in most systems these molecules are hidden in the ice.”

The discovery of dimethyl ether suggests that other COMs commonly detected in star-forming regions may also be lurking undetected in the ice of planet-forming discs.

“We are incredibly pleased that we can now start to follow the entire journey of these complex molecules from the clouds that form stars, to planet-forming discs, and to comets,” adds Nienke van der Marel, assistant professor at Leiden Observatory. “Hopefully with more observations we can get a step closer to understanding the origin of prebiotic molecules in our own Solar System.”