The Raw Materials for Life Form Early on in Stellar Nurseries

Life doesn’t appear from nothing. Its origins are wrapped up in the same long, arduous process that creates the elements, then stars, then planets. Then, if everything lines up just right, after billions of years, a simple, single-celled organism can appear, maybe in a puddle of water on a hospitable planet somewhere.

It takes time for the building blocks of stars and planets to assemble in space, and the building blocks of life are along for the ride. But there are significant gaps in our understanding of how all that works. A new study is filling in one of those gaps.

Stars form in Giant Molecular Clouds, vast stellar nurseries that can be hundreds of light-years across and contain millions of solar masses of gas and dust. These nurseries contain mostly hydrogen, the stuff of star formation. But they also contain carbon, and the carbon, hydrogen, and some other atoms combine to form complex molecules that are the rudiments of life.

New research shows how some important organic molecules can form in stellar nurseries. The article is titled “Five-membered ring compounds from the ortho-benzyne + methyl radical reaction under interstellar conditions” and is published in the journal Nature Astronomy. The lead author is Jordy Bouwman, research associate at the Laboratory for Atmospheric and Space Physics (LASP) and assistant professor in the Department of Chemistry at CU Boulder.

Life requires organic chemistry, and all the life we know of is carbon-based. That means carbon and its ability to form large, complex and durable molecules that can branch off into rings and chains is at the heart of life. Each carbon atom can form chemical bonds with four other atoms, and that means that carbon-based molecules can contain thousands of atoms. Unsurprisingly, carbon is present in all organic matter.

This image from ESA’s Herschel space observatory shows the distribution of gas and dust in the Taurus Molecular Cloud. Tangled filaments of gas and dust weave their way through the Cloud. The new study found certain organic compounds forming in Taurus that are links in the chemical chain stretching from the Big Bang to Life. Image Credit: ESA/Herschel/PACS, SPIRE/Gould Belt survey Key Programme/Palmeirim et al. 2013

In nature, chemistry evolved over time. The Universe began with only hydrogen and helium (and a little lithium.) Over time, more elements formed and that allowed more complex chemicals to form. Once carbon was synthesized in stars and spread out into the Universe, the stage was set for truly complex chemistry.

In the present-day Universe, all of the elements that can occur naturally already occur. Nature has put its cards on the table. The stage is set for chemistry to work its magic, creating all kinds of organic compounds, even in gas clouds.

In recent years, scientists have found different types of complex chemicals in the Taurus Molecular Cloud. This illustration shows some of the compounds found in TMC-1, an accreting stellar core in the larger TMC. Image Credit: M. WEISS / CENTER FOR ASTROPHYSICS | HARVARD & SMITHSONIAN

That’s what a team of researchers sees happening in the Taurus Molecular Cloud (TMC,) a stellar nursery about 440 light-years away. It’s called a molecular cloud because the hydrogen atoms are paired together into molecules (H2.) Scientists observe the TMC in detail because it’s a stellar nursery. It’s probably the closest star-forming region to Earth, and astronomers study it extensively. Telescopes like the Herschel Space Observatory have taken its portrait many times. The TMC also contains sub-regions called starless accreting cores, and one of them, called TMC-1, features in this new research.

TMC-1 is known for containing complex organic molecules (COMs.) There are surprisingly large amounts of what cosmochemists like lead author Bouwman call “five-membered ring compounds.” Each of these compounds is built on a pentagon of carbon atoms. The COMs in TMC-1 include compounds like fulvenallene and 1- and 2-ethynylcyclopentadiene.

Researchers found the complex, five-sided molecules fulvenallene (L) and ethynylcyclopentadiene (R) in the starless core TMC-1. Image Credit: NIH

Finding complex chemicals in GMCs is counterintuitive. They’re very cold environments, around -263 degrees Celsius (about -440 degrees Fahrenheit.) That’s only 10 degrees above absolute zero. The cold temperatures are what allow the clouds to collapse and form stars. If they were warmer, there would be outward pressure that inhibited the collapse. But chemical reactions normally require energy, so finding so many of them in frigid TMC-1 is puzzling.

“Researchers kept detecting these molecules in TMC-1, but their origin was unclear,” Bouwman said in a press release. While the larger TMC contains hundreds of young stars only one or two million years old, TMC-1 is a dense starless core that isn’t yet a star. If it were already a star, then its heat could conceivably drive the production of these molecules.

In 2021, researchers found another chemical that helps explain the presence of the pentagon-shaped compounds without any energy source. It’s called ortho-benzene, and it’s a small molecule based on six carbon atoms instead of five. It also has four hydrogen atoms. Its key property is that it can easily react with other molecules without needing a lot of heat.

“There’s no barrier to reaction,” Bouwman said. “That means it has the potential to drive complex chemistry in cold environments.”

The famous Pillars of Creation is in the Eagle Nebula, which is also a molecular cloud. The same type of complex chemistry is at work here as it is in other giant molecular clouds. Image Credit: NASA/ESA/CSA

But just because ortho-benzene has the potential to create the pentagon-shaped compounds in TMC-1 doesn’t mean that it is creating them. Bouwman and his colleagues, who are in the US, Netherlands, Switzerland, and Germany, needed a way to test the idea. They turned to a facility in Switzerland called the Swiss Light Source, a synchrotron at the Paul Scherrer Institute (PSI) in Switzerland. The researchers used UV light from the synchrotron in lab experiments to identify chemical compounds that might be created in stellar nurseries.

They saw that ortho-benzene, the same chemical found in the starless core TMC-1, combined with another type of chemical called methyl radicals to form more complex molecules. So far, so good.

It’s a good hint, but it didn’t yet explain the presence of the pentagon-shaped molecules they found in TMC-1.

“We knew we were onto something good,” Bouwman said.

This graphic shows how hexagonally-shaped ortho-benzyne molecules can combine with methyl radicals (white rectangle) to form a series of larger organic molecules, each containing a ring of five carbon atoms. The research shows this can happen in starless cores like TMC-1. (Credit: Henry Cardwell)

Next, the researchers turned to computer models of stellar nurseries spanning several light-years in space. Those models produced the same mix of organic molecules that astronomers observed in TMC-1 using telescopes. It appears that ortho-benzene is capable of driving the production of the pentagon-shaped fulvenallene and 1- and 2-ethynylcyclopentadiene.

This figure from the research article shows the reactants (top), reactive intermediates (middle), and the reaction products (bottom) for the main chemical species in the researchers’ work. Image Credit: Bouwman et al. 2023.

This research adds another link in the chemical chain reaching from the Big Bang to life. But the chain isn’t complete, and researchers like Bouwman have more work to do. We’re finding simple amino acids like glycine in space. How do organic molecules in space gain nitrogen atoms, which are critical components to amino acids, DNA, and life?

For now, there’s no answer to that question. But as this research shows, we can reasonably hope for an answer one day.

In any case, this work shows how the materials for life are wrapped up in the formation of stars, solar systems, and planets. The fact they’re nearly ubiquitous takes some of the mystery out of the appearance of life.

“Our findings may just change the view on what ingredients we have in the first place to form new stars and new planets,” Bouwman said.