Extensive mapping of chemical composition reveals carbon-rich, organic natal environments of planets

Scientists connect the dots between the formation of planets and what they are made of.

An international collaboration of scientists who created the Atacama Large Millimeter / Submillimeter Array (ALMA) has the most extensive mapping of the chemical composition of the protoplanetary disks about five nearby young stars in high resolution that produce images that capture the molecular composition associated with planetary births; and a roadmap for future studies on the composition of planet- and comet-forming regions. The new study provides clues as to the role of molecules in the formation of planetary systems and whether these emerging planetary systems have what it takes to harbor life. The results of the program, aptly called MAPS or Molecules with ALMA at Planet-forming Scales, will be featured in an upcoming 20-part special edition of. appear The supplementary series of the Astrophysical Journal.

Planets form in the dust and gas disks – also called protoplanetary disks – that surround young stars. The chemical makeup of these disks – or the molecules they contain – can have an impact on the planets themselves, including how and where planet formation takes place, the chemical makeup of the planets, and whether these planets have the organic makeup that is necessary for life is required. MAPS specifically examined the protoplanetary disks surrounding the young stars IM Lup, GM Aur, AS 209, HD 163296 and MWC 480, where evidence of ongoing planet formation has already been found. The project led to several exciting discoveries, including a link between dust and chemical substructures and the presence of large reservoirs of organic molecules in the inner disk regions of stars.

In the conception of this artist, planets are created from gas and dust in the protoplanetary disk that surrounds the young star. The gas is made up of many different molecules, including hydrogen cyanide and more complex nitriles – which are associated with the development of life on earth – and other organic and inorganic compounds. From simple organic to more complex compounds, the soup of molecules at a certain location in the disk shapes the future of the planet that is emerging there and determines whether or not that planet could support life as we know it. Photo credit: M.Weiss / Center for Astrophysics / Harvard & Smithsonian

“With ALMA we were able to see how molecules are distributed where exoplanets are currently assembling,” says Karin Öberg, astronomer at the Center for Astrophysics | Harvard & Smithsonian (CfA) and the Principal Investigator for MAPS. “One of the really exciting things we’ve seen is that the planet-forming disks around these five young stars are factories of a special class of organic molecules called nitriles that are involved in the creation of life here on Earth.”

Simple organic molecules such as HCN, C2H and H2CO were observed in unprecedented detail throughout the project thanks to the sensitivity and resolution of ALMA’s band-3 and band-6 receivers. “In particular, we were able to observe the amount of small organic molecules in the inner regions of disks, where rocky planets are likely to accumulate,” said Viviana V. Guzmán, astronomer at the Instituto de Astrofísica of the Pontificia Universidad Católica de Chile, first author on MAPS VI and a MAPS- Co-Principal Investigator. “We find that our own solar system is not particularly unique and that other planetary systems around other stars have enough basic ingredients to form the building blocks of life.”

Scientists also observed more complex organic molecules such as HC3N, CH3CN, and C.-C3H2 – especially those that contain carbon and therefore most likely serve as a starting material for larger, prebiotic molecules. Although these molecules have already been detected in protoplanetary disks, MAPS is the first systematic study of multiple disks with very high spatial resolution and sensitivity and the first study to find molecules on a small scale and in such significant quantities. “We found more of the large organic molecules than expected, a factor of 10 to 100 more, in the inner disks on the solar system scales, and their chemistry seems to be similar to that of comets in the solar system,” said John Ilee, an astronomer of the University of Leeds and lead author of MAPS IX. “The presence of these large organic molecules matters as they represent the stepping stone between simpler carbon-based molecules like carbon monoxide, which is found in abundance in space, and the more complex molecules necessary to create and sustain life. ”

Molecules, however, are not evenly distributed across planet-forming disks, as shown in MAPS III and IV, which showed that while the general disk composition appears to be similar to the solar system, high-resolution zooming reveals some compositional diversity leading to planet -to-planet-differences. “Molecular gas in protoplanetary disks is often found in groups of different rings and voids,” said Charles Law, CfA astronomer and lead author of MAPS III and IV. “But the same disk observed in different molecular emission lines often looks completely different , with each disc having multiple molecular faces. It also means that planets can form in different disks or even in the same disk in different places in radically different chemical environments. ”This means that some planets form with the necessary tools to build and sustain life, while other planets form in nearby may not do this.

One of these radically different environments takes place in the room environment Jupiter-like planets where scientists have found the gas to be poor in carbon, oxygen, and heavier elements while being rich in hydrocarbons like methane. “The chemistry seen in protoplanetary disks should be inherited through the formation of planets,” said Arthur Bosman, astronomer at the University of Michigan and lead author of MAPS VII could form extremely low-oxygen (carbon-rich) atmospheres, which calls into question the current expectations of the composition of the planets. “

All in all, MAPS offers just that: a map that scientists can track by connecting the points between gas and dust in a protoplanetary disk and the planets that will eventually form from them to create a planetary system. “The composition of a planet is a record of the place in the disk where it was formed,” said Bosman. “The combination of planet and disk composition enables us to look into the history of a planet and understand the forces that have shaped it.”

Joe Pesce, Astronomer and ALMA Program Officer at the National Science Foundation (NSF): “Whether life beyond Earth exists is one of the most fundamental questions facing humanity. We now know that planets can be found everywhere and the next step is to determine if they have the necessary conditions for life as we know it (and how common this situation might be). The MAPS program will help us better answer these questions. ALMA’s search for precursors of distant life complements studies conducted in laboratories and in places such as hydrothermal vents on Earth. “

Öberg added, “MAPS is the culmination of decades of work on the chemistry of planet-forming disks by scientists using ALMA and its precursors. Although MAPS had only examined five slices at the time, we had no idea how chemically complex and visually impressive these slices really were. For the first time, MAPS answered questions that we could not have imagined decades ago, and also presented us with many more questions that need to be answered. “

Learn more about the MAPS program on the Project website.

Highlighted Papers

“Molecules with ALMA at Planet-forming Scales (MAPS) I: Program overview and highlights”, K. Öberg et al., The supplementary series of the Astrophysical Journal, Preview [https://arxiv.org/pdf/2109.06268.pdf]

“Molecules with ALMA at Planet-Forming Scales (MAPS) III: Characteristics of radial Chemical Substructures”, C. Law et al., The supplementary series of the Astrophysical Journal, Preview [https://arxiv.org/pdf/2109.06210.pdf]

“Molecules with ALMA at Planet-Forming Scales (MAPS). IV: Emission Surfaces and Vertical Distribution of Molecules “, C. Law, The supplementary series of the Astrophysical Journal, Preview [https://arxiv.org/pdf/2109.06217.pdf]

“Molecules with ALMA at Planet-Forming Scales (MAPS) VI: Distribution of the small organics HCN, C2H and H2CO”, V. Guzmán et al., The supplementary series of the Astrophysical Journal, Preview [https://arxiv.org/pdf/2109.06391.pdf]

“Molecules with ALMA at Planet-forming Scales (MAPS) VII: Substellar O / H and C / H and superstellar C / O in planet-feeding gas”, A. Bosman et al., The supplementary series of the Astrophysical Journal, Preview [https://arxiv.org/pdf/2109.06221.pdf]

“Molecules with ALMA at Planet-Forming Scales (MAPS) IX:” Distribution and properties of the large organic molecules HC3N, CH3CN and c-C3H2 “, J. Ilee et al., The supplementary series of the Astrophysical Journal, Preview [https://arxiv.org/pdf/2109.06319.pdf]

About ALMA

The Atacama Large Millimeter / submillimeter Array (ALMA), an international astronomy institution, is a partnership of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), the US National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in collaboration with the Republic of Chile. ALMA is funded by ESO on behalf of its member states, by NSF in collaboration with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST), and by NINS in collaboration with Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

The construction and operation of ALMA is managed by ESO on behalf of its member states; from the National Radio Astronomy Observatory (NRAO), administered by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) ensures the uniform management and administration of the construction, commissioning and operation of ALMA.