Even More Complex Organic Molecules Have Been Found in a Protoplanetary Disc. Was Life Inevitable?

Will we ever understand life’s origins? Will we ever be able to put our finger on the exact moment and circumstances that lead to living matter? Will we ever pinpoint the spark? Who knows.

But what we can do is find out how widespread the conditions for life are and how widespread the molecular constituents for life are.

If a moment comes when we can point and say, “Look! Behold the Origins of Life!” it would be amazing. But scientific truth tends to come at us like clues along a winding path. Right now, we’re walking that path and finding the building blocks of life in more and more places.

We think peptides, the precursors to amino acids, can form on icy grains in space. According to one study, asteroid impacts can create chemical building blocks to life. On a comet, we’ve found phosphorous, one of the raw elements necessary for life, and we know that comets also host the amino acid glycine.

A new study expands our understanding of how widespread life’s building blocks are. The researchers found a large, complex, organic molecule in a protoplanetary disk. The molecule is called dimethyl ether, and while it’s not a building block for life on its own, it’s a precursor to even larger molecules that can lead to life.

Researchers at Leiden Observatory in the Netherlands discovered the molecule using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. They published their findings in a paper titled “A major asymmetric ice trap in a planet-forming disk III. First detection of dimethyl ether.” The paper is published in the journal Astronomy and Astrophysics, and the lead author is Nashanty Brunken, a Master’s student at Leiden Observatory.

Dimethyl ether has the formula CH3OCH3, simplified to C2H6O. It’s the simplest ether and a precursor to other organic molecules. Image Credit: By Benjah-bmm27 – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2117395

Ethers are common in organic chemistry and are widespread in biochemistry. Dimethyl ether is also called DME or methoxymethane. Astronomers have found it in star-forming molecular clouds but never in a planet-forming disk. “This work confirms the presence of oxygen-bearing molecules more complex than CH3OH (methanol) in protoplanetary disks for the first time,” the authors write in the paper. “It also shows that it is indeed possible to trace the full interstellar journey of complex organic molecules (COMs) across the different evolutionary stages of star, disk, and planet formation.”

“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.”

Nienke van der Marel, Leiden Observatory.

“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. It is very exciting to see how these findings fit into the bigger picture,” lead author Brunken said in a press release.

The disk is around the star Oph-IRS 48, about 400 light-years from Earth. Astronomers are very interested in the disk because the gas (traced by CO molecules) and small dust grains follow a complete disk ring structure around the star, but dust particles are gathered in a crescent shape. The unusual ring is an example of dust trapping and explains how dust particles can grow by clumping together, eventually forming planets, comets, and other bodies.

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

If this new study is accurate, then any rocky bodies that form around Oph-IRS 48 may form with some biological molecules on 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,” said Booth. “This was not known before as in most systems, these molecules are hidden in the ice.”

These images from the Atacama Large Millimeter/submillimeter Array (ALMA) show various gas molecules in the disc around the IRS 48 star, also known as Oph-IRS 48. The disc contains a cashew-nut-shaped region in its southern part, which traps millimetre-sized dust grains that can come together and grow into kilometre-sized objects like comets, asteroids and potentially even planets. Recent observations spotted several complex organic molecules in this region, including formaldehyde (H2CO; orange), methanol (CH3OH; green) and dimethyl ether (CH3OCH3; blue), the last being the largest molecule found in a planet-forming disc to date. Image Credit: ALMA (ESO/NAOJ/NRAO)/A. Pohl, van der Marel et al., Brunken et al.

The researchers may have also detected methyl formate, another complex molecule similar to dimethyl ether. Methyl formate is also a building block for larger organic molecules, but the detection isn’t as robust for dimethyl ether.

In separate recent research, scientists discovered that peptides, the precursors to amino acids, can form on the icy surfaces of dust grains, particularly with silicate and carbon atoms that attach to those grains. This goes alongside other research showing that amino acids themselves can form in the same environment. This newest research piles on more evidence showing that the chemical potential for life is widespread.

The molecules found in the Oph-IRS 48 disk may have formed before the star and the disk formed. These complex molecules can form in giant molecular clouds (GMCs,) which collapse to form stars. GMCs are cold, and simpler molecules like CO2—and even individual carbon atoms—can stick to dust grains. It forms an icy layer, and within that layer, chemical reactions can take place. Those reactions can create more complex molecules, including peptides. “The products are subsequently released back into the gas
phase if there is an increase in temperature resulting in thermal desorption,” the paper states.

As researchers find more complex organic molecules (COMs) in space, and as they piece together their formation, it’s changing our idea of space itself. While space is vast and cold and dark, the potential chemical origins of life appear to be widespread. Habitable planets and moons might be extremely rare—especially planets like Earth that have remained habitable for billions of years—but it’s looking more and more like any planet or moon that can support life likely has the necessary chemicals on hand.

“This work is an important puzzle piece in tracing the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.”

From “A major asymmetric ice trap in a planet-forming disk
III. First detection of dimethyl ether.”

There’s still a lot scientists don’t know about how these prebiotic molecules might eventually end up on planets. But if they’re as widespread as they seem to be, there could be multiple mechanisms and pathways of delivery to planets.

This image from the Atacama Large Millimeter/submillimeter Array (ALMA) shows the dust trap in the disc that surrounds the system Oph-IRS 48. The high asymmetry of the dust emission between the southern and northern parts of the disc (at least a factor of 130) is indicative of the presence of such a dust trap. The dust trap provides a haven for tiny particles in the disc, allowing them to clump together and grow to sizes that will enable them to survive independently. Image Credit: ALMA (ESO/NAOJ/NRAO)/Nienke van der Marel

“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. Hopefully, with more observations, we can get a step closer to understanding the origin of prebiotic molecules in our own Solar System,” said Nienke van der Marel, a Leiden Observatory researcher who also participated in the study.

“Hopefully, future observations of the IRS 48 icy dust-trap will allow for the detection of other COMs and more robust constraints on the column density and excitation conditions,” the authors write in their paper’s conclusion. “This work is an important puzzle piece in tracing the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.”

Future observations with even more powerful telescopes will tell astronomers more about these COMs and their path from giant molecular clouds to comet and planet formation. The cashew-shaped dust-trap is probably caused by the formation of a planet in the disk or maybe by a smaller companion star.

The ESO’s Extremely Large Telescope (ELT) will have enormous observing power, and astronomers should be able to see the very inner regions of the disk with it. That’s where rocky planets like Earth could be forming inside the frost line, and astronomers can use the ELT to study chemistry in that region.

Who knows what they’ll find? Maybe an Earth-like planet is forming there, surrounded by COMs and the building blocks for life.

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Evan Gough

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