Artist’s depiction of a protoplanetary disk with young planets forming around a star. The right-side panel zooms in to show various organic molecules that are accreting onto a planet. (M.Weiss/Center for Astrophysics | Harvard & Smithsonian)
One of the more persuasive arguments in favor of the potential existence of life beyond Earth is that the well-known chemical building blocks of that life are found throughout the galaxy. These chemical components aren’t all present in all examined solar systems and planets, but they are common and behave in ways familiar to scientists here.
And when it comes elements and compounds found on distant planets but not found here, there just aren’t many. That doesn’t mean they don’t exist — some unstable compounds in interstellar space, for instance — but rather that the cosmos holds many surprises but none have involved extraterrestrial elements or compounds near planets or stars.
This is in large part the result of how elements are formed in the universe. Other than hydrogen and helium, all other elements are forged in the thermonuclear explosion of stars that have exhausted their supply of fuel. These massive explosions (supernovae) then shoot the newly-formed elements out into space where they can and do collect in gas and dust clouds that will form other new stars. They are spread throughout the disks that form around new stars and over time they become components of new planets in formation.
This galactic evolution includes the bonding together of carbon-based organic compounds — the building blocks of life as we know it. They are an essential component to any theory of a planet’s habitability and, while their presence in space and star nurseries has been known for some time, they have remained a subject of great interest but limited detailed knowledge.
That is why an international team from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. set out to intensively study five disks forming around young stars to determine more precisely what organic compounds were present and available for objects developing into planets.
And the results are striking: The abundance of organic material detected was 10 to 100 times more than expected.
“These planet-forming disks are teeming with organic molecules, some of which are implicated in the origins of life here on Earth,” said team leader Karin Öberg. “This is really exciting; the chemicals in each disk will ultimately affect the type of planets that form and determine whether or not the planets can host life.”
This composite image of ALMA data from the young star HD 163296 shows hydrogen cyanide emission laid over a starfield. The MAPS project zoomed in on hydrogen cyanide and other organic and inorganic compounds in planet-forming disks to gain a better understanding of the compositions of young planets and how the compositions link to where planets form in a protoplanetary disk.
ALMA (ESO/NAOJ/NRAO)/D. Berry (NRAO), K. Öberg et al (MAPS)
The team also found, rather to their surprise, that specific organic compounds were not found evenly throughout the disk, but rather were found in clusters. This means, Öberg said, that some protoplanets are inherently more likely to develop into habitable planet simply because they came together in a section of the disk where organics associated with life were broadly present.
“Our overall findings go in two different directions: First that organics are widespread in the disks we studied, and second that because of the greater observing power we had available, we could see that they were
Karin Öberg is an astrochemist and head of the Öberg Astrochemistry Group at Harvard.
not at all evenly distributed in those disks. We learned this because the precision and resolution of the telescope was substantially greater than before.”
That telescope is the Atacama Large Millimeter/submillimeter Array (ALMA), a with 66 radio antenna telescope 16,500 feet high in the mountain desert of northern Chile. Observations with an earlier version of ALMA detected some organics but missed many more and gave an overall count that did not show the organics were clustered in specific areas.
Öberg said that the goal of the extensive disk study — begun in 2018 — was to both interpret the chemical makeup and dynamics of these nurseries of planets and ultimately to be able to make predictions about the potential habitability of evolving planets based on their chemical environment.
“If you know what kind of star and disk you have, you should have a deep understanding of the chemical composition of that disk, and the same for nascent planets in the system,” she said.
A series of 20 papers detailing the project, put together by 39 astronomers across 8 countries and 26 institutions, is appropriately named Molecules with ALMA at Planet-forming Scales, or MAPS, and was published last month in the open-access repository arXiv. The papers have also been accepted to The Astrophysical Journal Supplement as a November special edition series to showcase the high-resolution images and their implications. The paper on large organic compounds detected is here.
Öberg said the MAPS acronym refers to both the physical mapping of disks undertaken as well as the “mapping” of the evolutionary process by which planets are formed from the dust and gas of their disks.
The MAPS program focuses on five disks — around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 — in which dust substructures are detected and planet formation appears to be ongoing. The team observed these disks in 4 ALMA spectral setups, which together cover approximately 50 molecular lines from 20 different chemical species. These are the 20 molecular faces of the HD 163296 disk. (from MAPS I, Öberg et al. 2021).
CfA graduate student Charles Law led MAPS III, the study that mapped out the specific locations of 18 molecules — including hydrogen cyanide, and other nitriles (carbon-nitrogen compounds) connected to the origins of life — in each of the five disks.
The final maps of each disk showed that “understanding the chemistry occurring even in a single disk is much more complicated than we thought,” Law said. “Each individual disk appears quite different from the next one, with its own distinctive set of chemical substructures. The planets forming in these disks are going to experience very different chemical environments.”
Law said that the nitriles were a particularly important finding and that they generally were found in the inner disk regions.
Charles Law is a PhD student in the Department of Astronomy at Harvard University, advised by Karin Öberg and Qizhou Zhang. His research focuses on chemical complexity in protoplanetary disks and star-forming regions
While all elements heavier than helium are formed when stars run out of fuel and go supernova, producing the organic compounds is a next step that is an active area of research.
As Law described it, forming larger and more complex molecules typically happens on the surfaces of dust grains which are then released into the gas phase once the temperature increases (such as when a nearby protostar is formed).
The raw materials needed to form these the organic compounds come from molecular clouds that collapse in the formation of a star, or stars. “But one open question that remains,” he wrote, “is how many of the molecules are formed in the cloud and then are simply inherited as is during the disk phase, whereas how many new molecules are created (or destroyed) by chemical processing during the formation and lifetime of the disk.”
“You can imagine that the disk conditions, with higher densities and warmer temperatures, might lead to different kinds of chemistry that may substantially alter the composition of the gas from that inherited from the initial molecular cloud. This question remains an active area of research.”
Because the abundance of these organics in the five disks studied turned out to be 10 to 100 times larger than models initially predicted, he said, this means that the chemical ingredients for life might be commonplace in the majority of planet-forming disks throughout our galaxy.”
Part of the 66-antenna ALMA radio telescope array in Chile. (Wikipedia)
Another feature of the MAPS study was an effort to find newborn planets in the thick dust and gas of the disks. That effort was led by Richard Teague, a Submillimeter Array fellow at the CfA.
While astronomers are confident that planets form in protoplanetary disks, they can’t directly see them. The gas and dust, which can last three million years, shields developing planets from view.
“It’s like trying to see a fish underwater,” Teague says. “We know they’re there, but we can’t peer that far down. We have to look for subtle signs on the surface of the water, like ripples and waves.”
In protoplanetary disks, gas and dust naturally rotate around a central star. As described is a CfA release, the speed of the moving material, which astronomers can measure, should remain consistent throughout the disk. But if a planet is forming beneath the surface, Teague believes it can slightly disturb the gas traveling around it — causing a small deviation in velocity or the spiraling gas to move in an unexpected way.
Using this approach, Teague analyzed gas velocities in two of the five protoplanetary disks — around the young stars HD 163296 and MWC 480 — and small hiccups in velocity in certain portions of the disks revealed a young Jupiter-like planet embedded in each of the disks. The observations are detailed in MAPS XVIII.
As the planets grow, they will eventually “carve open gaps in the structure of the disks” so we can see them, Teague says, but the process will take thousands of years.
