EVERY STAR AND PLANETARY SYSTEM in the universe was once just a cloud of gas and dust. These humble dust clouds, astronomers have found, are rich with organic molecules necessary for life. Their cool temperatures, chemical compositions, and other factors make them really good at forming these vital compounds.
When one of these clouds gets dense enough, it collapses to become a spinning disk, and eventually, a star and its planets. The collapse, and the new star, heats up the gas and dust in the disk. If the disk gets too hot, it’s possible that the organic molecules would be destroyed, making it far less likely for planets born in this disk to have important molecules required for life to form.
“If you want to form a planet, whatever material is in the disk will set the composition of your forming planets,” astronomer Alice Booth of Leiden Observatory in the Netherlands tells Inverse.
But in a new study published May 10 in Nature Astronomy, Booth and colleagues describe evidence they found that complex organic molecules can survive in a warm planet-forming disk — promising news in the quest to understand where in the universe life could potentially arise.
WHAT’S NEW — Astronomers have often detected complex organic molecules in cool, star-forming clouds. And observations from our own Solar System imply that complex organic molecules could survive a cloud collapsing into a planet-forming disk, at least for a Sun-like system.
Booth and colleagues have now detected the organic molecule methanol in the warm, planet-forming disk around the star HD 100546, about 360 light-years away.
- HD 100546 is middle-aged for a star its size
- Astronomers have found evidence that at least two gas giant planets have formed in the system so far
- It’s what astronomers call an “A-type” star, meaning it’s bigger and has a hotter surface temperature than our Sun
And because methanol requires very cool temperatures to form in space, it probably didn’t form in the disk with that hot star around — it formed in the cloud phase and stuck around as the cloud collapsed.
HOW THEY DID IT — The team investigated the disk using the Atacama Large Millimeter / submillimeter Array observatory in Chile. In their ALMA data, the team looked for emission lines — specific wavelengths of light that are emitted by a given molecule — for several different compounds, including methanol. And they found the emission — a clear sign that the disk contains methanol.
“It’s a beautiful detection,” astrochemist Cécile Favre of the Université Grenoble Alpes in France, who was not involved in the study, tells Inverse.
WHY IT MATTERS — Among complex organic molecules, methanol is a pretty simple one, containing only six atoms per molecule. But the fact that researchers detected methanol in this disk means that organic molecules of greater complexity likely survived the transition from cool cloud to warm disk too.
“That means that when you start assembling planets and comets, you start with already a very high degree of chemical complexity,” astrochemist Karin Öberg of the Harvard-Smithsonian Center for Astrophysics, who was not involved in the study, tells Inverse. “Now, how you go from there to life is something we don't know. But it is exciting to know that we are already on our way towards building up organic, complex chemistry when planets are assembling.”
By showing that complex organic molecules such as methanol can stick around in planet-forming disks hotter than our Sun’s would have been, Booth and colleagues have shown that planets that form around hotter stars could have these building blocks necessary for life too.
WHAT’S NEXT — Booth says the team plans to make more observations of this planet-forming disk with ALMA, hopefully at higher resolutions, to try to see where in the disk the methanol is.
Booth, Öberg, and Favre all mentioned that another next step would be to search for methanol in other planet-forming disks, to see how common it is for complex organic molecules to stick around.
“Hopefully, this should hold for other systems,” Booth says. “And this means that the building blocks of what we need for life are kind of everywhere. And they're already there before we even form the stars.”
Abstract: Quantifying the composition of the material in protoplanetary disks is essential to determining the potential for exoplanetary systems to produce and support habitable environments. When considering potential habitability, complex organic molecules are relevant, key among which is methanol (CH3OH). Methanol primarily forms at low temperatures via the hydrogenation of CO ice on the surface of icy dust grains and is a necessary basis for the formation of more complex species such as amino acids and proteins. We report the detection of CH3OH in a disk around a young, luminous A-type star, HD 100546. This disk is warm and therefore does not host an abundant reservoir of CO ice. We argue that the CH3OH cannot form in situ, and hence that this disk has probably inherited complex-organic-molecule-rich ice from an earlier cold dark cloud phase. This is strong evidence that at least some interstellar organic material survives the disk-formation process and can then be incorporated into forming planets, moons and comets. Therefore, crucial pre-biotic chemical evolution already takes place in dark star-forming clouds.